CLC505 current-feedback operational amplifier with externally-adjustab

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CLC505 current-feedback operational amplifier with externally-adjustab

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Wideband Capable µPower Operation
CLC505 current-feedback operational amplifier with externally-adjustable supply current whose performance tuned meet precise requirements many high-speed applications. CLC505 provides small-signal bandwidth 150MHz while drawing supply current from power supplies. Reducing supply current decreases bandwidth only third; 50MHz +6). Please refer CLC505 data sheet full performance description over supply current range. following application note intended supplement CLC505 data sheet describing operation with quiescent supply currents below 1mA. Frequency Response Dependence Supply Current Application note OA-13 describes internal topology current-feedback amplifier dependence loop gain (and hence bandwidth) inverting-input impedance. ideal current-feedback amplifier, this impedance zero amplifier's frequency response completely independent signal gain. supply current CLC505 reduced below region, inverting-input impedance increases such degree that effect loop-gain begins dominate. understand impact this impedance, well similar increase output impedance (Ro) supply currents, amplifier's internal block diagram, figure resulting transfer function shown. This analysis considers only non-inverting configuration similar result obtained inverting configuration.
understanding transfer function given Equation central point this discussion. supply current's dependence enters into this equation through three internal terms, Z(s). represents output impedance unity-gain buffer found between amplifier's inputs, while represents output impedance output voltage buffer. Z(s) frequency-dependent transimpedance gain which converts error current (ierr), flowing through inverting input, voltage which buffered output. Both inverting input output pins voltageoutput structures consisting symmetric (PNP NPN)
z(s) z(s)
where: desired noninverting signal gain
z(s) will limit high frequency attenuation forward transimpedance gain, Z(s), become very small.
Ri1+ Loop Gain z(s)
ierr
Z(s)ierr
Figure current CLC505 analysis topology
1993 National Semiconductor Corporation
Printed U.S.A.
emitter followers (ref. very similar Class power buffer (ref. 293). Emitter-follower outputs show output impedance that directly proportional operating temperature inversely proportional transistor's quiescent current Vt/Ic, kT/q, ref. 398). supply current decreases, portion supply current allocated these stages also decreases causing increase both inverting input impedance, output impedance, Decreasing supply current will also increase open-loop gain, ZOL, while decreasing dominant pole frequency, However, product ZOL*wo remains relatively constant over supply current temperature.
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From transfer function shown Equation Rewriting Equation these terms Manipulating this into standard form
desired signal gain z(s) pole, forward transimpedance gain
feedback transimpedance; this inverting
error current, resulting from
Looking equation again, closed-loop response pole will (ZOL*wo)/Zt. supply current changed, ZOL*wo product remains relatively constant. Figure shows typical open-loop forward transimpedance gain, (20*log(|Z(s)|)), plotted over frequency supply current varied. Figure shows this same forward open-loop gain supply current plotted over full military temperature range. long these forward gain responses fall same line 20dB/decade roll-off region, ZOL*wo product remains constant. With constant ZOL*wo term, only element setting bandwidth transfer function equation expression, equation general, advantageous make small possible which will increase loop gain result improve harmonic distortion extend bandwidth. limit reduction comes when higher order poles Z(s) degrade phase margin unity-gain crossover loop gain. given supply current desired gain, decreasing increasing will decrease important limitation decreasing available output current drive. non-inverting configuration, Rf+Rg appears additional load parallel with while inverting configuration, only appears additional load parallel with
Loop gain
z(s)
then gain, high frequency gain,
260µA 600µA
100µA
|Z(s)|
Frequency (Hz)
Figure 20log|Z(s)| different supply currents Note that zero frequency shown Equation significantly higher frequency than pole frequency. Once operating frequency approaches this zero frequency, Equation predicts minimum gain, Amin. This generally observed practice, since zero frequency Equation typically much higher than frequencies which start show normal emitter-follower inductive characteristic. simplify this analysis, inductive characteristics have been neglected. should noted that inductive characteristics will continue roll closed-loop response with attenuations much greater than that predicted Amin high frequencies. zero shown transfer function Equation will neglected with rest this discussion focused closed-loop pole frequency.
25°C -55°C
125°C
|Z(s)|
Frequency (Hz)
Figure 20log|Z(s)| over temperature
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Letting
rewritten
computed measured results shown Table Figure shows small-signal frequency responses each these gains normalized enter graph same point y-axis. Gain Table Computed Expected Measured Measured -3dB -3dB 3.78k 6.50k 11.9k 32MHz 18.5MHz 10MHz 57MHz 26MHz 11.5MHz 285MHz 260MHz 230MHz
Equation emphasizes gain dependence supply currents, becomes large (500 1mA) cause first term Equation dominate. This part feedback transimpedance expression directly related desired signal gain, gain increased, increases, decreasing bandwidth. This bandwidth dependence gain analogous that observed with voltage-feedback amplifiers. such, configurations which first term equation dominant contributor gain-bandwidth (GBW) product characteristic will observed. Figure shows test circuit used measure supply current decreased from 100µA over gains +10, +20. very supply currents, slight DC-output currents offsets change performance. this reason, output DC-blocking capacitor used limit output currents.
6.8µF
test results good agreement with simplified analysis Figure highest gain tested, +20. lower gains, several effects combine extend bandwidth beyond that predicted this simplified analysis. Specifically, additional higher frequency poles open loop response come into play lower gains. These include both inductive characteristics output impedances higher order poles Z(s). This effect decreasing phase margin from theoretical assumed single pole analysis. Phase margins less than greater than will extend closedloop bandwidth without peaking.
Normalized Gain (1db/div)
Av=+5 Av=+10 Av=+20
0.1µF
CLC505
0.1µF
load
Frequency (Hz)
6.8µF
Figure Small signal frequency response gain (Icc 1mA) additional effect serves increase measured bandwidth desired signal level increased. frequency operation increases fast rise time signals applied), increase steady-state inverting-stage current observed increased Ierr required when operating these higher frequencies with reduced loop gain. This increasing error current, input swept over higher frequencies, decreases inverting input impedance. This frequency signal level dependence will decrease value increasing
Figure Test circuit Gain Bandwidth product measurement case, Equations were used predict small-signal -3dB bandwidth three gains +5,+10, +20. With 300k, 1mA, 500, approximate ZOL*wo product 2120E9. Compute from Equation expected -3dB bandwidth from Equation
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loop gain extending bandwidth. This effect particularly pronounced when Ri*Av term becomes large part total expression, relatively high noninverting inverting gains. Under these conditions, bandwidth actually increases signal level increased. Figure shows this effect case Figure with 1mA.
taken minimum achievable value good starting point estimating bandwidth capability CLC505 very supply currents. PSPICE simulation macromodel available from National used test performance under different operating conditions. This macromodel reasonably simulates most effects discussed earlier. Transient simulation will even show improved rise times higher gains signal swing increased.
Gain Bandwidth Product (MHz)
Gain (1db/div)
Vo=3.5Vpp -3dB=13.5MHz Vo=0.4Vpp -3dB=11.1MHz
1500 1350 1200 1050
Frequency (MHz)
(mA)
Figure Frequency response signal level given desired supply current, load impedance signal gain, close inspection feedback transimpedance expression Equation shows that optimum found that will minimize maximizing bandwidth loop gain. This relatively shallow minimum with resulting -3dB bandwidth significantly different than fixed Nevertheless, solving this optimum yields following. Table page shows required information predict gain-bandwidth product supply current. each supply current, internal parameters (Ri, ZOL*wo) shown. From this, optimum calculated using Equation measured small-signal bandwidth then recorded. measured -3dB bandwidths shown table agree very closely with those predicted from ZOL*wo/Zt (evaluating this expression from data given this table Equation Zt).
Figure Gain bandwidth product current resistor common illustrate wideband capability lowpower amplifiers through MHz-per-mA figure merit. Figure shows same data Figure with boundary regions decades MHz/mA shown. low-power Maxim amps also shown that claim superior MHz/ performance. Although certainly capable parts, Maxim amplifiers about decade lower performance than CLC505. CLC505, along with several other National wideband current-feedback amplifiers (such CLC406), push strongly above 100MHz/mA barrier. discussion thus assumed volt supplies. will discussed later, single supply operation also possible.
10mA
Supply Current (mA)
10MHz/mA
100MHz/mA
CLC406
Optimum
CLC505 3.4mA Data sheet spec point
CLC505 MAX403
1GHz/mA
0.1mA
MAX401
This estimate supply current represents very conservative estimate. signal gain decreased from +20V/V, will increase shown Table addition, measured bandwidth would increase signal level increased, discussed earlier, point that output-stage drive current slew limits come into consideration. supply current resulting Table plotted Figure This should
0.01mA 1MHz
10MHz
100MHz
1GHz
Gain Bandwidth Product (GBW)
Figure supply current
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Table Performance Supply Current (Vcc ±5V, 25°C, ZOLWo v=+20 Optimum 1.14k 1.42k 2.14k 2.60k 3.57k 4.52k v=+20 -3dB 11.4MHz 7.9MHz 6.0MHz 4.6MHz 2.7MHz 2.3MHz 1.6MHz 1.1MHz
300k 400k 500k 600k 900k 1.3M 1.6M
800µA 600µA 480µA 260µA 230µA 160µA 100µA
1.16k 1.97k 2.27k 3.27k 4.30k
1.93M 2.46M 2.92M 3.35M 4.73M 5.13M 6.80M
262kHz 249kHz 242kHz 238kHz 226kHz 225kHz 220kHz
2120E9 2121E9 2123E9 2127E9 2123E9 2128E9 2136E9
228MHz 158MHz 120MHz 92MHz 54MHz 46MHz 32MHz 22MHz
7.50M 217.5kHz 2131E9
Secondary Effects Supply Current Operation Besides having profound effect small signal performance, supply current operation CLC505 will also modify most other performance characteristics. most drastic effect available output current. supply current, CLC505 data sheet guarantees ±5mA 25°C. This specification should scaled down proportionately operation below 1mA. non-inverting slew rate retained with very power levels slew enhancement circuitry input buffer stage (e.g. supply current, 500V/µs particularly demanding condition +2). Both input bias currents will decrease with supply current input offset voltage temperature drift will become more pronounced. Recall that, current-feedback topology, input bias current terms unrelated both magnitude polarity. Bias current cancellation offset-current specification therefore ineffective. Please refer CLC505 data sheet more information these error terms 1mA. most subtle effect perhaps found with noise performance. reduced, amplifier's input referred noise terms show increase their noise corner frequencies. Also, additional gain term inverting noise current becomes appreciable. Specifically, inverting input impedance acts additional impedance gain inverting bias current noise. noise model discussed application note OA-12 (Noise Analysis National's Current-Feedback Amplifier's) does consider this effect would therefore understate total output noise. simulation macromodel will, however, show correct output noise including this effect.
Taking Advantage Voltage Feedback Characteristics Most design techniques developed voltagefeedback amplifiers applicable CLC505 operating below supply current. standard applications voltage-feedback amplifier, that directly possible with current-feedback part, simple integrator with direct capacitive feedback. Changing feedback resistor capacitor moving inverting integrator configuration will result following circuit, Figure transfer function.
ierr
ierr Z(s)
Figure Analysis circuit inverting integrator Neglecting high frequency zero Should feedback transimpedance zero higher order poles Z(s). Also, high frequency feedback impedance should Figure shows test circuit demonstrate this integrator operation, while Figure shows resulting integration square wave (100kHz) input output triangle wave.
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Again simulation macromodel CLC505 very effective analyzing performance these types circuits.
z(s)
Power Active Filters
Phase
Gain /Vi)
Frequency (Hz)
Phase
Gain
20MHz
Figure Integrator frequency response implement Sallen-Key type active filters, generally desirable have amplifier bandwidth least twenty times desired cutoff frequency. also desirable operate amplifier relatively gains. Figure shows test circuit used demonstrate CLC505's capability implementing very low-power single-supply high-frequency active filters.
Figure shows simulated gain phase integrator shown Figure Note that gain 66dB comparable other high-speed voltage-feedback amplifiers (such CLC420) while supply current this integrator very 500µA.
60µA
150µA
500µA
100kHz
Supply de-coupling shown
load
0.1µF 1.58k 6.37k 100pF
150pF
CLC505
CLC505
0.1µF
±4mV
600k
±40mV
20dB
270pF
D.C. Output Centering
0.1µF
Butterworth response 400kHz Amplifier Power 900µW
Figure power integrator test circuit
Figure Single supply power active filter low-power single-supply operation, signal nodes need coupled. three 0.1µF capacitors provide this function. This allows non-inverting input biased midpoint between supply pins, this case. capacitors also prevent currents from flowing output reduce amplifier gain which will hold output operating point equal non-inverting input (centered between supply pins.) least volts across part's supply pins required give some signal swing capability input stage from common-mode input range considerations. amplifier's gain been filter components have been adjusted allow amplifier's bandwidth (ref.
Output Voltage (0.2V/div)
Time (2µsec/div)
Figure Integrator output square wave input
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Figure shows frequency response just amplifier. this very power gain some peaking loss phase margin observed. This will effect filter performance however. 9MHz bandwidth more than adequate implement desired 400kHz Butterworth low-pass filter. Figure shows measured filter frequency response. desired cutoff achieved precisely. loss rolloff higher frequencies arises from direct signal coupling output through filter components after amplifier stopped controlling output voltage.
Conclusions Caveats CLC505 adjustable supply current offers highest MHz-per-mA performance levels available monolithic amplifier. simplified analysis good predicting gain-bandwidth product under variety supply current, gain, feedback resistor, loading conditions. PSPICE simulation model available from National does even better predicting performance over wide variety conditions. Although internal topology CLC505 uses current-feedback approach, very supply currents this part treated more like voltage-feedback amplifier having gain-bandwidth product. Very high-speed integrators active filters implemented exceptionally supply currents. leakage effects, part-to-part tolerance supply current fixed becomes greater desired nominal supply current decreased. 300k, National guarantees maximum 1.3mA supply current 25°C from nominal value. closer tolerance this, lower, supply currents required, please contact National further information. References: Ref. "Current Feedback Amplifiers", National Application Note Ref. "Analysis Design Analog Integrated Circuits", Gray Meyer, Wiley 1977.
Gain Phase -3dB 9.5MHz -118 -154
Amplifier Phase (36°/div)
Gain (dB)
Frequency (Hz)
50MHz
Figure Very power, single supply, amplifier frequency response
Gain
Filter Gain (5dB/div)
Phase
-3dB 400kHz
Filter Phase (90°/div)
Ref. "Simplified Component Value Pre-Distortion High Speed Active Filters", National Application Note OA-21
-180 -270 -360
Frequency (Hz)
50MHz
Figure Very power, single supply, active filter frequency response
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CLC505 current-feedback operational amplifier with externally-adjustab

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