Application Note October 2004 AN9787.1 Some designers (especially

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Application Note October 2004 AN9787.1 Some designers (especially

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Intuitive Approach Understanding Current Feedback Amplifiers
Application Note October 2004 AN9787.1
Some designers (especially digital designers doing analog design) intimidated current feedback amplifiers (CFAs), thus they don't take advantage CFA's superior frequency performance. CFAs have become pervasive because they have architectural advantage that delivers high bandwidth slew rate supply currents. designer without CFAs arsenal inadequately armed today's high speed design challenges. Most your voltage feedback amplifier (VFA) knowledge applicable design because ideal equations both amplifiers identical. attempts drive input error voltage zero, while attempts drive input error current zero, understanding feedback mechanism simplifies task understanding operation. characteristics will take some getting used closed loop bandwidth relatively independent closed loop gain, amplifier's stability dependent feedback resistor value. Both these differences offer significant advantages with very little drawback. simplified approach presented here adequate more scientifically inclined reader, thus, more detailed analysis (including in-depth mathematical derivations) desired reader encouraged peruse reference [1]. object determine proper value these resistors. When circuit configured non-inverting gain (Figure input circuit +IN. This terminal connects unity gain buffer input, characteristics high input impedance bias current. Conversely, input connects output same buffer. models output resistance buffer, usually small value which ignored. input characteristics buffer output which very impedance high current sinking/sourcing capability. high current capability buffer output enables large transient currents flow through external circuits, will shown later, this enables higher slew rates CFA. unity gain input buffer forces follow unconditionally. During quiescent operation only small current flows through -IN. This error current, which analogous error voltage VFA. current flowing through always mirrored (represented current source onto high impedance node where converted voltage transimpedance gain CFA. Transimpedance gain serves same function that open loop voltage gain serves VFA; i.e., driving force which closes feedback loop. amplifier large transimpedance gain will have error current because VOUT/Z. controlled current source simplified model represents transimpedance element, when current flows through terminating impedance, voltage developed input node output buffer. output buffer another unity gain buffer, provides current capability driving impedance loads.
Simplified Model
simplified model shown Figure external resistors, gain feedback elements which determine gain bandwidth performance, thus
VOUT
FIGURE THIS SIMPLIFIED MODEL DEMONSTRATES CURRENT-FEEDBACK AMPLIFIER (CFA) OPERATES, DIFFERS FROM VOLTAGE-FEEDBACK AMPLIFIER (VFA). CONTROLLED CURRENT SOURCE NODE REPRESENTS TRANSIMPEDANCE ELEMENT
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Application Note 9787
example, consider voltage follower infinity) where V+IN VOUT unit step voltage applied input, then V+IN V-IN VOUT -VIN -1V, thus -1V/RF. Notice that slew current, dependent magnitude output voltage change This error current sourced input buffer, equal current mirrored onto node causing voltage rise, consequently output voltage increases. feedback loop keeps VOUT rising until error current driven minimum value, this point V+IN VOUT within error tolerance.
VOUT
Analyze Amplifiers Just Like Amplifiers
same four "ideal amp" assumptions employed derive closed loop gain equations used derive ideal closed loop gain equations. definition, V-IN V+IN because element separating inputs well designed unity gain buffer. non-inverting input current enough neglected (+IN because this current buffer input current. inverting input current equals zero (-IN because feedback loop drives error current zero. long transimpedance gain large current negligible. prudent note that input currents totally uncorrelated, thus common technique canceling input current errors balancing impedance seen positive negative inputs recommended CFAs. transimpedance gain extremely high, which enables feedback loop function properly just like high open loop gain does VFA. feedback network constructed same VFA; feedback resistor (RF) connected from output inverting input, gain setting resistor (RG) connected from inverting input ground both cases. Because feedback network same VFA, because ideal assumptions valid both types, obvious that ideal closed loop gain equation must hold CFA. non-inverting gain formula, Equation based these assumptions; voltage across because tracks +IN, voltage divider rule yields: VOUT RG/(RG RF),
(EQ.
FIGURE INVERTING GAIN CONFIGURATION CURRENT-FEEDBACK AMPLIFIER INPUT FEEDBACK RESISTORS JUST LIKE VFA. CIRCUIT'S GAIN EQUATION (VOUT/VIN) -(RF/RG)
Frequency Response
non-ideal gain equation non-inverting taken from Reference Equation repeated here Equation Notice that input buffer output resistance, designated Equation
(EQ.
input buffer output resistance zero, which goal every designer, Equation reduces
(EQ.
transimpedance gain contains more poles, these poles make transfer function frequency dependent. were independent frequency, were purely resistive, would independent frequency. Equation form classic closed loop feedback equation VOUT/VIN A/(1 sole determining factor stability feedback system loop gain, often mathematically expressed loop gain Equation quantity Z/ZF [1]. Thus, fundamental conclusion that CFA's stability completely dependent transimpedance feedback impedance. This very different from where stability dependent closed loop gain a/ACL). Another important note about CFA's loop gain that being denominator precludes from being zero ohms linear circuit. Therefore, required even unity gain applications, capacitive feedback undesirable because will cause zero some frequency. Bode plot magnitude plot gain phase, evaluates stability very effectively. This plot transimpedance gain plot CFA, where would voltage gain plot VFA. Bode plot loop gain Equation (with transimpedance plotted two-pole transfer function) given Figure notice that numerator denominator loop gain plotted
AN9787.1
When used inverting gain configuration (V+IN V-IN input current VIN/RG (see Figure feedback current, which must equivalent input current because current flows into inputs, equal -VOUT/RF. Equating currents yields:
(EQ.
Application Note 9787
separately added graphically obtain final result. criteria stability that loop gain less than before -180 degrees phase shift accumulated; i.e., -180.
20LOG|Z| AMPLITUDE (dB) 20LOG|10ZF| COMPOSITE CURVE 10ZF COMPOSITE CURVE LOG(f)
AMPLITUDE (dB)
20LOG|Z| 20LOG|ZF|
PHASE (DEGREES)
COMPOSITE CURVE LOG(f)
PHASE (DEGREES)
-120 >60o -180 PHASE MARGIN
-120 -180
PHASE MARGIN
FIGURE ADJUSTING VALUE DESIGNER MAKE TRADE-OFF BETWEEN STABILITY BANDWIDTH. BANDWIDTH INVERSELY PROPORTIONAL
FIGURE THIS BODE PLOT USEFUL STABILITY ANALYSIS. WITH VFA, AMPLITUDE SCALE WOULD REPRESENT VOLTAGE GAIN, CASE CFA, AMPLITUDE SCALE SHOWS AMPLIFIER'S TRANSIMPEDANCE GAIN
When composite curve crosses phase shift -120 degrees, this particular would have degrees phase margin, would very stable with that value Bode plot, |ZF| subtracts from curve moving composite curve down from curve, moving gain crossing point left into area less phase shift. Thus, conclusion made that increasing stabilizes reducing loop gain without impacting open loop phase shift (i.e., phase margin increased, Figure Note, also from Figure that amplifier's bandwidth (BW1, BW2) decreases increases. These critical conclusions because they indicate that value adjusted trade stability bandwidth, this basis conclusion that bandwidth inversely proportional actual practice designer will never decrease much that approaches instability, because gain peaking overshoot increase dramatically long before circuit becomes unstable. Note, when increasing closed loop gain circuit, should decreased rather than increasing
Inspecting Equation reveals that closed loop gain does affect stability bandwidth because does impact loop gain pole-zero locations. This means that bandwidth independent closed loop gain, except minor effect neglected term (see Equation When taken into consideration, bandwidth becomes slightly dependent closed loop gain, still much more independent closed loop gain than This phenomenon explains CFAs make much better high frequency, high gain amplifiers than VFAs usually emitter-follower type output impedance which exhibits zero transfer function high frequencies. This explains CFAs tend become peaky high frequencies especially when there significant capacitance inverting input.
Slew Rate Pulse Performance
Slew rate measure amplifier's ability transition from output voltage another response input voltage change. Fast slew rates required good pulse amplifiers because pulses contain fast rise fall times. error current multiplied transimpedance gain form unbuffered output voltage, sooner error current driven zero faster amplifier gets final output voltage. earlier derivation established formula unity gain error current (VOUT -VIN)/RF. Thus, decreased increased, more current available slew internal high impedance node faster slew rates result. input step size doubled current available slewing doubled, hence rise/fall time virtually independent step size. internal current source which limits maximum slew rate regardless voltage step size. usual method increasing slew rate manufacturer increase this internal current source which also increases supply current.
AN9787.1
Application Note 9787
input buffer must first slew follow input signal, input buffer's slew rate critical part overall slew rate. high gain configurations input buffer does through large voltage change compared output, it's slew rate does contribute much error. gain configurations input buffer significantly degrade slew rate. example: unity gain applications CFA's input buffer must slew inverting input same amount output, this often limits unity gain CFA's slew rate. inverting gain configurations virtual ground input buffer doesn't have slew much, inverting gain configuration fastest slew rate. Circuit configurations using feedback capacitors diodes lend themselves CFAs because these feedback components will approach zero impedance some operating point oscillation will result. Diodes feedback loop often replaced input output clamp diodes. Feedback capacitors neutralized putting resistor equal optimum value series with them. classic inverting integrator cannot implemented CFA, non-inverting integrator often used place. Basically speaking, harder than with equivalent bandwidth; it's just little different.
References Convert From VFAs CFAs
usually replace most high frequency applications with better performance lower cost [3]. There salient points aware during replacement, most cases will require either minor board changes. remember that must always have feedback resistor. can't substituted directly unity gain applications where output shorted inverting input stability feedback resistor. While higher values fine VFAs, most CFAs optimized higher than optimum values degrade performance dramatically. value that yields stable operation always found. best start with manufacturer's recommended value, graphical techniques used select values wider bandwidth higher closed loop gain (reduce rather than increase desired. Remember, reducing increases bandwidth while increasing ringing overshoot. Don't drop value much oscillation will result. Other common factors that destabilize CFAs when replacing VFAs excessive capacitance inverting input output. common source ringing oscillation high frequency amplifier long board trace length attached inverting input lead; this situation exacerbated with CFA. Intersil documents available internet, site http://www.intersil.com AN9420 Application Note, Intersil, "Current Feedback Amplifier Theory Applications" Bode "Network Analysis Feedback Amplifier Design", VanNostrand Inc., 1945 AN9663 Application Note, Intersil, "Converting From Voltage-Feedback Current-Feedback Amplifiers"
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AN9787.1

Application Note October 2004 AN9787.1 Some designers (especially

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