Amplifiers With Feedback

Amplifiers with Feedback

Electronic Engineering

© University of Wales Newport 2009 This work is licensed under a Creative Commons Attribution 2.0 License.

Amplifier Compensation

The following presentation is a part of the level 5 module -- Electronic Engineering. This resources is a part of the 2009/2010 Engineering (foundation degree, BEng and HN) courses from University of Wales Newport (course codes H101, H691, H620, HH37 and 001H). This resource is a part of the core modules for the full time 1 st year undergraduate programme.

The BEng & Foundation Degrees and HNC/D in Engineering are designed to meet the needs of employers by placing the emphasis on the theoretical, practical and vocational aspects of engineering within the workplace and beyond. Engineering is becoming more high profile, and therefore more in demand as a skill set, in today’s high-tech world. This course has been designed to provide you with knowledge, skills and practical experience encountered in everyday engineering environments.

Contents Generalised Feedback Configuration Voltage Series Feedback Current Series Feedback Current Shunt Feedback Voltage Shunt Feedback Summary Credits

In addition to the resource below, there are supporting documents which should be used in combination with this resource. Please see:Clayton G, 2000, Operational Amplifiers 4th Ed, Newnes James M, 2004, Higher Electronics, Newnes

Generalised feedback configuration

VinVoutVa

ß

A

Amplifier Forward Gain AFeedback Network ßLoop Gain A x ßClose Loop Gain

AA

1


AdvantagesGain SensitivityBandwidth

DisadvantagesGain ReductionPossible Instability

Today we will look at the way that the feedback is applied. There are four possible configurations (topologies):

Voltage-Series Feedback (voltage amplifier)

Current-Series Feedback (transconductance amplifier)

Current Shunt Feedback (current amplifier)

Voltage –Shunt Feedback (transresistance amplifier)

There are two parts

First part - voltage or current - this tells us how the output is sampled and hence the output of the amplifier. Voltage is sampled in parallel and Current is sampled in series.

Second part – series or shunt – this tells us how the feedback is applied to the input. Series is feeding back voltage to be subtracted and Shunt is feeding back current which is subtracted.


Voltage Series Feedback

+Va-

+Vf-

+Vin-

+Vout-

Voltage Amplifier

Rin

Rout

AVa

Feedbackß


Finally

Series connection on the input, increases input resistance.

Iin

VinRIN

VaARinIinVoutRinIinVfRinIinVin

ARinIinRinIinARinIinVin 1

)( ARinIin

VinRIN 1

Input Resistance

Resistance looking into the input is RIN, the input current is Iin.


Output Resistance

Resistance looking in on the output is ROUT. Iout is the current flowing in on the output. No input applied.

Rout

VaAVoutIout

VoutVa

Rout

VoutAVoutIout

A

Rout

Iout

VoutROUT 1

Voltage (parallel) sampling on the output decreases the output resistance.


Current Series Feedback

+Va-

+Vf-

+Vin-

Transconductance Amplifier

Rin

RoutAVa

Feedbackß

Iout


Input Resistance

As the feedback is applied in the same way as the previous topology the effect is the same i.e. resistance increases.


Output Resistance

Resistance looking in on the output is ROUT. Vout is the voltage across the output. No input applied.

Rout

VoutVaAIout IoutVa

Rout

VoutIoutAIout AIout

Rout

Vout1

ARoutIout

VoutROUT 1

Current (series) sampling on the output increases the output resistance.


Current Shunt Feedback

Ia

If

Iin

CurrentAmplifier

Rin

RoutAIa

Feedbackß

Iout


Input Resistance

Resistance looking into the input is RIN, the input voltage is Vin.

Iin

VinR IN

Rin

VinA

Rin

VinIaA

Rin

VinIout

Rin

VinIf

Rin

VinIin

ARin

VinIin 1

Finally

Shunt (parallel) connection on the input, decreases input resistance.

)(

A

Rin

Iin

VinRIN 1


Output Resistance

As the sample is derived in the same way as the previous topology the effect is the same i.e. resistance increases.


Voltage Shunt Feedback

Ia

If

Iin

TransresistanceAmplifier

Rin Rout

AIa

Feedbackß

+Vout-


Input Resistance

As the feedback is applied in the same way as the previous topology the effect is the same i.e. resistance decreases.

Output Resistance

As the sample is derived in the same way as the first topology the effect is the same i.e. resistance decreases.


Summary

Topology Voltage series

Current series

Current shunt

Voltage shuntParameter

Gain decrease decrease decrease decrease

Bandwidth increase increase increase increase

Input Resistance increase increase decrease decrease

Output Resistance decrease increase increase decrease

In each case when feedback is applied the input and output resistance become more ideal for that particular amplifier.


This resource was created by the University of Wales Newport and released as an open educational resource through the Open Engineering Resources project of the HE Academy Engineering Subject Centre. The Open Engineering Resources project was funded by HEFCE and part of the JISC/HE Academy UKOER programme.

© 2009 University of Wales Newport

This work is licensed under a Creative Commons Attribution 2.0 License.

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