1

Feedback

Microelectronic Circuits - Fifth Edition Sedra/Smith 2Copyright 2004 by Oxford University Press, Inc.

Figure 8.1 General structure of the feedback amplifier. This is a signal-flow diagram, and the quantities x represent either voltage or current signals.

Microelectronic Circuits - Fifth Edition Sedra/Smith 3Copyright 2004 by Oxford University Press, Inc.

Figure E8.1

Microelectronic Circuits - Fifth Edition Sedra/Smith 4Copyright 2004 by Oxford University Press, Inc.

Figure 8.1 General structure of the feedback amplifier. This is a signal-flow diagram, and the quantities x represent either voltage or current signals.

A ~ open loop gain of the amplifier

~ feedback ratio

xs ~ signal from source

xo ~ output signal

xf ~ feedback signal

xi ~ input signal to amplifier

Microelectronic Circuits - Fifth Edition Sedra/Smith 5Copyright 2004 by Oxford University Press, Inc.

Figure 8.1 General structure of the feedback amplifier. This is a signal-flow diagram, and the quantities x represent either voltage or current signals.

xf = xo

xi = xs - xf

xo = xi

xo = xs - xf)

xo = xs - A xo)

)xo = A xs

1o

fs

x AA

x A

Gain with Feedback: Closed Loop Gain

~ loop gain

Microelectronic Circuits - Fifth Edition Sedra/Smith 6Copyright 2004 by Oxford University Press, Inc.

Figure 8.1 General structure of the feedback amplifier. This is a signal-flow diagram, and the quantities x represent either voltage or current signals.

1

ln ln ln 1

1 1

1

f

f

f

f

AA

A

A A A

A

A A A A

Gain Desensitivity

11

1 1 0

f

f

Af

f

AA A

A A A

AA

A A

Microelectronic Circuits - Fifth Edition Sedra/Smith 7Copyright 2004 by Oxford University Press, Inc.

Figure 8.1 General structure of the feedback amplifier. This is a signal-flow diagram, and the quantities x represent either voltage or current signals.

1

1

1 11

M

H

M

Hf

M

H

AA s

s

AA s s

A sAA ss

Bandwidth Extension

Microelectronic Circuits - Fifth Edition Sedra/Smith 8Copyright 2004 by Oxford University Press, Inc.

Figure 8.1 General structure of the feedback amplifier. This is a signal-flow diagram, and the quantities x represent either voltage or current signals.

1

111

1

1 1

M

H Mf

M H M

H

M Mf

H M

As A

A sA s As

A AA s

s A

Bandwidth Extension

Microelectronic Circuits - Fifth Edition Sedra/Smith 9Copyright 2004 by Oxford University Press, Inc.

Figure 8.1 General structure of the feedback amplifier. This is a signal-flow diagram, and the quantities x represent either voltage or current signals.

1

1 1 1

~ closed loop midband gain1

1 ~ closed loop bandwidth

closed loopM Mf

H M

closed loop

Mclosed loop

M

closed loop H M

GA AA s

ss AB

AG

A

B A

Bandwidth Extension

Microelectronic Circuits - Fifth Edition Sedra/Smith 10Copyright 2004 by Oxford University Press, Inc.

Figure 8.2 Illustrating the application of negative feedback to improve the signal-to-noise ratio in amplifiers.

Noise Reduction

s

n

VS

N V

Microelectronic Circuits - Fifth Edition Sedra/Smith 11Copyright 2004 by Oxford University Press, Inc.

Figure 8.2 Illustrating the application of negative feedback to improve the signal-to-noise ratio in amplifiers.

Noise Reduction

1 2 1

1 2 1 21 1o s n

A A AV V V

A A A A

Superposition:

Microelectronic Circuits - Fifth Edition Sedra/Smith 12Copyright 2004 by Oxford University Press, Inc.

Figure 8.2 Illustrating the application of negative feedback to improve the signal-to-noise ratio in amplifiers.

Noise Reduction

1 2

1 22

1

1 2

1

1

ss

feedback nn

A AV

VA ASA

AN VVA A

Microelectronic Circuits - Fifth Edition Sedra/Smith 13Copyright 2004 by Oxford University Press, Inc.

Reduction in Nonlinear Distortion

xf = xo

xi = xs - xf

xo = xi + xnl

xo = xs - xf) + xnl

xo = xs - A xo) + xnl

)xo = A xs + xnl

1

1 1o s nl

Ax x x

A A

1 1 1

1 1A

o s nl s

Ax x x x

A A

Microelectronic Circuits - Fifth Edition Sedra/Smith 14Copyright 2004 by Oxford University Press, Inc.

Figure 8.4 The four basic feedback topologies: (a) voltage-mixing voltage-sampling (series–shunt) topology; (b) current-mixing current-sampling (shunt–series) topology; (c) voltage-mixing current-sampling (series–series) topology; (d) current-mixing voltage-sampling (shunt–shunt) topology.

Microelectronic Circuits - Fifth Edition Sedra/Smith 15Copyright 2004 by Oxford University Press, Inc.

Effects of Negative Feedback on Input and Output Impedances

Microelectronic Circuits - Fifth Edition Sedra/Smith 16Copyright 2004 by Oxford University Press, Inc.

1

1

if i

oof

Z Z A

ZZ

A

Microelectronic Circuits - Fifth Edition Sedra/Smith 17Copyright 2004 by Oxford University Press, Inc.

1

1

iif

of o

ZZ

A

Z Z A

Microelectronic Circuits - Fifth Edition Sedra/Smith 18Copyright 2004 by Oxford University Press, Inc.

1

1

if i

of o

Z Z A

Z Z A

Microelectronic Circuits - Fifth Edition Sedra/Smith 19Copyright 2004 by Oxford University Press, Inc.

1

1

iif

oof

ZZ

A

ZZ

A