ELECTRONIC CIRCUITS-Il

UNIT I- FEED BACK AMPLIFIERS:

Types of feedback — Effect of feed back on noise, distortion, gain, input and output

impedance of amplifiers — Analysis of Current feedback and voltage feedback

amplifiers.

INTRODUCTION

The Voltage gain, Input impedance, Output impedance, Bandwidth etc., are

few important characteristics of an Amplifier.

These parameters are nearly constant for a given amplifier. Sometimes, we are

required to change these parameters. This can be achieved by a technique

known as feedback.

When a part or fraction of output is combined to the input, feedback is said to

exist. Thus the process of combining a fraction of output energy back into the

input is known as feedback.

The feedback technique is broadly classified as positive and negative

feedback.

NEGATIVE FEEDBACK:

If the net effect of feedback is to reduce the magnitude of the input signal, it is

called as negative feedback. [ called as Inverse or Degenerative Feedback.]

Negative feedback reduces the gain of the amplifier.

ADVATAGES OF NEGATIVE FEEDBACK:

Stabilization of Gain

Reduction in Distortion

Reduction in Noise

Increase in Input Impedance

Decrease in Output Impedance

Increases the range of Uniform Amplification.

POSITIVE FEEDBACK:

If the net effect of the feedback is to increase the magnitude of the input signal,

it is called as Positive.

Direct or Regenerative Feedback. This positive f/b has the disadvantage of

increased distortion and instability.

So, positive feedback is seldom used in amplifiers.

1.1 TYPES OF FEEDBACK:

The f/b amplifiers are classified into general classes:

VOLTAGE FEEDBACK

CURRENT FEEDBACK

High Voltage Feedback, the voltage fed back to the input terminal is

proportional to the output voltage.

In Current Feedback, the voltage fed back to the input terminal is proportional

to the current through the load.

GENERAL THEORY OF FEEDBACK:

The feedback amplifier has two parts i.e.,

Amplifier

Feedback circuit

The feedback circuit usually consists of resistors. This returns a fraction (say j3 ) of

the output voltage back to the input.

Let A be the gain of the amplifier i.e. the ratio of output voltage V to the input

voltage V This is the gain of the amplifier without feedback.

The feedback network extracts a voltage Vç jW from the output V of the

amplifier.

This voltage is added(positive fib) or subtracted (negative Q’b) from the signal

voltage V . Now,

FOR POSITIVE FEEDBACK:

If the feedback signal 4 is in phase with input signal 4 then the net effect of the

feedback will increase the input signal given to the amplifier i.e. = hence, the

input voltage applied to the basic amplifier is increased thereby increasing 4

exponentially.

This type of feedback is said to be positive or regenerative feedback.

Gain of the amplifier with positive feedback is:

EFFECTS OF FEEDBACK:

1) Stabilization of Gain:

The gain of the amplifier may change due to the changes in the parameters of the

transistor or the supply voltage variation.

Negative feedback stabilizes the gain of the amplifier against these factors.

2) Reduction in distortion: The negative feedback reduces the non-linear distortion in

the output signal.

3) Reduction in noise: It reduces the level of noise generated within the amplifier.

4) Change in input impedance: It increases the input impedance of the amplifier.

5) Change in output impedance: It decreases the output impedance of the amplifier.

6) Increase in Bandwidth: The negative feedback decreases the lower cut off

frequency f while increases the upper cut off frequency f i.e. it increases the

bandwidth of the amplifier.

These factors are discussed one by one:

Equation (2) shows that the gain A of the feedback amplifier is independent of

internal gain A and depends only on feedback fraction /3. [ in turn depends on

the passive elements such as resistors.

The values of resistors remain fairly constant because they can be selected

very precisely with almost zero temperature co efficient of resistance.

When there is a certain change in the internal resistance of the amplifier due to

some reasons, we now can find the corresponding percentage change in the

overall gain of the feedback amplifier.

Differentiating equation (1) with respect to A , we get

Therefore the sensitivity is defined as the ration of percentage change in voltage gain

with feedback to the percentage change in voltage gain without feedback.

The reciprocal of the term sensitivity is called desensitivity, i.e. desensitivity is

(l+Af3)

1.3 REDUCTION IN NON-LINEAR DISTORTION:

A large signal stage has non-linear distortion.

The reason is that the voltage gain changes at various points in the cycle. The

use of negative feedback in large signal amplifiers reduces the non-linear

distortion.

Let the amplifier with gain A produces a distortion D with feedback. Suppose

when feedback is applied, the gain becomes Af and the distortion becomes D

Now,

Fraction of output distortion feedback to the input = j3D

After amplification, the distortion output = A3D

This result is of great importance in the design of high-power audio amplifiers.

Feedback must be employed in such equipment to reduce the harmonic

distortion to the low levels required.

The required distortion is usually less than 1 percent of the amplifier power

output.

1.4 REDUCTION u NOISE:

Noise is always present in voltage as there is a noise voltage in the amplifier.

The output of the amplificr consists of output signal and noise voltage (say N).

After the amplification of negative feedback, the noise voltage Nf is given by,

1.5 CHANGE IN INPUT IMPEDENCE:

Due to the application of negative feedback, the input impedance increases.

High input impedance is desirable in an amplifier because it will not load the

input voltage source.

16 CHANGE IN OUTPUT IMPEDENCE:

The output impedance decreases due to the application of negative feedback.

Output impedance is desirable in amplifier because it is capable of delivering

Here Z is the output impedance of the amplifier without feedback. In order to

find the output impedance of the amplifier with feedback, short circuit the

input source V and connect a voltage source V at the output terminal. The

output side has been replaced by an equivalent voltage source AI3VQ.

Let I be the current from the applied source.

For the output circuit,

1.7 TYPES OF’ NEGATIVE FEEDBACK CONNECTIONS:

There are four different combinations in which negative feedback may be

accomplished, as given below:

• Voltage-series feedback

• Voltage-shunt feedback

• Current-series feedback Current-shunt feedback

The effect of negative feedback on the input and output resistances differ with the

methods of sampling and mixing. The above mentioned four classification are done

on that basis only.

I is observed that:

Negative feedback using output voltage sampling regardless mixing, tends to

decrease the output resistance.

Negative feedback using output current sampling regardless mixing tends to

increase the output resistance.

Negative feedback using series mixing, regardless of sampling, tends to

increase the input resistance.

Negative f using shunt mixing, regardless of the method of sampling, tends to

decrease the input resistance.

1.7.1 VOLTAGE SERIES FEEDBACK:

A block diagram of a voltage series feedback is shown below:

The effect of negative feedback on the input and output resistances differ

with the methods of sampling and mixing.

The above mentioned four classification are done on that basis only.

1.7.1 VOLTAGE SERIES FEEDBACK:

A block diagram of a voltage series feedback is shown below:

I is observed that:

Negative feedback using output voltage sampling regardless mixing, tends to

decrease the output resistance.

Negative feedback using output current sampling regardless mixing tends to

increase the output resistance.

Negative fcedback using series mixing, regardless of sampling, tends to

increase the input resistance.

Negative f using shunt mixing, regardless of the method of sampling, tends to

decrease the input resistance.

Here, the input to the feedback network is in parallel with the output of the

amplifier.

A fraction of the output voltage through the feedback network is applied in

series with the input voltage of the amplifier.

The shunt connection at the output reduces the output resistance R the series

connection at the input increases the input resistance.

In this case, the amplifier is a true voltage amplifier The voltage feedback

factor is given by fi = V / V Output resistances.

From the above diagram, we have,

Example of voltage series feedback The common collector or emitter follower is an example of voltage series

feedback since the voltage developed in the output is in series with the input

voltage as far as the base emitter junction is concerned.

1.7.2 VOLTAGE SHUNT FEEDBACK:

It is called shunt-derived, shunt-fed feedback connection. Here, a fraction of

the output voltage is supplied in parallel with the input voltage through the feedback

network. The feedback signal I is proportional to the output voltage V therefore, the

feedback factor is given by, fi = I / V this type of amplifier is called a trans resistance

amplifier.

A voltage shunt feedback circuit is shown below

Example of voltage shunt feedback:

The collector feedback biased common emitter amplifier is an example of

voltage-shunt feedback circuit.

1.7.3 CURRENT-SERIES FEEDBACK:

In current series feedback, a voltage is developed which is proportional to the

output current.

This is called curre feedback even though it is a voltage that subtracts from the

input voltage.

Because of the series connection at the input and

One of the most common methods of applying the current-series feedback is to

place a resistor Re between the emitter lead of a common emitter amplifier and

ground.

As the common emitter amplifier has a high gain, this is most often used with

series negative feedback do that it can afford to lose some gain.

Such a circuit is shown below.

When R is properly bypassed with a large capacitor Ce, the output voltage is V

and the voltage gain without feedback is A. resistor R provides d.c. bias

stabilization, but no a.c. feedbaêk.

When the capacitor Ce is removed, an ac. voltage will be developed across R

due to the emitter current flowing through R and this current is approximately

equal to the output collector current.

This voltage drop across R will serve to decrease the input voltage between b

and emitter, so that the output voltage will decrease to V the gain of the amplifier

with negative feedback is now A Simplified diagram:

Therefore, we find that there is a large decrease in voltage gain due to negative

feedback. output resistance (R)

The expression for the output resistance R looking back into the collector

involves R and all the h-parameters.

For values of Re in the order of R and h an approximate expression for R is

R 1+hfe/hoe = 1/h

This has usually a large value in the range of Mg, so the overall output

resistance R taking load resistance into consideration.

Examples of current-series feedback:

The CE amplifier with Re in the emitter lead and FET CS amplifier stage

with source resistor R are the best example for current series feedback circuit

input.

It is called a series-derived, shunt

feedback

The shunt connection at the input reduce F the input resistance and the series

connection at the output increases the output resistance. This is a true current

amplifier.

The current feedback factor is given by fl L/L

Input and output resistances:

The below circuit diagram shows the: current-shunt feedback circuit used to

calculate input and output resistances I

Since current in tile output circuit due to feedback opposes the current, I due

applied voltage V.

Thus this type of feedback decreases the input resistance and increases

output resistance

As this type of feedback has the least desirable effects, this connection

not be considered at all for practical applications.