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ECE 3120 Microelectronics II Dr. Suketu Naik

Chapter 10

Feedback

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ECE 3120 Microelectronics II Dr. Suketu Naik

Operational Amplifier Circuit Components

1. Ch 7: Current Mirrors and Biasing

2. Ch 9: Frequency Response

3. Ch 8: Active-Loaded Differential Pair

4. Ch 10: Feedback

5. Ch 11: Output Stages

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ECE 3120 Microelectronics II Dr. Suketu Naik

Basic Feedback Topologies

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ECE 3120 Microelectronics II Dr. Suketu Naik

10.3 The Four Basic Feedback Topologies

Voltage Amplifiers, Current Amplifiers, Trans-conductance Amplifiers, Trans-resistance Amplifiers

Voltage Amplifier Current Amplifier

Trans-conductance Amplifier Trans-resistance Amplifier

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ECE 3120 Microelectronics II Dr. Suketu Naik

Feedback Voltage Amplifier

(Series-Shunt Feedback)

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ECE 3120 Microelectronics II Dr. Suketu Naik

10.3.1 Voltage Amplifiers

Voltage amplifiers – accept input voltage and yield output

voltage.

VCVS (Voltage Controlled Voltage Source)

Thevenin Equivalent

Voltage-mixing / voltage-sampling – is the topology most

suitable for voltage amps

Is also known as series-shunt feedback

Provides high input resistance/low output resistance.

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ECE 3120 Microelectronics II Dr. Suketu Naik

Figure 10.6: Block diagram of a feedback voltage amplifier. Here the appropriate

feedback topology is series–shunt.

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ECE 3120 Microelectronics II Dr. Suketu Naik

Examples of Series-Shunt Feedback

Series-Shunt Feedback

Input to the amplifier Vi must

decrease (increase) as the signal

Vs increases (decreases)

This can happen if Vf increases

(decreases):

e.g. same change in polarity for

Vs

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ECE 3120 Microelectronics II Dr. Suketu Naik

Application of Feedback Voltage Amplifier

One of many (100s) of

applications: rain gauge

Non

inverting

Op amp

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ECE 3120 Microelectronics II Dr. Suketu Naik

Input resistance increases because Vf subtracts from Vs,

resulting in smaller signal Vi at the input (in series).

Low Vi causes input current Is to be smaller

Higher input resistance (Vs/Is)

Output resistance decreases because feedback works to

keep Vo as constant as possible (in parallel)

DVo and DIo change together

Lower output resistance

10.3.1 Voltage Amplifiers

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ECE 3120 Microelectronics II Dr. Suketu Naik

10.4 The Feedback Voltage Amplifier

Series-shunt is appropriate feedback for voltage amplifier.

Unilateral open-loop amplifier (circuit A).

Ideal Voltage-Sampling, voltage-mixing feedback network (b circuit)

Input resistance Ri , Open Circuit Gain A, Output resistance Ro

Figure 10.12: The series–shunt feedback amplifier: (a) ideal structure; (b)

equivalent circuit

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ECE 3120 Microelectronics II Dr. Suketu Naik

10.4.1 The Ideal Case

(10.17) closed-loop gain:

(10.18) input current:

(10.19) input resistance:

(10.20) output resistance:

(10.21) current-x:

1

1

1

of

s

sii

i i

if i

xof

x

x ix

o

V AA

V A

VVI

R A R

R A R

VR

I

V AVI

R

b

b

b

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ECE 3120 Microelectronics II Dr. Suketu Naik

Figure 10.13: Determining the output resistance of the feedback amplifier of Fig.

10.12(a): Rof = Vx /Ix.

Output Resistance of Feedback Amplifier

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ECE 3120 Microelectronics II Dr. Suketu Naik

10.4.2 The Practical Case

In practical case, feedback network will not be ideal

VCVS

Source and load resistances will affect A, Ri, and Ro.

Source and load resistances should be lumped with basic

amplifier

Expressed as two-port network

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ECE 3120 Microelectronics II Dr. Suketu Naik

10.4.2 The Practical Case

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ECE 3120 Microelectronics II Dr. Suketu Naik

Series-Shunt Feedback Amp: How to find A and β

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ECE 3120 Microelectronics II Dr. Suketu Naik

10.4.3 Summary

Ri and Ro are the input and output resistances, respectively, of the A circuit in Figure 10.15(a)

Rif and Rof are the input and output resistances, respectively, of the feedback amplifier, including Rs and RL (see Figure 10.14a)

The actual input and output resistances of the feedback amplifier exclude Rs and RL. These are denoted Rin and Rout in Figure 10.14(a) and can be determined via equations (10.25) and (10.25)

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ECE 3120 Microelectronics II Dr. Suketu Naik

List of Problems

Feedback Voltage Amplifier:

expl10.4 (read-only): Series-shunt feedback amplifier

p10.31: Series-shunt feedback amplifier

p10.36 (simulation only): Series-shunt feedback amplifier

with BJTs

p10.39 (simulation only): Series-shunt feedback in active-

loaded differential amplifier with MOSFETs