Feedback - Memorial University of Newfoundland

Microelectronic Circuits, Seventh Edition Sedra/Smith Copyright © 2015 by Oxford University Press

CHAPTER 11

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Figure 11.1 General structure of the feedback amplifier. This is a signal-flow diagram, and the

quantities x represent either voltage or current signals.


Figure 11.2 Determining the loop gain by breaking the feedback loop at the output of the basic amplifier,

applying a test signal xt, and measuring the returned signal xr : Aβ ≡ −xr/xt.


Figure 11.3 (a) A noninverting op-amp circuit for Example 11.1. (b) The circuit in (a) with the op amp

replaced with its equivalent circuit.



Figure 11.4 Application of negative feedback reduces the midband gain, increases fH, and

reduces fL, all by the same factor, (1 + AMβ), which is equal to the amount of feedback.


Figure 11.5 Illustrating the application of negative feedback to improve the signal-to-interference ratio in amplifiers.



Figure 11.7 Block diagram of a feedback voltage amplifier. Here the appropriate feedback topology is series–shunt.


Figure 11.8 Examples of a feedback voltage amplifier. All these circuits employ series–shunt feedback.

Note that the dc bias circuits are only partially shown.


Figure 11.9 Breaking the conceptual feedback loop in (a) to determine the loop gain requires the termination of the loop as

shown in (b), to ensure that the loop conditions do not change.


Figure 11.10 Determining: (a) the feedback factor β; and (b) the loop gain Aβ for the feedback voltage amplifier of Fig. 11.8(b).


Figure 11.11 Example 11.3. (a) A series–shunt feedback amplifier; (b) the feedback loop obtained by setting Vs = 0 and

replacing the op amp with its equivalent-circuit model; (c) breaking the feedback loop to determine the loop gain Aβ = −Vr/Vt.


Figure 11.12 The series–shunt feedback amplifier: (a) ideal structure; (b) equivalent circuit.


Figure 11.13 Determining the output resistance of the feedback amplifier of Fig. 11.12(a): Rof = Vx/Ix.


Figure 11.14 (a) Block diagram of a practical series–shunt feedback amplifier. (b) The circuit in (a)

represented by the ideal structure of Fig. 11.12(a). (c) Definition of R11 and R22. (d) Determination of

the feedback factor β. (e) The A circuit, showing the open-loop resistances Ri and Ro.


Figure 11.14 continued


Figure 11.15 Circuits for Example 11.4.


Figure 11.16 (a) Series–shunt feedback amplifier for Example 11.5; (b) the A circuit; (c) the β circuit.


Figure E11.8


Figure 11.17 Example 11.6.


Figure 11.18 The feedback transconductance amplifier (series–series).


Figure 11.19 The feedback transconductance amplifier (series–series).






Figure 11.22 The feedback transresistance amplifier (shunt–shunt).


Figure 11.23 The feedback transresistance amplifier (shunt–shunt).


Figure 11.24 (a) A feedback transresistance amplifier; (b) the β circuit; (c) determining β; (d) the A circuit.


Figure E11.19


Figure 11.25 The feedback current amplifier (shunt–series).


Figure 11.26 The feedback current amplifier (shunt–series).


Figure 11.27 Circuit for Example 11.10.


Figure 11.27 continued



Figure 11.28 The Nyquist plot of an unstable amplifier.


Figure 11.29 Relationship between pole location and transient response.


Figure 11.30 Effect of feedback on (a) the pole location and (b) the frequency response of an amplifier having a

single-pole, open-loop response.


Figure 11.31 Root-locus diagram for a feedback amplifier whose open-loop transfer function has two real poles.


Figure 11.32 Definition of ω0 and Q of a pair of complex-conjugate poles.


Figure 11.33 Normalized gain of a two-pole feedback amplifier for various values of Q. Note that Q is determined

by the loop gain according to Eq. (11.70).


Figure 11.34 Circuits and plot for Example 11.11.


Figure 11.35 Root-locus diagram for an amplifier with three poles. The arrows indicate the pole movement as A0β is increased.


Figure E11.26


Figure 11.36 Bode plot for the loop gain Aβ illustrating the definitions of the gain and phase margins.


Figure 11.37 Stability analysis using Bode plot of |A|.



Figure 11.39 (a) Two cascaded gain stages of a multistage amplifier. (b) Equivalent circuit for the interface between the two stages in (a). (c)

Same circuit as in (b), but with a compensating capacitor CC added. Note that the analysis here applies equally well to MOS amplifiers.


Figure 11.40 (a) A gain stage in a multistage amplifier with a compensating capacitor connected in the feedback path, and (b)

equivalent circuit. Note that although a BJT is shown, the analysis applies equally well to the MOSFET case.


Figure P11.2


Figure P11.3


Figure P11.9


Figure P11.24


Figure P11.29


Figure P11.31


Figure P11.32


Figure P11.33


Figure P11.40


Figure P11.41


Figure P11.43


Figure P11.45


Figure P11.47


Figure P11.48


Figure P11.52


Figure P11.53


Figure P11.54


Figure P11.55


Figure P11.56


Figure P11.57


Figure P11.58


Figure P11.63


Figure P11.64


Figure P11.65


Figure P11.68


Figure P11.69


Figure P11.70


Figure P11.72


Figure P11.73


Figure P11.77


Figure P11.78


Figure P11.79


Figure P11.80


Figure P11.81


Figure P11.82


Figure P11.106

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