Amplifiers with Feedback Electronic Engineering © University of Wales Newport 2009 This work is licensed under a Creative Commons Attribution 2.0 License .
May 21, 2015
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
Amplifiers with Feedback
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.
Amplifiers with Feedback
Voltage Series Feedback
+Va-
+Vf-
+Vin-
+Vout-
Voltage Amplifier
Rin
Rout
AVa
Feedbackß
Amplifiers with Feedback
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.
Amplifiers with Feedback
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.
Amplifiers with Feedback
Current Series Feedback
+Va-
+Vf-
+Vin-
Transconductance Amplifier
Rin
RoutAVa
Feedbackß
Iout
Amplifiers with Feedback
Input Resistance
As the feedback is applied in the same way as the previous topology the effect is the same i.e. resistance increases.
Amplifiers with Feedback
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.
Amplifiers with Feedback
Current Shunt Feedback
Ia
If
Iin
CurrentAmplifier
Rin
RoutAIa
Feedbackß
Iout
Amplifiers with Feedback
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
Amplifiers with Feedback
Output Resistance
As the sample is derived in the same way as the previous topology the effect is the same i.e. resistance increases.
Amplifiers with Feedback
Voltage Shunt Feedback
Ia
If
Iin
TransresistanceAmplifier
Rin Rout
AIa
Feedbackß
+Vout-
Amplifiers with Feedback
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.
Amplifiers with Feedback
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.
Amplifiers with Feedback
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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Amplifiers with Feedback