Download - BTEC HNC - Electronics - Investigate Circuits With Feedback

Investigate Circuits with FeedbackElectronics

By Brendan Burr

Brendan Burr BTEC Higher National Certificate in ElectronicsInvestigate Circuits with Feedback

Table of Contents

TABLE OF CONTENTS - 2 -

TASK 1 - 5 -

1.1. Derive from first principles an expression for the closed loop voltage gain of a F/B amplifier. - 5 -

Solution:- - 5 -

TASK 2 - 6 -

2.1 Describe the following types of negative feedback : - 6 -

Voltage - 6 -Solution:- - 6 -

Current - 6 -Solution:- - 6 -

Shunt - 7 -Solution:- - 7 -

Series - 7 -Solution:- - 7 -

2.2 Summarize the effects of the above types on circuit performance in the form of a table. - 8 -

Solution:- - 8 -

TASK 3 - 9 -

3.1. Design only 1 of the following types of amplifier circuits, that uses negative feedback:- - 9 -

(a) Operational Amplifier OR (b) Emitter Follower - 9 -Solution:- - 9 -

3.2 Produce a circuit diagram of the resultant design showing all component values. - 9 -

Solution:- - 9 -

3.3 Use a 1 kHz input sine wave signal of amplitude 5 V peak to peak. to test your design using Croc Clips or MultiSim. - 9 -

Solution:- - 9 -

3.4 Record the results of the simulation in the form of input and output waveforms. - 9 -

Solution:- - 9 -2


TASK 4 - 10 -

4.1 Use Croc Clips or MultiSim to simulate a single stage circuit. Investigate the effect of applying feedback in terms of performance of the Voltage Gain of a Non Inverting Operational Amplifier circuit in the following 2 configurations:- - 10 -

(a) Open Loop - 10 -Solution:- - 10 -

(b) Closed Loop - 10 -Solution:- - 10 -

4.2 Record your results in the form of :- - 10 -

(a) Circuit diagrams - 10 -Solution:- - 10 -

(b) Voltage waveforms - 11 -Solution:- - 11 -

(c) Table of input v output voltage levels - 13 -Solution:- - 13 -

(d) Table of Voltage gains - 13 -Solution:- - 13 -

4.3 Use Croc Clips or MultiSim to simulate a multi stage circuit. - 14 -

Investigate the effects of applying feedback in terms of performance of the Input Resistance of a Unit Gain Voltage Follower (first stage) connected to an Inverting Operational Amplifier circuit (second stage) in the following 2 configurations : - 14 -

(a) Open Loop - 14 -Solution:- - 14 -

(b) Closed Loop - 14 -Solution:- - 14 -

4.4 Record your results in the form of :- - 14 -

(a) Circuit diagrams - 14 -Solution:- - 14 -

(b) Voltage waveforms - 15 -Solution:- - 15 -

(c) Table of input voltage v current - 17 -Solution:- - 17 -

(d) Table of input resistance values - 17 -Solution:- - 17 -

3


EVALUATION - 18 -

CONCLUSION - 18 -

APPENDIX 1 - 19 -

APPENDIX 2 - 20 -

APPENDIX 3 - 21 -

APPENDIX 4 - 22 -

APPENDIX 5 - 23 -

BIBLIOGRAPHY - 24 -

Books - 24 -

Catalogues - 24 -

Websites - 24 -

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Task 1

1.1. Derive from first principles an expression for the closed loop voltage gain of a F/B amplifier.

(You must Include a block diagram as well as showing all calculations)

Solution:-

Derivation of Expression

Starting Point

The total input voltage at the terminals of the amplifier is given by:

But:

OR

5


Task 2

2.1 Describe the following types of negative feedback :

Voltage

Solution:-

See Appendix 1 for the Topology

Voltage feedback is where the output voltage is fed back into the input of an amplifier. This can act as a source to increase stability, bandwidth, input impedance and reduce distortion of the voltage waveform.

As negative feedback, the change in voltage input is attenuated to ensure any large gains do not affect the output by a dramatic amount.

The circuit design in Appendix 1 shows that the input of the amplifier samples the output voltage.

The circuit in Appendix 1 is also known as the series-shunt circuit because the connections are in series to the input and then the feedback is shunted (in parallel) to the output.

Current

Solution:-


Current feedback is supplied back to the input of the amplifier from the output. This can act as a source to increase stability, bandwidth, input impedance and reduce distortion of the current waveform.

As negative feedback, the change in current input is attenuated to ensure any large gains do not affect the output by a dramatic amount.

The circuit design in Appendix 2 shows that the input of the amplifier samples the output current.

The circuit in Appendix 2 is also known as the shunt-series circuit because the feedback connections are shunted (in parallel) to the input and then the feedback is in series to the output.

6


Shunt

Solution:-


Shunt feedback is where the input to the amplifier is current and the output is voltage.

This type of amplifier is often used as a current – to – voltage converter.

The circuit in Appendix 3 is also known as the shunt-shunt circuit because the feedback connections are shunted (in parallel) to the input and then the feedback is also shunted (in parallel) with the output.

Series

Solution:-


Series feedback is where the input to the amplifier is voltage and the output of the amplifier is current.

This type of amplifier is often used as a voltage – to – current converter.

The circuit in Appendix 4 is also known as the series-series circuit because both the input and output connections are in series with one another.

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2.2 Summarize the effects of the above types on circuit performance in the form of a table.

Solution:-

Current Feedback

Type Current Gain (AI) Input Impedance (ZIN)

Output Impedance (ZOUT)

Shunt VoltageShunt Current

DecreasedDecreased

DecreasedDecreased

DecreasedIncreased

Voltage Feedback

Type Voltage Gain (AV) Input Impedance (ZIN)

Output Impedance (ZOUT)

Series VoltageSeries Current

ReducedReduced

IncreasedIncreased

DecreasedIncreased

8


Task 3

3.1. Design only 1 of the following types of amplifier circuits, that uses negative feedback:-

(a) Operational Amplifier OR (b) Emitter Follower

Solution:-

Chosen Circuit:- Emitter Follower

3.2 Produce a circuit diagram of the resultant design showing all component values.

Solution:-

See Appendix 5

3.3 Use a 1 kHz input sine wave signal of amplitude 5 V peak to peak. to test your design using Croc Clips or MultiSim.

Solution:-

See Appendix 5

3.4 Record the results of the simulation in the form of input and output waveforms.

Solution:-

See Appendix 5

9


Task 4

4.1 Use Croc Clips or MultiSim to simulate a single stage circuit. Investigate the effect of applying feedback in terms of performance of the Voltage Gain of a Non Inverting Operational Amplifier circuit in the following 2 configurations:-

(a) Open Loop

Solution:-

See Question 4.2 (a)

(b) Closed Loop

Solution:-


4.2 Record your results in the form of :-

(a) Circuit diagrams

Solution:-

10


(b) Voltage waveforms

Solution:-

11


12


(c) Table of input v output voltage levels

Solution:-

Open Loop Closed LoopVoltage In Voltage Out Av Voltage In Voltage Out Av

1 uV 20 mV 20,000 dB’s 1 uV 2u V 2 dB’s10 uV 200 mV 20,000 dB’s 10 uV 20 uV 2 dB’s

100 uV 2 V 20,000 dB’s 100 uV 200 uV 2 dB’s1 mV 20 V 20,000 dB’s 1 mV 2 mV 2 dB’s

10 mV 200 V 20,000 dB’s 10 mV 20 mV 2 dB’s100 mV 2,000 V 20,000 dB’s 100 mV 200 mV 2 dB’s

1 V 20,000 V 20,000 dB’s 1 V 2 V 2 dB’s10 V 200,000 V 20,000 dB’s 10 V 20 V 2 dB’s

(d) Table of Voltage gains

Solution:-

See 4.2 (c) Table

13


4.3 Use Croc Clips or MultiSim to simulate a multi stage circuit.

Investigate the effects of applying feedback in terms of performance of the Input Resistance of a Unit Gain Voltage Follower (first stage) connected to an Inverting Operational Amplifier circuit (second stage) in the following 2 configurations :

(a) Open Loop

Solution:-


(b) Closed Loop

Solution:-


4.4 Record your results in the form of :-

(a) Circuit diagrams

Solution:-

14


(b) Voltage waveforms

Solution:-

15


16


(c) Table of input voltage v current

Solution:-

Closed LoopStage Voltage

InVoltage

OutOverall

AvCurrent Rin

1 1 uV -10 uV -10 dB’s 0 A Ω2 1 uV -10 uV -10 dB’s 100 pA 10 KΩ1 10 uV -100 uV -10 dB’s 0 A Ω2 10 uV -100 uV -10 dB’s 1 nA 10 KΩ1 100 uV -1 mV -10 dB’s 0 A Ω2 100 uV -1 mV -10 dB’s 10 nA 10 KΩ1 1 mV -10 mV -10 dB’s 0 A Ω2 1 mV -10 mV -10 dB’s 100 nA 10 KΩ1 10 mV -100 mV -10 dB’s 0 A Ω2 10 mV -100 mV -10 dB’s 1 uA 10 KΩ1 100 mV -1 V -10 dB’s 0 A Ω2 100 mV -1 V -10 dB’s 10 uA 10 KΩ1 1 V -10 V -10 dB’s 0 A Ω2 1 V -10 V -10 dB’s 100 uA 10 KΩ1 10 V -100 V -10 dB’s 0 A Ω2 10 V -100 V -10 dB’s 1 mA 10 KΩ

Open LoopStage Voltage

InVoltage

OutOverall Av Current Rin

1 1 nV -400 mV -400,000 dB’s 0 A Ω2 1 nV -400 mV -400,000 dB’s 0.1 pA 10 KΩ1 10 nV -4 V -400,000 dB’s 0 A Ω2 10 nV -4 V -400,000 dB’s 1 pA 10 KΩ1 100 nV -40 V -400,000 dB’s 0 A Ω2 100 nV -40 V -400,000 dB’s 10 pA 10 KΩ1 1 uV -400 V -400,000 dB’s 0 A Ω2 1 uV -400 V -400,000 dB’s 100 pA 10 KΩ1 10 uV -4 KV -400,000 dB’s 0 A Ω2 10 uV -4 KV -400,000 dB’s 1 nA 10 KΩ1 100 uV -40 KV -400,000 dB’s 0 A Ω2 100 uV -40 KV -400,000 dB’s 10 nA 10 KΩ1 1 mV -400 KV -400,000 dB’s 0 A Ω2 1 mV -400 KV -400,000 dB’s 100 nA 10 KΩ1 10 mV -4 MV -400,000 dB’s 0 A Ω2 10 mV -4 MV -400,000 dB’s 1 uA 10 KΩ

(d) Table of input resistance values

Solution:-

See 4.4 (c) Table

17


EvaluationFor Task 1, I had to derive an expression for a closed loop voltage gain in a feedback amplifier. I managed to use my notes to comprehensively answer this question.I struggled to find suitable examples for Task 2 from internet based sources. I did eventually find examples which corresponded to the handouts we had been given in class which made the task much clearer.I found that Task 3 took a while to get correct. I wasn’t sure of what the input voltage was supposed to be as I had copied 3 different values down in my notes. After clarifying this I could then move on to analyse the output voltages. Another reason why it took so long to get correct was because I connected the input signal voltage to Channel A of the Oscilloscope and then connected the output signal voltage to Channel B of the same Oscilloscope, following the handout. This caused the two waveforms to merge and produce a single waveform which didn’t provide any relevant information. I accidentally, but fortunately found the Grapher application in the MultiSim 10 Software. This provides a slightly more detailed Graph which can be used to show multiple traces from various Oscilloscopes if necessary. By using this application I could view the input and output voltage signals simultaneously and compare whether there was a difference in amplitude or if there was any phase shift in the output waveform. I noticed there was a slight lagging in the output waveform, but after one complete cycle the output was back in phase with the input. I think this is due to the transistor, where it doesn’t draw current until 0.7V has been reached.I had to redo most of Task 4 due to my assignment type-up becoming corrupt. I successfully managed to represent the Open and Closed Loop circuits in both single stage and multi stage configurations. The outputs were representative to the amount of gain calculated which indicated that the results I was receiving were correct. The only thing I had to change was the +/- supply voltage to the Op Amps. This was because the waveforms would top out at the supply voltage, so if the output was +/-10V and the supply was +/-9V then the output voltage would only reach +/-9V and the sinusoidal waveform would have flat peaks.

ConclusionTo conclude this assignment, I felt that it went well, apart from having to redo the majority of Task 4. Whilst working in parallel to other assignments I feel I have managed the work load appropriately to get this assignment in on time and to a high standard.I have checked my workings to ensure they meet the relative gains that are expected in the formulae. I enjoyed understanding the theory of the Emitter Follower as I soon realised the results that I was getting were actually correct, as the Gain was supposed to be approximately 1 as this circuit is often used as a Unity Gain Buffer.Overall I will continue to put in the effort required to receive a decent grade in this unit.

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Appendix 1

19


Appendix 2

20


Appendix 3

21


Appendix 4

22


Appendix 5

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Bibliography

Through guidance from my lecturer, the following text books, catalogues and websites I was able to complete this assignment:

Books

BTEC National Engineering (Mike Tooley & Lloyd Dingle) ISBN: 978-0-7506-8521-4Success in Electronics (Tom Duncan & John Murray)ISBN: 0-7195-4015-1Higher Engineering Mathematics (John Bird) ISBN: 0-7506-8152-7

Catalogues

N/A

Websites

http://en.wikipedia.org/wiki/Negative_feedback_amplifierhttp://en.wikipedia.org/wiki/Electronic_amplifier#Input_and_output_variableshttp://en.wikipedia.org/wiki/Two-port_network#Hybrid_parameters_.28h-parameters.29http://www.electronics.dit.ie/staff/ypanarin/Lecture%20Notes/K235-1/3%20Feedback.pdf

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http://www.electronics.dit.ie/staff/ypanarin/Lecture%20Notes/K235-1/3%20Feedback.pdf

http://www.electronics.dit.ie/staff/ypanarin/Lecture%20Notes/K235-1/3%20Feedback.pdf

http://en.wikipedia.org/wiki/Two-port_network#Hybrid_parameters_.28h-parameters.29

http://en.wikipedia.org/wiki/Two-port_network#Hybrid_parameters_.28h-parameters.29

http://en.wikipedia.org/wiki/Electronic_amplifier#Input_and_output_variables

http://en.wikipedia.org/wiki/Negative_feedback_amplifier

Download - BTEC HNC - Electronics - Investigate Circuits With Feedback

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