ece 2133 electronic circuitsstaff.iium.edu.my/zahirulalam/courses/ece2133/feedback.pdf · ece 2133...

ECE 2133

Electronic Circuits

Dept. of Electrical and Computer Engineering

International Islamic University Malaysia

Chapter 12

Feedback and Stability

Introduction to Feedback

1-4

© Electronic Circuits

Introduction to Feedback

Harold Black, an electronics engineer from Western Electric Company, invented the feedback amplifier in 1928 while searching for methods to stabilize the gain of amplifiers for use in telephone repeaters.

In a feedback system, a signal that is proportional to the output is fed back to the input and combined with the input signal to produce a desired system response.

Feedback can be either negative or positive.

In negative feedback, a portion of the output signal is subtracted from the input signal.

Tends to maintain a constant value of amplifier voltage gain against variations in transistor parameters, supply voltages, and temperature.

In positive feedback, a portion of the output signal is added to the input signal.

Used in the design of oscillators and other applications.

1-5


Advantages of Negative

Feedback Gain sensitivity

Variations in the circuit transfer (gain) as a result of changes in transistor parameters are reduced by feedback

Bandwidth extension

The bandwidth of a circuit that incorporates negative feedback is larger than that of the basic amplifier

Noise sensitivity

Increase the signal-to-noise ration if noise is generated within the feedback loop

Reduction of nonlinear distortion

At large signal levels, distortion may appear in the transistor output signal due to its nonlinear characteristics. Negative feedback reduces this distortion

Control of impedance levels

The input and output impedances can be increased or decreased with the proper type of negative feedback circuit

1-6


Disadvantages of Negative

Feedback

Circuit Gain

The overall amplifier gain, with negative feedback, is reduced compared to the basic amplifier used in the circuit

Stability

The feedback circuit may become unstable (oscillate) at high frequencies

Basic Concepts of Feedback

1-8


Basic Feedback Circuit

S current or voltage.

A open loop gain of a basic amplifier

Sfb feedback signal by sampling the output signal

Sɛ error signal by subtracting the feedback signal from the input source signal

Error signal is the input to the basic amplifier and amplified to produce the output signal

1-9


Assumptions

The input signal is transmitted through the amplifier only, none through the feedback network

The output signal is transmitted back through the feedback network only, none through the amplifier

There are no loading effects in the ideal feedback system

The feedback network does not load down the output of the basic amplifier

The basic amplifier and feedback network do not produce a loading effect on the input signal source

1-10


Ideal Closed-Loop

Signal Gain

A amplification factor

feedback transfer function

Af closed-loop transfer function

T loop gain

S* can be either voltage or currents or a combination of both

T is (+) for negative feedback but can be a complex number also

1-11


Ideal Closed-Loop

Signal Gain

If the loop gain is large so that A >> 1, the overall gain of

the feedback is a function of the feedback network only

For a large loop gain

1-12


Gain Sensitivity

Consider the feedback transfer function is a constant

The percent change in the closed-loop gain Af is less than the corresponding percent change in the open-loop gain A by the factor (1+ A)

Ideal Feedback Topologies

1-14


Preview

There are four feedback topologies, based on the parameter to be amplified (voltage or current) and the output parameter (voltage or current).

Series-Shunt (voltage amplifier).

Shunt-Series (current amplifier)

Series-Series (transconductance amplifier)

Shunt-Shunt (transresistance amplifier)

1-15


Preview

1-16


Series-Shunt Configuration

The circuit is a voltage-controlled voltage source and is an ideal voltage amplifier.

The feedback circuit samples the output voltage and provides a feedback voltage in series with the source voltage.

An increase in the output voltage produces an increase in the feedback voltage, which in turn decreases the error voltage due to the negative feedback.

The smaller error voltage is amplified producing a smaller output voltage.

Which means that the output signal tends to be stabilized.

1-17



The output of the feedback network is an open circuit

Vfb feedback voltage, v (= Vfb/Vo) voltage feedback transfer function

Source resistance RS is negligible

Avf closed-loop voltage transfer function

The magnitude of Avf is less than that of Av, the advantage is that Avf becomes independent of the individual transistor parameters

1-18



A series input connection results in an increased input resistance compared to that of the basic voltage amplifier. This eliminates loading effects on the input signal source due to the amplifier

1-19



A shunt output connection results in a decreased output resistance compared to that of the basic voltage amplifier. This eliminates loading effects on the output signal when an output load is connected

1-20



1-21


Shunt-Series Configuration

The circuit is a current-controlled current source and is an ideal current amplifier.

The feedback circuit samples the output current and provides a feedback signal in shunt with the signal current.

An increase in the output current produces an increase in the feedback current, which in turn decreases the error current due to the negative feedback.

The smaller error current is amplified producing a smaller output current.

Which means that the output signal tends to be stabilized.

1-22



The output of the feedback network is a short circuit

Ifb feedback current, i (= Ifb/Io) feedback current transfer function

Source resistance RS is large

Aif closed-loop current transfer function

The magnitude of Aif is less than that of Ai, the advantage is that Aif becomes independent of the individual transistor parameters

1-23



A shunt input connection decreases the input resistance compared to that of the basic amplifier. This eliminates loading effects on the input signal current source due to the amplifier

1-24



A series output connection increases the output resistance compared to that of the basic voltage amplifier. This eliminates loading effects on the output signal when an output load is connected

1-25



1-26


Series-Series Configuration

The feedback circuit samples a portion of the output current and converts it to a voltage.

This feedback circuit is a voltage-to-current amplifier.

1-27



The output of the feedback network is a short circuit

Vfb feedback voltage, z (= Vfb/Io) resistance feedback transfer function

Neglecting the affect of Source resistance RS

Agf closed-loop current-to-voltage transfer function or transconductance gain

1-28



Input resistance increases compared to that of the basic amplifier.

gzii

iif

gzi

i

ii

gz

i

gz

gz

oz

fbi

ARI

VR

AR

V

R

VI

A

VV

AV

VAV

IV

VVV

1

1

1

1

)(

1-29



The output resistance increases compared to that of the basic amplifier.

ogzx

xof

ogzx

oxzgxogxx

RAI

VR

RAI

RIAIRVAIV

1

1

1-30



1-31


Shunt-Shunt Configuration

The feedback circuit samples a portion of the output voltage and converts it to a current.

This feedback circuit is a current-to-voltage amplifier.

1-32



The output of the feedback network is an open circuit

Ifb feedback current, g (= Ifb/Vo) conductance feedback transfer function

Source resistance RS is very large

Azf closed-loop voltage-to-current transfer function or transresistance gain

1-33



Input resistance decreases compared to that of the basic amplifier.

zg

i

i

iif

zg

iiii

zg

i

zgogfbi

A

R

I

VR

A

RIRIV

A

II

IAIVIIII

1

1

1

1-34



The output resistance decreases compared to that of the basic amplifier.

zg

o

x

xof

o

zgx

o

xgzx

o

zxx

A

R

I

VR

R

AV

R

VAV

R

IAVI

1

1

1-35



Feedback Networks

1-37


Voltage Amplifiers

1

2

21

222

111

1R

R

v

vA

R

vv

R

v

R

vv

R

vvi

R

v

R

vi

i

ov

oii

oiofb

ifb

Av is very large

v is feedback transfer function

.

1-38


Voltage Amplifiers

Av is the open-loop voltage gain of the basic amplifier. For Ro 0

Ri is very large

1-39


Voltage Amplifiers

.

=

vvAT

Av is positive and v is also positive, thus the loop gain is positive for negative feedback

fbivoii VVVVAVRIV ,,

1-40


Current Amplifiers

Ai is open-loop current gain and very large

if RS >> Rif, then Ii’ Ii and I is negligible,

Assuming V1 is at virtual ground

1-41


Current Amplifiers

Ai is very large


1-42


Solving for Ifb and substitute to

and rearranging to find the closed-loop current gain

Current Amplifiers

.

1-43


Transconductance Amplifiers

Ag is open-loop transconductance gain and very large

Neglecting base current

Ag is very large

1-44


Transconductance Amplifiers

Assume Ic Ie ad Ri is very large

1-45


Transresistance Amplifiers

Az is open-loop transresistance gain and very large

Az is very large


Ifb = Ii

1-46


Transresistance Amplifiers

Vo = -AzI, I = Ii – Ifb, and Vo = -Az(Ii – Ifb)


ece 2133 electronic circuitsstaff.iium.edu.my/zahirulalam/courses/ece2133/feedback.pdf · ece 2133...

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