Current-Feedback Myths Debunked Arne Buck July 1992 A(s

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Current-Feedback Myths Debunked Arne Buck July 1992 A(s

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Mystery needlessly surrounds operation current feedback operational amplifiers. Many engineers refuse design with these amps misunderstandings which easily rectified. Much been written date internal circuitry current feedback amps. These open-loop "tutorials" obfuscate current feedback works closed-loop circuit. Practical circuits closed-loop feedback systems which yield classical control theory analysis. Analog circuit designers comfortable with voltage feedback amps closed-loop circuit with familiar ideal approximations feedback affords. will shown that current feedback amps analyzed analogous fashion. Once this closed-loop similarity appreciated, easy that most circuits commonly built with voltage feedback amps realized with current feedback amp, with better results high frequencies. Refer Figure review open-loop terminal characteristics voltage feedback amplifier. Ideally non-inverting input impedance infinite, inverting input impedance. output voltage source, output impedance which zero. This voltage source controlled potential difference between input terminals. This error voltage, hence term voltage feedback. Feedback will drive error voltage zero. open-loop dynamics contained A(s). This A(s) dimensionless gain, often represented units volts volt decibels.
Current-Feedback Myths Debunked
Arne Buck
July 1992
A(s) approaches infinity, closed-loop gain -(R2/R1). frequency response closed-loop circuit determined denominator transfer function. Both noise gain R2/R1) circuit frequency-dependent source (A(s)) appear denominator, linking closed-loop gain bandwidth.
OA-20 Fig2 Figure Voltage Feedback Inverting Gain
familiar Bode plot this circuit shown Figure amplifier typically compensated with dominant low-frequency pole ensure stability down specified minimum gain, often unity. region where onepole approximation open-loop response valid, phase around degrees. This gain-bandwidth product region. intersection zero-slope noise gain line open-loop gain curve determines closed-loop system -3dB bandwidth. high gain circuit will have less bandwidth than lower gain circuit. circuit moves lower gains, bandwidth increases, phase margin lost stability suffers.
|A(s)|
Open Loop Gain Loop Gain (1+R2/R1) Loop Bandwidth Open Loop Phase
Assumptions
Z(s)
A(s)[V1-V2] V1-V2 error voltage
Figure Voltage Feedback Fig1 OA-20 typical voltage feedback circuit shown Figure inverting amplifier. transfer function developed from following equations: -A(s)V2, V2)/R1
-180° Phase Margin
Figure Bode Plot
OA-20 Fig3
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open-loop terminal characteristics current feedback amplifier depicted Figure There unitygain buffer between inputs. This buffer ideally infinite input impedance zero output impedance. Thus non-inverting input impedance current feedback infinite, inverting input impedance zero. output voltage source, output impedance zero. This voltage source controlled current inverting input. This error current, hence current feedback. Feedback forces error current zero. open-loop dynamics determined Z(s). This Z(s) current controlled voltage source which units transimpedance, ohms.
Assumptions
Z(s)
have gain-bandwidth product. closed-loop bandwidth determined feedback resistor, closed-loop gain. could entertain idea "feedback-resistor-bandwidth" product. intersection zero-slope feedback resistor line openloop transimpedance curve yields closed-loop -3dB bandwidth. circuit with higher feedback resistor will have reduced bandwidth. This good overcompensate current feedback amp. feedback resistor twice manufacturer's recommended value will circuit bandwidth half. feedback resistance, impedance, reduced lower value, there loss phase margin. seen from transfer function Figure negative loop transmission, (R2/(Z)s), equals loop unstable.
|Z(s)|
Open Loop Gain Loop Gain
Iinv
Z(s)Iinv Iinv error current
Figure Current Feedback
OA-20 Fig4
Loop Bandwidth
inverting amplifier employing current feedback shown Figure transfer function derived from following equations: Z(s)Iinv, (Vi/R1) Iinv -(Vo/R2)
Open Loop Phase
Z(s)
Phase Margin
Sets Gain Determines Frequency Response OA-20 Fig5 Figure Current Feedback Inverting Gain
-180°
Z(s) approaches infinity, closed-loop gain -(R2/R1). Notice that only feedback resistor appears characteristic equation, term with Z(s). closed-loop gain been decoupled from frequency response determining term transfer function. Only feedback resistor affects closed-loop frequency response. Bode plot circuit shown Figure current feedback amplifier also compensated with dominant low-frequency pole. This pole usually higher frequency than that voltage feedback amp. current feedback commonly compensated maximally flat response specified closed-loop gain with specified feedback resistor. phase approximately degrees where this one-pole approximation valid. ideal current feedback does
Figure Bode Plot
OA-20 Fig6
design trade-offs between current feedback voltage feedback differ. Voltage feedback allows freedom choice feedback resistor impedance) expense sacrificing bandwidth gain. Current feedback maintains high bandwidth over wide range gains cost limiting feedback impedance. example, common error using current feedback short inverting input output attempt build voltage follower. This circuit will oscillate. circuit perfectly stable recommended feedback resistor used place short. Similarly, integrator commonly accomplished placing capacitor between inverting input output. high frequencies capacitor impedance easily have impedance less than
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that required stability. proper feedback resistor series with feedback capacitor will stabilize amplifier, introduce high frequency zero into integrator transfer function. Another aspect current feedback amps which causes much consternation open-loop inverting input impedance. This feature, which causes decoupling closed-loop gain bandwidth, often viewed making current feedback amps unsuitable differential amplifiers. fact, inverting input impedance result better high-frequency differential amplifier than similar circuit built with voltage feedback amp. First, consider closed-loop driving-point impedance amp, regardless nature error signal. circuit equations find this closed-loop impedance shown Figure resistor, openloop inverting input impedance. Note that simply resistance. voltage feedback amplifier will have approaching infinity; current feedback approaches zero. test current, applied inverting input inverting node currents summed. find closed-loop inverting input impedance either amplifier type simply substitute proper form output voltage,
Iinv
output voltage derived differently current feedback amp. When -V2(Z(s)/R) substituted, result Figure also. seen that mechanisms force inverting input impedance value, ideally zero. When Z(s) very large, Zinv(s) goes zero. addition, goes zero does closed-loop inverting input impedance. topology input buffer keeps small very high frequencies. Thus current feedback have better virtual ground inverting input than voltage feedback amp, especially high frequencies. summary above discussion tabulated Figure voltage difference between input terminals zero. Voltage feedback drives this difference zero. current feedback amplifier input buffer forces input terminals equal voltages. Both amplifier types have high non-inverting input impedance, non-inverting current small. voltage feedback high open-loop inverting input impedance, thus inverting current approaches zero. Current feedback forces inverting current zero. Both amps display similar input voltage current characteristics. Only mechanism forcing these zero differs.
Voltage Feedback
Current Feedback
V1-V2
Figure Comparison Closed-Loop Operation
open loop inverting input impedance
OA-20 Fig9
OA-20 Fig7
Figure Closed-Loop Inverting Input Impedance case voltage feedback amp, -V2A(s) this circuit. result, Zinv(s), Figure familiar result that when A(s) approaches infinity, incremental inverting impedance approaches zero. This incremental virtual ground which much firstorder analysis based. Traditional A(s) Zinv Z(s) Currenty Feedback Zinv Z(s) Figure Inverting Input Impedance Comparison
seen that from closed-loop standpoint both current voltage feedback amps allow same ideal assumptions made. Voltage feedback gain-bandwidth product which limits lowest stable gain. Current feedback displays "feedback-resistor-bandwidth" product which limits lowest stable feedback impedance. inverting input impedance ideal voltage feedback closed-loop circuit zero. Feedback accomplishes this dividing high open-loop impedance high loop gain. open-loop inverting input impedance ideal current feedback zero. practical current feedback finite inverting input impedance, less than 100. Feedback reduces this further dividing initially open-loop inverting input impedance loop transmission. result better incremental ground inverting input very high frequencies. both amplifier types used closed-loop topologies, same methods analysis equally applicable. that this seen, current feedback amps easily designed into amplifiers arbitrary gain
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(inverting non-inverting), integrators, differential amplifiers (Figure 10), current-to-voltage converters, commonly known transimpedance amplifiers (Figure 11).
bias
-Iin
R1/R2(V1-V2)
OA-20 Fig11 Figure Transimpedance Amplifier with Current Feedback
Figure Difference OA-20 Fig10
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Current-Feedback Myths Debunked Arne Buck July 1992 A(s

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