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EXPT. NO. :5

TITLE : To simulate Voltage series, voltage shunt and current shunt feedback

topologies.

AIM : To simulate Voltage series, voltage shunt and current shunt feedback topologies.

APPARATUS :

1. PC with simulation software loaded.

2. Printer.

THEORY :

In the feedback process a part of output is sampled and fed back to the input. Thus at the input of an amplifier two signals will be simultaneously present. One of them is the original signal and other is the fed back signal itself. Feedback is defined as the process in which a part of output signal (voltage or current) is returned back to the input. The amplifier that operates on the principle of feedback is known as feedback amplifier.

TYPES OF FEEDBACK

1. Positive feedback 2.Negative feedback.

If the original input signal and the feedback signal are in phase, the feedback is called as ‘positive feedback’ .However if these two signals are out of phase then feedback is known as ‘negative feedback’. Positive feedback is used in oscillators and negative feedback is used in amplifiers.

CLASSIFICATION OF AMPLIFIERS BASED ON FEEDBACK TOPOLOGY

The amplifiers are classified into four categories based on the magnitudes of the input and output impedences relative to the source and load impedances respectively. The categories are as follows:

1. Voltage amplifier 2.Current amplifier

3. Tran conductance amplifier 4.Transresistance amplifier

FEEDBACK TOPOLOGIES :

We can combine the sampling and mixing techniques to yield the following feedback configurations:

1. Voltage series feedback 2.Voltage shunt feedback

3. Current shunt feedback

Signal or ratio

Type of feedback

Voltage series

Current series

Current shunt

Voltage shunt

Xo

Voltage

Current

Current

Voltage

Xs, Xf, Xd

Voltage

Voltage

Current

Current

A

Av

Gm

Ai

Rm

Î’

Vf/Vo

Vf/Io

If/Io

If/Vo

CURRENT SERIES FEEDBACK :

The block diagram of a voltage series feedback amplifier is shown in fig.1 .Here we use the combination of ‘Current sampling’ and ‘series mixing’. Therefore

Current series feedback= Current sampling +series mixing.

VOLTAGE SHUNT FEEDBACK:

The block diagram of a voltage shunt feedback amplifier is shown in fig.2 Here we use the combination of ‘voltage sampling’ and ‘shunt mixing’ Therefore,

Voltage shunt feedback=voltage sampling +shunt mixing. Voltage shunt feedback is present in transsresisitance amplifiers.

CURRENT SHUNT FEEDBACK :

The block diagram of a current shunt feedback amplifier is shown in fig.3. Here we use the combination of ‘current sampling’ and ‘shunt mixing’. Therefore,

Voltage shunt feedback=current sampling +shunt mixing. Current shunt feedback is present in current amplifiers.

Advantages of negative feedback:

1. Negative feedback stabilizes the gain of the amplifier.

2. There is significant increase in bandwidth of the amplifier.

3. Distortions in the amplifier output are reduced.

4. Input resistance increases for certain feedback configurations.

5. Output resistance decreases for certain feedback configurations.

6. Operating point is stabilized.

Disadvantages of negative feedback:

1. Reduction in gain.

2. Reduction in input resisistance in case of voltage shunt and current shunt type amplifiers.

3. Reduction in output resisistance in case of current shunt and current series type amplifiers.

4. More no. of amplifier stages are required to be used,in order to maintain the required gain.

Application of negative feedback:

1. Almost all the electronic amplifiers.

2. Regulated power supplies.

3. Wideband amplifiers (amplifiers having a large bandwidth).

4. Circuits like bootstrapping.

Effect of negative feedback on Ri and Ro.

Type of feedback

Effect on input resistance

Effect on output resisitance

Voltage series

Increases

Decreases

Voltage shunt

Decreases

Decreases

Current shunt

Decreases

Increases

CALCULATIONS:

RESULT:

1.VOLTAGE SERIES:

Sr.No

Parameter

With feedback

Without feedback

2.VOLTAGE SHUNT:

Sr.No .

Parameter

With feedback

Without feedback

3.CURRENT SHUNT:

Sr.No .

Parameter

With feedback

Without feedback

COMMENTS AND CONCLUSION :------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

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