# (p.438) APPENDIX D FEEDBACK AMPLIFIERS

# (p.438) APPENDIX D FEEDBACK AMPLIFIERS

The most basic amplifier is a gain device itself, *e.g.* a bipolar transistor, JFET, or MOSFET. None of these is perfectly linear, so any amplifier utilizing these devices will also exhibit nonlinearity. Furthermore, the overall gain depends on device parameters that can vary among nominally identical devices.

Feedback amplifiers can provide predictable gain and significantly improve linearity by making the gain dependent primarily on linear components, *i.e.* resistors or capacitors, whose values are independent of signal amplitude.

# D.1 Gain of a feedback amplifier

Figure D.1 shows the principle. A portion of the output is fed back to a summing circuit at the input. The net voltage applied to the amplifier input

*i.e.*the gain with feedback

*A*

_{v}≫ 1/

*A*

_{fb}, the gain of the overall system (“closed loop gain”)

*A*

_{v}and determined by the feedback network alone. If the feedback network is an attenuator (

*A*

_{fb}< 1),

*e.g.*formed by resistors, the overall gain is set by the resistor values and is independent of the amplifier gain.

However, note that when the amplifier gain *A* _{v} is marginal it cannot be ignored. For example, when *A* _{v} = 1/*A* _{fb} (*i.e.* the amplifier gain equals the nominal closed loop gain)

# D.2 Linearity

Feedback linearizes the amplifier response. For a deviation Δ *A* _{v} in open loop gain

*A*

_{CL}is the nominal closed loop gain. Thus, deviations from linearity are reduced by the factor

# D.3 Bandwidth

Similarly, the bandwidth is also improved. The frequency dependent gain of a single-pole amplifier with a corner frequency *f* _{c}

*A*

_{fb}

*A*

_{v0}) this result can be rewritten as

*A*

_{fb}

*A*

_{v0}). The gain–bandwidth product and unity gain frequency remain unchanged whether or not feedback is applied. By a similar calculation, a lower corner frequency is reduced by the factor 1/(1 +

*A*

_{fb}

*A*

_{v0}).

# D.4 Series and shunt feedback

Feedback can be applied as either voltage or current, as illustrated in Figure D.2. In series feedback the feedback signal is applied as a voltage in series with the input. The feedback signal is commonly applied to the inverting input of a differential amplifier input. Shunt feedback adds currents, so the feedback signal connects directly to the input to form a current summing node.

# D.5 Input and output impedance

Although for simplicity in analysis we often use amplifier models with infinite input resistance (*i.e.* no current ever flows into the amplifier input), in reality all amplifying devices have a finite input impedance (impedance because the input generally does not appear purely resistive). A MOSFET input, for example, appears capacitive at low frequencies.

## (p.441) D.5.1 Series feedback

Consider the configuration in the left panel of Figure D.2. Without feedback the input current *i* _{i} = *v* _{i}/*Z* _{i0}, where *Z* _{i0} is the input impedance of the amplifier without feedback. With feedback the voltage applied to the amplifier input is reduced

## D.5.2 Shunt feedback

Again consider an amplifier with an infinite input impedance and a voltage gain *A* _{v}, but now the output is fed back directly to the input through an impedance *Z* _{f}, as shown in the right panel of Figure D.2. Since the amplifier has an infinite input impedance, any input current *i* _{i} must flow through the feedback impedance *Z* _{f}. Thus, the input current

*A*

_{v}≫ 1

This is the mechanism that leads to the notion of “virtual ground”. For example, an amplifier with a gain of 10^{5} and a feedback resistor of 10^{4} Ω yields
(p.442)

It is straightforward to extend the result to an amplifier with a finite input impedance *Z* _{a}. As indicated in Figure D.3 the input current splits into two components, each corresponding to a component of the input impedance,

## D.5.3 Output impedance

The same reasoning can be applied to the output impedance. In this case shunt feedback takes the voltage directly from the output, as in Figure D.2. Any reduction in the output signal with increasing load current due to a finite output

For a constant input voltage *V* _{B} applied to the base, the collector voltage

*dV*

_{BE}= −

*dI*

_{C}

*R*

_{E}and the output resistance of the transistor

*r*

_{o}=

*dV*

_{CE}/

*dI*

_{C}, the change in output voltage

*dI*

_{C}/

*dV*

_{BE}=

*g*

_{m}and

*dV*

_{CE}= −

*dI*

_{C}

*R*

_{E}, the output resistance with feedback becomes

# D.6 Loop gain

The quantity *A* _{fb} *A* _{v} that improves linearity, extends bandwidth, and affects the input or output impedance is called the loop gain. It can be measured by breaking the feedback loop at any convenient point and measuring the total gain between the break, as illustrated in Figure D.5. The original input signal source should remain connected, but with zero signal.

Since the benefits that accrue from negative feedback depend on the loop gain, as the loop gain decreases with *A* _{v} (*f*) beyond the either the lower or upper corner frequency, the effects of negative feedback decrease. For small loop gains the overall response follows the open loop response.

# D.7 Stability

The amount of feedback is limited by stability considerations. In reality, amplifiers have multiple corner frequencies, so eventually the additional phase shift attains 180° and negative feedback turns into positive feedback, leading to self-oscillation. Figure D.6 illustrates the frequency response of an amplifier in open loop and closed loop operation. Two values of closed loop gain are shown. For *A* _{CL1} the corner frequency is in the regime where the open loop phase shift is 90°, so the total phase shift of the inverting amplifier is 180° + 90° = 270°. At frequencies beyond the second corner frequency the additional phase shift is 180°, so the amplifier is noninverting and potentially unstable if too much feedback is applied. The stability criterion is that the loop gain may not exceed unity at the frequency where the additional phase shift is 180°. However, to ensure a safety margin, a phase margin of 45°, *i.e.* an additional phase shift of 135° is generally accepted to be the minimum. This is shown for closed loop gain *A* _{CL2}.

Figure D.7 shows the open loop gain and phase *vs.* frequency (left) of a commercial operational amplifier. Two corner frequencies with an extended 90° phase shift regime are apparent. The right panel shows the gain *vs.* frequency for various closed loop gains with the open loop gain for comparison. Beyond their respective corner frequencies the closed loop gain curves follow the open loop response. Reduced phase margin at closed loop gains of 3 and 5 results in pronounced

*A*

_{CL}= 3, 5, 10, and 20. The peaking in the frequency response for

*A*

_{CL}= 3 and 5 translates into ringing in the time response.

The stability of a feedback circuit can be assessed either by inspection of the open loop gain and phase (Bode 1940) or by plotting the imaginary part of the loop gain *vs.* the real part (Nyquist 1932). If the resulting curve encloses the point (1,0) – *i.e.* one on the real axis – the circuit is unstable. This is illustrated in Figure D.9. A closed loop gain *A* _{CL} = 2 leads to self-oscillation, whereas reducing the feedback to obtain *A* _{CL} = 3 achieves stability, but the small phase margin leads to pronounced peaking in the frequency response (Figure D.7) and ringing in the pulse response (Figure D.8).