Electronics · Ch 4 — Feedback in Amplifiers

Chapter Summary

Feedback is the process of taking a part of an amplifier's output and feeding it back to the input; the returned sample modifies the net input to control the output.
The open-loop gain (no feedback) is $A = V_o/V_i$ ; the closed-loop gain (with feedback) is $A_f = V_o/V_s$ . The feedback factor is $\beta = V_f/V_o$ and the loop gain is $A\beta$ .
Positive feedback ( $V_f$ in phase with $V_s$ , $V_i = V_s + V_f$ ) increases gain but worsens distortion, stability, frequency response and drift — used in oscillators.
Negative feedback ( $V_f$ $180^\circ$ out of phase, $V_i = V_s - V_f$ ) reduces gain but gives highly stabilised gain, less distortion, reduced noise, increased bandwidth and adjustable impedances — used in amplifiers.
There are four types of negative feedback: voltage series, voltage shunt, current series and current shunt. All four reduce the voltage gain; their effects on input and output impedance are summarised in Table 4.1.
For a voltage series negative feedback amplifier the recurring factor is $(1+A\beta)$ : gain $A_f = \dfrac{A}{1+A\beta}$ (and $A_f \approx 1/\beta$ when $A\beta \gg 1$ , giving stable gain); input impedance $Z_{if} = Z_i(1+A\beta)$ (increases); output impedance $Z_{of} = \dfrac{Z_o}{1+A\beta}$ (decreases); bandwidth $BW_f = BW(1+A\beta)$ (increases). …