Current Feedback Amplifiers Voltage Feedback Amplifiers .6.2 High Spee

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Current Feedback Amplifiers Voltage Feedback Amplifiers .6.2 High Spee

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High Speed Amplifiers
Current Feedback Amplifiers Voltage Feedback Amplifiers .6.2 High Speed Amplifier Applications Integrator .6.18 Transimpedance Amplifier .6.23 State Variable Filter .6.28 Sallen Pass Filter .6.30 Difference Amplifier .6.33 Instrumentation Amplifier .6.39
Contributing Author: Bonnie Baker
WHICH BEST APPLICATION?
VERR
AOL(s)
VOUT
IERR
Z(s)
VOUT
Voltage Feedback Amplifier
Current Feedback Amplifier
commonly asked question, when comes matters current feedback amplifiers "Why need application?" quick response that question could "You adjust gain with external resistors without adjusting bandwidth". regular user voltage feedback amplifiers would have difficult time with this statement because been ingrained with fact that closed-loop gain directly related closed-loop bandwidth (Gain Bandwidth Product). most designers, bandwidth specification criterion determining which right application. Current feedback amplifiers also rumored have higher bandwidths, faster slew rates lower distortion than their voltage feedback amplifier cousins. other hand, misconceptions what current feedback amplifier leads designers believe that amps difficult design with because mismatched inputs transimpedance open-loop gain. This tutorial designed address common misconceptions about current feedback amplifier prove worthiness variety applications.
VOLTAGE FEEDBACK AMPLIFIER BLOCK DIAGRAM
VOUT VERR
VERR
VERR AOL(s)
VOUT
voltage feedback amplifier most prolific amplifier market. Dependent characteristics specific amplifier, they used high speed well precision applications. Since preferred frame reference most analog designers voltage feedback amplifier, comparison analysis begins with topology shown above. This simplified block diagram illustrates many characteristics voltage feedback amplifier. Starting with input segment diagram, inputs voltage feedback amplifier evenly matched. consequence, input bias currents very close being same magnitude difference between input bias currents usually small. Additionally, input impedances amplifier close equal relatively high. high speed voltage feedback amplifiers, input bias currents 100µA uncommon. small error voltage input amplifier gained open-loop gain, (s), which usually fairly high. common this gain range 60dB, however, some high speed voltage feedback amplifiers have open-loop gains 90dB better. open-loop gain frequency dependent starts rolling relatively frequency. resulting open-loop voltage output product open-loop gain times input voltage error. compensation capacitor, contributes stability amplifier well dominates slew rate performance.
VOLTAGE FEEDBACK AMPLIFIER SPECIFICATIONS
Voltage Feedback Stable Voltage Feedback Unity Gain Stable
GBWP 400MHz
PART V/µs 1000
GBWP 200MHz
PART V/µs
OPA643 OPA621 OPA675 OPA676 OPA651
OPA642 OPA640 OPA655 OPA646 OPA620
GBWP 400MHz
PART V/µs
GBWP 200MHz
PART V/µs
OPA654 OPA641 OPA678 OPA637
OPA628 OPA650 OPA2650 OPA4650 OPA671
Some voltage feedback amplifiers designed stable closed-loop gains +2V/V internally compensated stable closed-loop gains +1V/V. Voltage feedback amplifiers that stable gains +2V/V higher, typically have wider bandwidths, higher slew rates, lower input voltage noise. Gain Bandwidth Product (GBWP) amplifier defined frequency which open-loop gain amplifier would gain curve were extrapolated 20dB/decade from dominant pole. This figure merit does account poles zeros amplifier's transfer function higher frequency. Consequently, possible operate amplifier with wider bandwidth than predicted GBWP because decrease phase margin. trade-off this increased bandwidth gain peaking some instances instability. amplifiers listed above have been separated into GBWP classes assist product selection process. term GBWP meaning when discussing bandwidth performance current feedback amplifier (CFB), will shown later.
NOTE: Bold indicates PRODUCT.
CURRENT FEEDBACK AMPLIFIER BLOCK DIAGRAM
VOUT IERR Z(s)
IERR
VOUT IERR Z(s)
current feedback amplifier's block diagram illustrates this amplifier differs from voltage feedback amplifier. inputs current feedback amplifier matched, consequently input bias currents different along with input impedances. Typically, current feedback amplifier's input bias current micro ampere region. ratio between input bias currents dependent current feedback input stage topology vary from inverting input higher input bias current magnitude very input resistance (ideally zero) compared non-inverting input. other hand, non-inverting input buffered, high impedance magnitude input bias current lower than inverting input bias current. buffer's gain approximately +1V/V bandwidth significantly wider than bandwidth remaining internal stages amplifier. small error current from inverting input current feedback amplifier gained open-loop transimpedance amplifier, Z(s), which usually fairly high. resulting open-loop output voltage current feedback amplifier product open-loop transimpedance (Z(s)) times input current error (IERR).
CURRENT FEEDBACK AMPLIFIER SPECIFICATIONS
PART OPA658 OPA648 OPA2658 OPA4658 OPA644 OPA623 OPA603
(MHz)
(V/µs) 1700 1200 1700 1700 2500 2100 1000
Current feedback amplifiers usually optimized specified operate specific closed-loop gain, which typically +2V/V. This approach current feedback design used offer best amplifier majority applications that amplifier will used sometimes misleading. Current feedback amplifiers used over wide range gains. current feedback amplifiers unity gain stable. family, current feedback amplifiers touted having higher slew rate than voltage feedback amplifiers. This generalization true large fast input signals. tail current current feedback amplifier's inverting input mirrored tail current internal that supplies current compensation capacitor controlling slew rate. With high input dV/dt signals delta current compensation capacitor increased, consequently slew rate higher than input signal small.
TYPICAL NON-INVERTING AMPLIFIER
VOUT
VOUT
Although internal topology voltage feedback amplifier current feedback amplifier seem quite different, both amplifier types suitable this typical non-inverting amplifier configuration. frequency gain, VOUT/ resistors, feedback mechanism voltage feedback amplifier VERR, which very small, conjunction with high open-loop gain, feedback input resistors. feedback mechanism current feedback amplifier IERR, which also very small, conjunction with high transimpedance gain, feedback resistor. both cases, error signal small, gain component large negative feedback used control system.
NON-INVERTING AMPLIFIER WITH VOLTAGE FEEDBACK AMPLIFIER
VOUT VERR
VOUT
VERR
VERR
VOUT (VIN VERR)
AOL(s)
voltage feedback amplifier analyzed across frequency spectrum non-inverting circuit show above. using simple model discussed earlier, equations quickly come calculation. These calculations assume there contributions frequency behavior circuit from input bias currents amplifier. Since this analysis assumes amplifier operating linear region this small signal analysis, this good assumption. open-loop gain amplifier modeled single pole system. single pole AOL(s) equation represents dominant pole. Typically, this pole occurs 100s region. This formula accurate representation open-loop gain over entire frequency spectrum, however, adequate purposes this discussion. open-loop gain symbolized with variable, element, represents effective impedance open-loop gain equation. used frequency dominant pole. conjunction with internal bias currents also dominates slew rate response voltage feedback amplifier. Rigorous calculation transfer function reveals characteristics limitations voltage feedback amplifier this closed-loop system.
CALCULATION CONCLUSIONS
VOUT RIN) AOL(s) RIN)
Gain Frequency Behavior
expected, calculation proves that closed-loop gain equal 1+RF /RIN. frequencies open-loop gain amplifier sufficiently high allow ignoring gain error. frequency increases, AOL(s) begins decrease finally becomes dominant controlling factor gain circuit. calculation intersection open-loop gain, AOL(s) noise gain (GN), (1+RF /RIN) gives close approximation bandwidth closed-loop amplifier circuit. Gain peaking, which caused phase response amplifier feedback circuit, increase bandwidth closed-loop system expense increased instability. Careful examination denominator this equation points dependence closed-loop bandwidth ratio input feedback resistors (RF) circuit.
Gain
VOLTAGE FEEDBACK AMPLIFIER BODE PLOT
VOUT RIN) AOL(s) RIN)
frequency
transfer function non-inverting amplifier shown graphically above. open-loop gain plot amplifier assumes single pole system, which completely realistic. Even though this case, generalization closed-loop gain closed-loop bandwidth shown here still true. closed-loop gain increases, closed-loop bandwidth decreases. circuit designer needs take this characteristic under consideration when selecting right amplifier application. guarantee stability phase margin must greater than degrees. open-loop gain curve represented single pole system, phase margin would degrees higher. Voltage feedback amplifiers current feedback amplifiers have high frequency poles zeros open-loop transfer function. case voltage feedback amplifiers, increases closed-loop bandwidth usually decreases phase margin. decrease phase margin cause gain peaking, giving higher than expected closedloop frequency response well overshoot ringing with step function response.
6.10
VOLTAGE FEEDBACK AMPLIFIER
BENEFITS
Matched inputs accuracy
DISADVANTAGES
Bandwidth tied desired gain
VOUT
GAIN RIN) AOL(s)
Matched inputs benefit when using voltage feedback amplifiers high speed applications. high impedance saving grace times when line termination otherwise difficult. addition, offset voltages offset currents relatively compared current feedback amplifier topology. These offsets gained closed-loop network. These characteristics benefit, recalling that this class amplifier typically used high speed applications offsets issue because coupling techniques used circuit. High speed applications typically value resistors because bandwidth limitations parasitics encountered when high value resistors used. Micro amps bias current times ohms resistance will give millivolts offset. possible disadvantage voltage feedback amplifier intimate relationship between bandwidth closed-loop gain. Additionally, harmonic distortion typically good current feedback amplifier over wide range gains.
6.11
CURRENT FEEDBACK AMPLIFIER NON-INVERTING GAIN
VOUT IERR Z(s)
IERR Z(s) VOUT
VOUT
IERR
Z(s)
current feedback amplifier also used non-inverting circuit shown above. using simple model discussed earlier, equations quickly come calculation. These calculations assume there contributions frequency behavior circuit from input offset voltage buffer stage amplifier. Since this analysis assumes amplifier operating linear region this small signal analysis, these good assumptions. open-loop transimpedance amplifier modeled single pole system. single pole equation slide represents dominant pole. Typically, this pole occurs 100s kilohertz region. This formula accurate representation open-loop transimpedance over entire frequency spectrum, however, adequate purposes this discussion. open-loop transimpedance symbolized with variable, used derive frequency dominant pole. conjunction with transient inverting error current also dominates slew rate response current feedback amplifier. Rigorous calculation transfer function reveals characteristics limitations current feedback amplifier this closed-loop system.
6.12
CALCULATION CONCLUSIONS
VOUT Z(s) RIN)
Gain Frequency Behavior
gain this circuit same regardless whether current feedback voltage feedback amplifier used. bandwidth closed-loop response, when current feedback amplifier used, dependent feedback resistor, conjunction with transimpedance amplifier. resistor, does effect bandwidth circuit would voltage feedback amplifier used circuit. This fundamental difference closed-loop response between amplifier topologies allows each have advantage disadvantage, case dependent circuit topology selected.
6.13
DESIGN PROBLEM NON-INVERTING
OPA658
VOUT
Define Select Define Select
required bandwidth your amplifier required closed-loop gain
circuit design problem using current feedback amplifier uses simple, straight forward process. Initially, required closed-loop bandwidth must determined required application. From that specification, appropriate current feedback amplifier selected. current feedback specification sheet, appropriate feedback resistor suggested. closed-loop gain then determined dictated application. appropriate input resistor, RIN, selected according closed-loop gain requirements. event closed-loop gain changed, selected, leaving constant. circuit design problem more complex when using voltage feedback amplifier. required closed-loop bandwidth gain must known from beginning. appropriate amplifier then selected estimating closed-loop bandwidth gain from typical performance curves given specification sheet. event that gain needs adjusted, possible that complicated compensation techniques required that another amplifier, with different bandwidth characteristics, will needed.
6.14
CURRENT FEEDBACK AMPLIFIER BODE PLOT
Gain VOUT
RIN) Z(s)
Ideally, closed-loop frequency bandwidth independent changes possible adjust closed-loop gain without changing bandwidth. This conclusion reached using assumptions that take into account first order effects current feedback amplifier closed-loop system. Ideally, current feedback amplifier viewed single pole transimpedance system with infinite impedance non-inverting input zero impedance inverting input. Additionally, buffer gain between inverting non-inverting input +1V/V with zero offset voltage. When these assumptions used, easy derive relationship between feedback resistor, input resistor, RIN, closedloop bandwidth performance discussed previous sections. When these assumptions re-examined, shown that second order effects have small impact closed-loop bandwidth. equation below, alpha represents gain input buffer, which typically +0.996V/V opposed +1V/V. represents non-zero output impedance input buffer, which ranges from depending particular amplifier used.
VOUT(s) VIN(s)
(1+RF/RIN) +RS(1+RF/RIN))/Z(s
From formula above, easy limitations current feedback amplifier's frequency response performance. Because effects closedloop bandwidth does vary slightly with changes RIN. addition, frequency gain attenuated alpha.
6.15
CURRENT FEEDBACK AMPLIFIERS
VOUT Z(s) GAIN
BENEFITS
Ease Design Dominant pole higher/lower distortion Bandwidth dependent
DISADVANTAGES
Bias Current Current Noise
current feedback amplifier offers more ease design process than voltage feedback amplifier. dominant pole current feedback open-loop transimpedance gain higher frequency than voltage feedback open-loop gain dominant pole. Consequently, current feedback amplifiers have lower gain distortion signal increases frequency. bandwidth current feedback amplifier closed-loop configuration dependent adjustable with feedback element. Some disadvantages current feedback amplifier are, input bias currents mismatched current noise inverting input higher than non-inverting input.
6.16
DESIGN PROBLEM INVERTING
VOUT
Current Feedback
VOUT Z(s) /RIN
Voltage Feedback
VOUT (1+nRF/RIN) AOL(s) /RIN
Following similar methods calculation, gain frequency response inverting amplifier circuit derived. Note that bandwidth circuit with voltage feedback amplifier changes with changes input resistance. Also note that closed-loop bandwidth smaller than expected. Since current feedback amplifiers depend closed-loop frequency response, possible change gain without changing bandwidth. This amplifier circuit implements simple summing function. Both current feedback voltage feedback amplifier will work this circuit, however, number inputs changed fly, configuration with voltage feedback amplifier will change signal bandwidths. event that multiplexed inputs required, effective input resistance changes according number signals multiplexed particular time. This easily illustrated transfer function circuit with voltage feedback amplifier. this case, numerator ratio that determines frequency response voltage feedback gain formula. current feedback amplifier performs best this situation because relative immunity changes effective non-inverting gain.
6.17
INTEGRATOR AMPLIFIER
OPA650
VOUT
BEST WITH VOLTAGE FEEDBACK
Integrators natural many circuit applications, using current feedback amplifier this configuration would mistake. voltage feedback best choice when consider that feedback element, does exist. With current feedback amplifiers, capacitive element alone (without resistance series) feedback loop will make circuit unstable. voltage feedback amplifiers, unity gain stability requirement. closed-loop gain this circuit higher frequencies dominated capacitors equal where equal parallel combination amplifier input capacitance input resistor parasitic capacitance equal capacitance feedback loop circuit. insure stability with these amplifiers, high frequency gain equation, must equal greater than specified stable gain amplifier used.
6.18
INTEGRATORS USING CURRENT FEEDBACK AMPLIFIER
Lossy integrator
OPA658
VOUT
Noisy integrator
OPA658
VOUT
possible current feedback amplifiers integrators long required feedback resistance circuit. first diagram required feedback resistor placed feedback loop series with integrating capacitor. appropriate feedback resistor this circuit recommended manufacturer's value. Although this circuit will perform integration function, there some degradation signal bandwidth. second diagram, required feedback resistor placed series with inverting input amplifier. current feedback amplifier loop requirements fulfilled with position Although, stability achieved with relatively high voltage noise source introduced into circuit. Typically, current noise from inverting input current feedback amplifier significantly higher than non-inverting input same amplifier well higher than most voltage feedback amplifiers. This current noise multiplied resistor, then multiplied closed-loop noise gain circuit.
6.19
NANOSECOND INTEGRATOR
27pF OPA660 Buffer
VOUT
OPA660
Sample Hold
transconductance configured "nanosecond integrator" illustrated here with OPA660. This circuit process incoming pulses that have amplitude +/-2.5V short duration. OPA660 will respond risetime. transconductance amplifier (OTA), like OPA660, voltage-controlled current source, which particularly useful when load capacitor. relationship between voltage across load capacitor, output current transfer function integrator VOUT
VBEdt
where integration capacitor, voltage across input terminals OPA660 transimpedance OPA660, adjustable with external resistor, output voltage equal time integral input voltage. constants influence output voltage; transconductance amplifier external capacitor, transconductance, which essentially gain varied over wide range. This allows some flexibility voltage level pulses selection integration capacitor, equation above shows that capacitor reciprocal affect output voltage, consequently, smaller capacitor higher voltage, VOUT integrated signal feedback path emitter through low-pass filter, This counteracts effects bias currents buffer integrating output offset voltage adjusted zero using potentiometer,
6.20
NANOSECOND INTEGRATOR TEST RESULTS
Channel Input 2V/DIV
Channel Output 2V/DIV 10ns/DIV
test results previous circuit shown here. charges integration capacitor, linearly according equation: (VBE With: voltage across capacitor base emitter voltage OPA660 section transconductance time integration capacitor
voltage across capacitor increases linearly predicted equation, shown with channel diagram above. input pulse, voltage sampled sample/hold amplifier. delay between input pulse charging capacitor approximately 250ps. This corresponds group delay time OPA660. group delay time calculated taking frequency where open-loop gain reduced calculated using equation: f0dB avoid integration error, signal delay time should less than 1/20th pulse width.
6.21
SERIES INPUT RESISTORS HIGH SPEED AMPLIFIERS
RINPUT
VOUT
WHY?
OPA622, OPA623, OPA660, OPA2662, BUF600, BUF601, MPC10X, SHC615
Sometimes high-speed amplifiers need series input resistor, because package parasitics become more more apparent higher signal frequencies. Package parasitics mainly leadframe pins, bondwire itself. pins bondwire modeled high frequency inductors, with small capacitors between each. adds parasitic capacitance from bondpad substrate. together, these parasitics form resonant circuits, with high values resonant frequencies range 700MHz 1GHz. Most problems that created these parasitics occur high impedance input Even overall bandwidth much less than resonant frequency, transistors input stage still affected. indication problems associated with parasitics higher than expected gain peaking amplifier. series input resistor will help prevent excessive gain peaking problems even oscillation dampening parasitic circuit. Typical values this resistor between 250. value vary widely because different PC-board parasitics that will this problem. rule, however, exists: smaller package less parasitics smaller associated effects. Therefore, designers should choose SOIC packages over packages whenever possible.
6.22
APPLICATION TRANSIMPEDANCE
+VBIAS Light OPA655 VOUT
BEST WITH VOLTAGE FEEDBACK
Photodiode preamp circuits used wide variety applications involving sensing light converting that information useful voltage. photodiode that sensing light configured with without bias voltage. speed response time important, photodiode typically configured with reverse bias voltage lower junction capacitance, shown figure. Voltage feedback amplifiers natural transimpedance amplifier circuits. Typically, photodiode selected responsivity physical dimensions. gain then adjusted changing this done with current feedback amplifier circuit, bandwidth would change with gain adjustments, which could inefficient. other hand, current feedback amplifiers used this circuit. erroneous conclude that current feedback amplifier appropriate because input bias current. difference between input bias currents between voltage feedback amplifiers current feedback amplifiers that great. instance, input bias currents OPA642 voltage feedback amplifier typically 18µA input bias currents OPA644 current feedback amplifier typically non-inverting input 20µA inverting input. unity gain bandwidths both amplifiers close same, 450MHz 300MHz, respectively. Additionally, wrongly perceived detrimental current this type application. IERR with current feedback amplifier, like VERR with voltage feedback amplifier relatively small generally does interfere with overall operation circuit. input bias currents micro ampere range large cause unacceptable offset errors circuit, alternative circuits implemented. topology, would discrete transistors high speed amplifier. Another design approach would voltage feedback amplifier with input like OPA655. OPA655 input voltage feedback amplifier class having typical unity gain bandwidth 400MHz input bias currents 5pA.
6.23
DISCRETE INPUT HIGH SPEED AMPLIFIER
VBIAS
OPA603
high input bias currents current feedback amplifiers buffered with JFETs give desirable combination constant bandwidth with minimum input loading. this circuit JFETs configured source followers need biased zero volts VGS. only real trick this circuit compensation resistor, manufacturer's data sheets, optimum value feedback resistor (RFB recommended order concurrently achieve wideband stable performance. this circuit, summation plus JFET transconductance should equal RFB. feedback resistor, then selected optimize dynamic response photodiode. shown figure, JFET buffered OPA603 (current feedback amplifier) configured photodiode transimpedance amplifier. 2N5911 input transistors resistively biased since there common-mode swing. compensation resistor, selected order achieve degrees phase margin unity-gain. feedback capacitor helps eliminate peaking that would normally result from feedback pole created parasitic capacitance inverting input. With values listed below, circuit 2MHz bandwidth. Special attention should paid circuit layout. with current feedback amplifiers inverting input should kept capacitance possible. Ground planes should removed vicinity inverting input. junction should soldered close amplifier possible resistor lead lengths should kept short possible. These practices should observed feedback network, also, frequency operation expected region. 1.42K Rphotodiode 100M 100K Cphotodiode 10pF 2N5911
used adjust output offset zero usually range. 6.24
JFET AMPLIFIER TRANSIMPEDANCE CIRCUIT
BPW34 +2.5V 1004 1/2RF 1/2RF
Light OPA655
VOUT
complete circuit implementation transimpedance amplifier using OPA655 shown above. case transimpedance circuit exhibits gain peaking, very difficult implement appropriate compensation capacitor, risk lessening signal bandwidth. single feedback resistor equal higher than 0.5M used, feedback capacitor need have value well below 1pF. split feedback resistor allows feedback capacitance parasitics around amplifier have manageable values tenths reference circuit, such REF1004-2.5 which volt reference, added circuit provide stable reverse bias voltage across sensor diode. OPA655 also used cable driver, using termination resistor. tested configurations their results shown below.
BPW34 Capacitance 38pF (VBIAS=1V) 38pF (VBIAS=1V) 27pF (VBIAS=2.5V) 20pF (VBIAS=5V) 16pF (VBIAS=10V) 16pF (VBIAS=10V) 16pF (VBIAS=10V)
309K 249K 274K 274K 274K 309K 549K
6.25
f-3dB
1.7MHz 1.9MHz 1.9MHz 2.0MHz 2.2MHz 1.88MHz 2.18MHz
REDUCING CIRCUIT FEEDBACK CAPACITANCE
OPTION
VBIAS
OPTION
final transimpedance amplifier solution should sufficiently stable with wide enough bandwidth accommodate speed input signal. variables this design problem photodiode, amp's feedback network. high speed applications, difficult achieve optimum results pole feedback capacitor, feedback resistor, order increase pole frequency feedback loop increase bandwidth response, must designed value, typically less than 2pF. Since this uncommonly low, circuit options recommended achieve this performance. Option T-network uses capacitors trim capacitor design value capacitor. effective capacitance this circuit equal (C1C2) C3). inexpensive sub-pico farad capacitance required, option recommended. With this series resistor topology, resistance adds discrete and/or parasitic capacitances divide. effective resistance this network 1/2RF 1/2RF effective capacitance this circuit (C1C2) C2). This option implemented with capacitor, such discrete capacitor this situation, would replaced parasitic capacitance resistor PCB, which could easily 0.2pF using standard RN55D resistors careful layout techniques. consequence, circuit designed with feedback capacitance lower than capacitance achievable with single resistor discrete capacitor.
6.26
JFET TRANSIMPEDANCE AMPLIFIER FREQUENCY PERFORMANCE
OPA655
small signal bandwidth this transimpedance amplifier illustrated this diagram. Tests were performed using Network Analyzer, 8753A. both cases feedback resistor, 274K. With trace photodiode reverse biased with 2.5V, causing parasitic capacitance across photodiode 27pF. -3dB bandwidth this trace measured 1.927315MHz. effective capacitance feedback loop ~0.151pF. Notice small amount gain peaking approximately 1dB. trace photodiode reverse biased with causing parasitic capacitance across photodiode 20pF. signal bandwidth slightly increased 2MHz. achieve this performance, care should taken remove ground plane from areas where inverting input feedback resistors are. OPA655 high speed voltage feedback amplifier input stage ensure input bias currents resulting errors very noise making device good choice high speed integrators transimpedance stages. Transimpedance amplifiers used variety applications, ranging from precision measurements, such medical blood analyzer circuits, high speed designs, such fiber optic receiver circuits. risk over generalization, precision circuits typically require amplifiers with offset voltage, input bias current, input capacitance, voltage noise current noise. With high speed designs, amplifier's slew rate, bandwidth, input capacitance circuit's parasitic capacitance critical achieve high speed performance. combination precision speed becomes challenging because limited selection amplifiers available market.
6.27
STATE VARIABLE FILTER
NEEDS BOTH TYPES
state variable filter excellent choice topology pass, band pass high pass filters needed concurrent outputs. From previous discussion, configured integrators should voltage feedback amplifiers. wild card this circuit. Since adjusts gain circuit, current feedback amplifier more suitable rendering wider overall bandwidth circuit. This becomes critical with high pass filter.
6.28
STATE VARIABLE FILTER FREQUENCY RESPONSE
SIMULATED
MEASURED
PERFORMANCE RESULTS
results frequency performance state variable filter high pass output shown this slide. graph left shows Spice simulation circuit performance. curves, bottom most curve shows performance circuit with voltage feedback amplifier, OPA642, used three amplifiers, most curve shows performance circuit using current feedback amplifier, OPA644, position circuit voltage feedback amplifiers, OPA642, graph right shows actual performance circuits. Note attenuation both plot responses around 200MHz. This behavior caused layout parasitics.
6.29
SALLEN ORDER PASS
VOUT
BEST WITH CURRENT FEEDBACK
This 2nd-order Sallen-Key pass filter distinguished from other filter topologies non-inverting gain passive positive feedback network. circuit (for equal 1/(3 where closed-loop gain RG). When compared State-Variable filter configuration, this filter more sensitive component tolerances gain accuracy dependent ratio useable range confined filter configured unity gain (ie. open shorted), current feedback voltage feedback amplifier used application. Additionally, large gain implemented circuit voltage feedback amplifier bandwidth will show reduction with increasing gains making current feedback topology better choice. transfer function this circuit using ideal amplifier VOUT (1/R1 1/R2 G)/R2 where,
6.30
SLEW RATE
Large Signal
Voltage Feedback Input
Current Feedback
Small Signal
Voltage Feedback Current Feedback Input
Voltage feedback amplifiers have slew rate limiting built into internal circuitry. slew rate dominated tail currents bias strings size compensation capacitor. slew rate independent input swing edge rate. Current feedback amplifiers have same slew rate limiting constraints voltage feedback amplifier. major advantage current feedback amplifier absence slew rate limiting. This accomplished with unique topology input stage current feedback amplifier. input signal amplifier changes current becomes available charge internal compensation capacitance. This current proportional initial voltage imbalance divided size initial voltage lessens, slew rate reduced well.
6.31
COMPOSITE AMPLIFIER
OPA627
OPA603
VOUT
USES BOTH TYPES
composite amplifier uses precision voltage feedback amplifier, OPA627, OPA603 current feedback amplifier utilize best qualities both amplifiers. Precision voltage feedback amps such OPA627 have excellent performance with precision specifications where closed-loop gain compared openloop gain amplifier. However, starting relatively frequencies, loop gain rolls-off 20dB/decade signal frequency increases. This produce significant errors higher frequencies where loop gain very low. Current feedback amplifiers, such OPA603, have good dynamic performance both high gains. This because feedback component (R2) sets bandwidth input resistor (R1) then sets closed-loop gain. Unfortunately, (VOS, dVOS CMR, etc.) performance current feedback amps poor compared precision voltage feedback amp. combination amplifier types this circuit render some impressive results. example, overall gain 20V/V signal bandwidth 30MHz obtained placing OPA603 gain 12V/V 1.02K). Slew rate full-power response OPA627 boosted composite amplifier. Since OPA603 adds gain output OPA627, slew rate OPA627 increased gain OPA603. resulting slew rate 730V/µs settling time 0.01% 520ns. Care must taken when selecting feedback resistor OPA603. Excessive phase shift through that amplifier will cause instability. This composite amplifier takes advantage more precise OPA627 (voltage feedback amplifier) faster slewing OPA603 (current feedback amplifier).
6.32
HIGH SPEED DIFFERENCE AMPLIFIER
VINVIN+ VOUT
first pass, would voltage feedback amplifier would only amplifier type appropriate difference amplifier configuration. First impressions indicate that errors bias currents current feedback amplifier would cause significant limitations. Actually, that overriding concern this high speed application where resistors circuit value errors typically coupled signal. This circuit will work with both voltage feedback amplifier current feedback amplifier, with major limitation; input signal termination when either amplifier used. both cases, input resistance looking into difference amplifier equal inverting input non-inverting input. should equal order match input loads when true independent sources differentiated. case current feedback amplifier, further restriction placed values resistors circuit. must equal manufacturer's recommended resistor value insure good dynamic performance. Although voltage feedback amplifier feedback resistor restricted value current feedback amplifier, proper termination would require that feedback resistor assuming gain one. Depending amplifier output drive capability output load, this issue.
6.33
INPUT TERMINATION DIFFERENCE AMPLIFIER
VINVIN+
VOUT
resistor network added order overcome problems with impedance sources circuit matching. this design exercise, source resistance, usually known prior design difference amplifier. Additionally, current feedback amplifier suggested manufacturer order achieve optimum bandwidth performance. order properly terminate source resistance, should equal parallel combination This termination requirement attenuates signal 50%. Consequently, gain inverting signal should then 2V/V. This achieved making twice that non-inverting input terminated similar fashion, causing attenuation signal. This made possible setting source resistance equal parallel combination R4). Additionally, equal order make termination resistors, equivalent inverting well non-inverting inputs. Further attenuation signal done with voltage divider formed non-inverting signal then gained where impedance looking from inverting input amplifier back towards source, complete design equations listed below: RS))
6.34
COMMON-MODE REJECTION DIFFERENCE AMPLIFIER
IDEAL VOUT
Assuming ERR),
VOUT
common-mode rejection difference amplifier equally dominated common-mode rejection amplifier common-mode rejection network around amplifier. precision circuits common-mode rejection amplifier usually ignored because precision amplifiers typically have CMRR performance range. Evaluation resistive network reveals that discrete designs limited their common-mode rejection performance resistive network. Given assumptions above, results calculations common-mode rejection ratio network table below:
0.001% 0.0032% 0.01%
CMRR 100dB 90dB 80dB 40dB
With high speed amplifiers, CMRR figures range 50dB 60dB performance area. Consequently, necessary consider amplifier performance well resistor performance. This evaluation also holds true where does necessarily equal
6.35
COMMON-MODE REJECTION WITH VOLTAGE FEEDBACK AMPLIFIER
VERR OPA650
VOUT
CMRR (VCM VERR)
voltage feedback amplifier, common-mode rejection device determined changes error voltage, VERR, with changes common-mode voltage input amplifier, VCM. Very little done about this limitation voltage feedback amplifier difference amplifier configuration. CMRR amplifier limiting portion circuit, usually, another voltage feedback amplifier, with higher CMRR performance selected.
6.36
DIFFERENCE AMPLIFIER WITH CURRENT FEEDBACK AMPLIFIER
OPA658
VOUT
with voltage feedback amplifier, current feedback amplifier dominate CMRR performance difference amplifier. four resistors perfectly matched, common-mode rejection difference amplifier usually limited 60dB, depending amplifier selected. current feedback amplifier differs from voltage feedback amplifier that common-mode rejection this circuit substantially improved lower frequencies. This achieved adjusting gain non-inverting signal such accommodate gain error, buffer portion input stage current feedback amplifier. transfer function difference amplifier, with buffer gain included evaluation VOUT R4(1+R2 /R1) ((R3 calculation illustrates, most effectively counter gain error alpha. alpha equal one, common-mode rejection dependent resistive network. Typical common-mode rejection ratio specifications current feedback amplifier, OPA658, measure 50dB, given theory, possible adjust common-mode rejection difference amplifier that uses current feedback amplifier 30dB better, adjusting resistor.
6.37
DIFFERENCE AMPLIFIER CMRR IMPROVEMENT
20dB/
405.2 50.5
START STOP
order prove theory, OPA658 current feedback amplifier used with matched resistive network through 402. commonmode rejection performance amplifier measured shown most curve diagram above. then adjusted maximize common-mode rejection performance. Note that frequency CMRR improved while high frequency rejection unaffected variance this test example, adjusted resistor, changed 405.2. This adjusted resistance circuit counteracts attenuation input stage caused input buffer gain,
6.38
HIGH SPEED INSTRUMENTATION AMPLIFIER
INOPA655 OPA655 OPA651 VOUT
By-pass capacitors shown
difference amplifier's input impedance difficult problem design around. solution impedance matching problems high speed circuits implement high speed instrumentation amplifier using three topology. This achieved with OPA655s input amplifiers OPA651 difference amplifier. gain equation this topology common-mode rejection capability circuit dominated common-mode rejection difference amplifier which usually dominated resistors, through high frequencies, roll this circuit affected input amplifiers well output amplifier. gain increases input stage adjusting circuit bandwidth limitation dominated input amplifiers, OPA655. amplifiers this circuit voltage feedback amplifiers. linear commonmode range input amps range approximately +/-2.4V with +/-5V supplies. output voltage increases, linear input range will limited output voltage swing input amplifiers. This circuit configured gain +2V/V. component values this circuit listed below:
6.39
HIGH SPEED FREQUENCY RESPONSE
1dB/
196.868135 136.957021
START
.500
STOP
300.000
frequency response this circuit illustrated above. bottom most plot represents performance circuit gain +1.5V/V, where left open. most plot represents performance circuit gain +3.5V/V. change bandwidth circuit result effects gain bandwidth product OPA655.
6.40
HIGH SPEED COMMON-MODE REJECTION RATIO
common-mode rejection instrumentation amplifier gain shown this plot. 100MHz common-mode rejection measured -23dB.
6.41
INSTRUMENTATION AMPLIFIER WITH CORRECTION
1/4OPA4650
1/4OPA4650
1/4OPA4650
0.1µF 0.1nF 1/4OPA4650
VOUT
This circuit shows high speed voltage feedback amplifier standard three instrumentation amplifier configuration with DC-correction feedback circuitry. gain two, instrumentation amplifier bandwidth approximately 120MHz. gain resistors, where gain circuit equal Amplifier configured difference amplifier with four resistors equal 402. optimal feedback resistor value amp, OPA4650, illustrated this figure resistors should well matched achieve good common-mode rejection. Wide band amps known have only moderate specifications, like input bias currents range, offset voltage several maximum input offset voltage OPA4650 +/-5.5mV. increase accuracy this circuit remaining fourth amplifier OPA4650 quad amplifier (A4) used correction loop. Configured integrator, amplifier senses component output corrects with voltage opposite polarity into non-inverting node time constant integrator with transfer function VOUT 1/RICI integral higher values integration capacitance used recommended that another capacitor smaller values, like 0.1µF parallel. This will counteract inductive behavior 0.1nF capacitor higher frequencies. could argue that integrator could built using low-speed, precision amp. This good choice terms good common mode rejection over frequency. `cold' side resistor should constant impedance throughout entire frequency range. output impedance OPA4650 only 0.08 0.1MHz. case output impedance amplifier shows much inductive behavior, series circuit placed from output ground. compensate error voltage caused resistor similar value placed non-inverting input disadvantage that adds more voltage noise circuit, however, capacitor parallel will shunt most this noise.
6.42
400MHz INSTRUMENTATION AMPLIFIER
-VIN +VIN OPA660 BUF601 VOUT
18pF
example versatility transconductance amplifier, OPA660 shown here. segment OPA660 operates similar transistor. input high impedance, input impedance follows voltage changes input. differentiation between ideal transistor that inputs nearly same voltage, with small offset difference. When input sinks sources current output also sinks sources current. This 400MHz differential amplifier difficult design, involving extensive complex hardware. OPA660 makes easy with stable operation -60dB commonmode rejection 1MHz. OPA660 configured open-loop structure feedback loop) with identical, high-impedance inputs. When differential voltage applied input, current flows through That current mirrored high impedance collector output producing corresponding output voltage across gain according gain equation: 2/gm where transconductance OTA. depends quiescent current which controlled chosen +/-20mA quiescent current making 120mA/V. buffer amplifier, BUF601, used drive lowimpedance loads decouple output from load. Both inputs output terminated systems. This adapted other termination requirements replacing R11. resistors placed series with high-impedance inputs order reduce gain peaking. Capacitor parallel with compensates parasitic capacitance present OTA's collector (C), consequently expanding circuits achievable bandwidth.
6.43
400MHz INSTRUMENTATION AMPLIFIER PERFORMANCE
shown bandwidth this circuit approximate 400MHz. Without capacitor -3dB frequency roll-off circuit would 85MHz. common-mode gain frequency shown second plot. commonmode rejection approximately -63dB frequencies -20dB 400MHz. addition, noise voltage density 7.7nVHz makes possible process very small signals.
6.44
DRIVING MULTIPLE VIDEO LOADS
OPA2650 OPA2658
Video distribution amplifiers used drive same video signal into multiple loads. this application dual high-speed amplifier, like voltage feedback OPA2650 current feedback amplifier OPA2658, used this circuit. standard video amplitude peak-to-peak, with video portion signal swinging between +0.7V synchronization portion signal swinging between -0.3V Consequently, most video driver applications require larger positive output voltage swing capability. Considering complementary transistor (NPN PNP) push-pull output stage built high-speed process, transistors known weak part when comparing ability sink source current. This means that high-speed amps tend have higher output currents sourcing than sinking. This performance characteristic used advantage with video driver applications. OPA2650 (dual voltage feedback amplifier) sourcing current capability +75mA sinking capability -65mA room temperature. OPA2658 (dual current feedback amplifier) source +90mA sink -60mA room temperature.
6.45
COMPARISON
Closed-Loop Bandwidth Feedback Network Slew Rate Dominant Noise Source Input Impedance Bias Current Relatively unaffected closed-loop gain performance resistor values input edge rate Inverting input current noise Asymmetrical
Varies according relation performance independent resistor values Fixed internally Input voltage noise Symmetrical
errors difficult cancel
errors canceled matched input impedance
summarize, closed-loop bandwidth current feedback amplifier relatively unaffected closed-loop gain long changed significantly. There input error resistance, impedance inverting input consequently requiring some adjustments obtain near ideal results. other hand, closed-loop bandwidth when using voltage feedback amplifier varies with gain approximately 20dB/ decade. From another perspective, current feedback amplifier's performance feedback resistor value. possible restrict bandwidth circuit using higher which would improve dynamic response without adjusting gain. With voltage feedback amplifier circuit, dynamic response dependent ratio input resistor, feedback resistor, independent values. Typically slew rate current feedback amplifier better than voltage feedback amplifier. With qualifier, slew rate current feedback amplifier input edge rate. case voltage feedback amplifier, slew rate internally independent input signal edge rate. dominant noise source current feedback amplifier inverting input current. With voltage feedback amplifier, current noise very voltage noise dominates performance. errors input impedance input bias current mismatch current feedback inputs hand hand. These mismatches cause errors that difficult reduce.
6.46
PRODUCT SELECTION TREES
Voltage Feedback Stable Voltage Feedback Unity Gain Stable Current Feedback 1000V/µsec OPA658 (650MHz) OPA648 (600MHz) OPA2658 (500MHz, dual) OPA4658 (450MHz, quad) OPA644 (300MHz) OPA623 (290MHz) OPA603 (160MHz) GBWP 400MHz OPA643 =1000V/µsec) OPA621 =350V/µsec) OPA675 =350V/µsec) OPA676 =350V/µsec) OPA651 =300V/µsec) GBWP 400MHz OPA654 =750V/µsec) OPA641 =650V/µsec) OPA678 =350V/µsec) OPA637 =100V/µsec) GBWP 200MHz OPA642 =380V/µsec) OPA640 =350V/µsec) OPA655 =300V/µsec) OPA646 =180V/µsec) OPA620 =175V/µsec)
GBWP 200MHz OPA628 =310V/µsec) OPA650 =240V/µsec) OPA2650 =240V/µsec, dual) OPA4650 =240V/µsec, quad) OPA671 =100V/µsec)
High speed amplifiers found with voltage feedback current feedback topologies with variety bandwidths slew rates. tables above summarize Burr-Brown product offering. voltage feedback amplifiers separated into general categories, first amplifiers that stable gain high second amplifiers capable unity gain stability. Each voltage feedback categories further subdivided into gain bandwidth product groupings. This separation along with slew rate capability amplifier useful first order product selection. current feedback amplifiers listed third column ordered according small signal bandwidth closed-loop gain +2V/V. Slew rate specified because strong dependency input signal characteristics.
6.47
PRODUCT SELECTION GUIDE
Circuit Requirements
Fast Slew Wideband High Gain Errors Distortion Noise Transimpedance Ease Design
Maybe Maybe Maybe Maybe Maybe
This table briefly summarizes some general rules thumb selection proper amplifier application. circuit requires fast slewing amplifier, particularly large signals current feedback amplifier will typically slew faster than voltage feedback amplifier. However, recently, voltage feedback amplifiers have been introduced with quite good slew performance very good bandwidth. application requires amplifier that high closed-loop gain, voltage feedback amplifier would more appropriate amplifier socket. Current feedback amplifiers optimized gain, typically +2V/ possible amplifiers higher gains expense loosing gain accuracy. Current feedback amplifiers, typically, have lower harmonic distortion across closed-loop bandwidth. Applications, such video, require good dynamic performance making current feedback amplifier many times preferred amplifier.
6.48

Current Feedback Amplifiers Voltage Feedback Amplifiers .6.2 High Spee

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