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OA-20 Current-Feedback Myths Debunked Current-Feedback Myths Debunked OA-20 National Semiconductor Current-Feedback Myths OA-20 Arne Buck Debunked July 1992 Introduction Mystery needlessly surrounds the operation and use of cur- Vi R1 R2 R2 rent feedback operational amplifiers. Many engineers refuse − Vo = R1 to design with these op amps due to misunderstandings V R V2 i 1+ 2 Vo R which are easily rectified. 1+ 1 V1 () Much has been written to date on the internal circuitry of As current feedback op amps. These open-loop “tutorials” ob- fuscate how current feedback works in a closed-loop circuit. 01280002 Practical op amp circuits are closed-loop feedback systems which yield to classical control theory analysis. Analog circuit FIGURE 2. Voltage Feedback Op Amp Inverting Gain designers are comfortable with voltage feedback op amps in a closed-loop circuit and with the familiar ideal op amp The familiar Bode plot of this circuit is shown in Figure 3. The approximations feedback affords. It will be shown that cur- amplifier is typically compensated with a dominant rent feedback op amps can be analyzed in an analogous low-frequency pole to ensure stability down to a specified fashion. Once this closed-loop similarity is appreciated, it is minimum gain, often unity. In the region where the one-pole easy to see that most circuits commonly built with voltage approximation of the open-loop response is valid, the phase feedback op amps can be realized with a current feedback is around −90˚. This is the gain-bandwidth product region. op amp, and with better results at high frequencies. The intersection of the zero-slope noise gain line and the open-loop gain curve determines the closed-loop system −3 Refer to Figure 1 to review the open-loop terminal charac- dB bandwidth. A high gain circuit will have less bandwidth teristics of a voltage feedback amplifier. Ideally the than a lower gain circuit. As the circuit moves to lower gains, non-inverting input impedance is infinite, as is the inverting bandwidth increases, phase margin is lost and stability suf- input impedance. The output is a voltage source, the output fers. impedance of which is zero. This voltage source is controlled by the potential difference between the two op amp input terminals. This is the error voltage, hence the term voltage feedback. Feedback will drive the error voltage to zero. The | Open Loop Gain open-loop dynamics are contained in A(s). This A(s) is a A(s) | dimensionless gain, often represented in units of volts per Loop Gain volt or decibels. 20 log (1+R2/R1) V1 + Assumptions 0f Loop Bandwidth + Z i ∞ f Z(s) V Z - ∞ o i Open Loop Phase V 2 Zo = 0 - Vo = A(s)[V1-V2] Θ° V1-V2 is the error voltage 01280001 Phase Margin -180° FIGURE 1. Voltage Feedback Op Amp 01280003 A typical voltage feedback circuit is shown in Figure 2, the inverting amplifier. The transfer function is developed from FIGURE 3. Bode Plot the following equations: V1 =0,Vo = −A(s)V2,(Vi–V2)/R1 =(V2–Vo)/R2. The open-loop terminal characteristics of a current feedback As A(s) approaches infinity, the closed-loop gain is −(R2/R1). amplifier are depicted in Figure 4. There is a unity-gain buffer The frequency response of the closed-loop circuit is deter- between the two op amp inputs. This buffer ideally has mined by the denominator of the transfer function. Both the infinite input impedance and zero output impedance. Thus noise gain (1 + R2/R1) of the circuit and the the non-inverting input impedance of the current feedback frequency-dependent source (A(s)) appear in the denomina- op amp is infinite, and the inverting input impedance is zero. tor, linking the closed-loop gain and bandwidth. The output is a voltage source, so the output impedance is © 2002 National Semiconductor Corporation AN012800 www.national.com zero. This voltage source is controlled by the current out of phase margin. As can be seen from the transfer function in the inverting input. This is the error current, hence current Figure 5, if the negative of the loop transmission, (R2/(Z)s), OA-20 feedback. Feedback forces the error current to zero. The equals −1 the loop is unstable. open-loop dynamics are determined by Z(s). This Z(s) is a current controlled voltage source which has units of transim- pedance, ohms. | Open Loop Gain Z(s) | V1 + Assumptions Loop Gain + Z i ∞ - 20 log x1 Z(s) Z i = 0 V 2 Zo = 0 R2 - Vo = Z(s)Iinv f I 0 inv I is the error current inv Loop Bandwidth 01280004 f FIGURE 4. Current Feedback Op Amp Open Loop Phase The inverting amplifier employing a current feedback op amp is shown in Figure 5. The transfer function is derived from the following equations: Θ° V1= 0, Vo = Z(s)Iinv, (Vi/R1) + Iinv = -(Vo/R2) Phase Margin -180° Vi R1 R2 − R2 Vo = R1 V R V2 i 1+ 2 01280006 Vo Zs() V1 FIGURE 6. Bode Plot R1 – Sets the Gain The design trade-offs between current feedback and voltage R2 – Determines the Frequency Response feedback differ. Voltage feedback allows freedom of choice 01280005 of the feedback resistor (or impedance) at the expense of sacrificing bandwidth for gain. Current feedback maintains FIGURE 5. Current Feedback Op Amp Inverting Gain high bandwidth over a wide range of gains at the cost of limiting the feedback impedance. As Z(s) approaches infinity, the closed-loop gain is −(R2/R1). Notice that only the feedback resistor appears in the char- For example, a common error in using a current feedback op acteristic equation, in the term with Z(s). The closed-loop amp is to short the inverting input to the output in an attempt gain has been decoupled from the frequency response de- to build a voltage follower. This circuit will oscillate. The termining term of the transfer function. Only the feedback circuit is perfectly stable if the recommended feedback re- resistor affects the closed-loop frequency response. sistor is used in place of the short. Similarly, an integrator is commonly accomplished by placing a capacitor between the A Bode plot for the circuit is shown in Figure 6. A current inverting input and output. At high frequencies a capacitor feedback amplifier is also compensated with a dominant has a low impedance and can easily have an impedance low-frequency pole. This pole is usually at a higher fre- less than that required for stability. The proper feedback quency than that of a voltage feedback op amp. A current resistor in series with the feedback capacitor will stabilize the feedback op amp is commonly compensated for maximally amplifier, and introduce a high frequency zero into the inte- flat response at a specified closed-loop gain and with a grator transfer function. specified feedback resistor. The phase is approximately −90˚ where this one-pole approximation is valid. The ideal current Another aspect of current feedback op amps which causes feedback op amp does not have a gain-bandwidth product. much consternation is the low open-loop inverting input im- The closed-loop bandwidth is determined by the feedback pedance. This feature, which causes the decoupling of resistor, not the closed-loop gain. One could entertain the closed-loop gain and bandwidth, is often viewed as making idea of a “feedback-resistor-bandwidth” product. The inter- current feedback op amps unsuitable for use as differential section of the zero-slope feedback resistor line and the amplifiers. In fact, the low inverting input impedance can open-loop transimpedance curve yields the closed-loop result in a better high-frequency differential amplifier than a −3dB bandwidth. A circuit with a higher feedback resistor will similar circuit built with a voltage feedback op amp. have reduced bandwidth. This is a good way to First consider the closed-loop driving-point impedance of an over-compensate the current feedback op amp. A feedback op amp, regardless of the nature of the error signal. The resistor of twice the manufacturer’s recommended value will circuit and equations to find this closed-loop impedance are cut the circuit bandwidth in half. As the feedback resistance, shown in Figure 7. The resistor, R, is the open-loop inverting or impedance, is reduced to a lower value, there is a loss of input impedance. Note that R is simply a resistance. A www.national.com 2 OA-20 voltage feedback amplifier will have an R approaching infin- small to very high frequencies. Thus a current feedback op ity; in a current feedback op amp R approaches zero. A test amp can have a better virtual ground at the inverting input current, IT, is applied to the inverting input and the inverting than a voltage feedback amp, especially at high frequencies. node currents are summed. To find the closed-loop inverting A summary of the above discussion is tabulated in Figure 9. input impedance of either amplifier simply substitute the The voltage difference between the input terminals is zero. proper form of the output voltage, Vo. Voltage feedback drives this difference to zero. The current feedback amplifier input buffer forces the two input terminals to equal voltages. Both amplifier types have a high Z = 0 V o 1 + non-inverting input impedance, so the non-inverting current Vo is small. A voltage feedback op amp has a high open-loop V2 R inverting input impedance, thus the inverting current ap- - Iinv proaches zero. Current feedback forces the inverting current R2 to zero. Both op amps display similar input voltage and IT current characteristics.
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