electrical engineering ba (b), analog electronics, et064g 7.5 credits et065g 6 credits

1

Muhammad Amir Yousaf

Electrical Engineering BA (B), Analog Electronics,

ET064G 7.5 CreditsET065G 6 Credits

Muhammad Amir Yousaf2

Exam question• An amplifier have the following open-loop gain:

•  Draw a detailed asymptotic bode-diagram for the open-loop gain. Both the amplitude and phase should be clearly visualized. (2p)

• What is the gain-bandwidth product for the amplifier? (1p)• What is the lowest amplification the amplifier can be used (AVmin),

without any stabilization network assuming 45˚ phase margin, (ΦM).(2p)

• Stabilize the amplifier with a dominant pole compensation so that it can be useful as a 20 dB amplifier with ΦM>45˚.

MHz

fj

kHz

fj

kHz

fj

AV

11

1

3001

1

1001

1101 5

0

Muhammad Amir Yousaf3

1010 100100 1K1K 10K10K 100K100K 1M1M 10M10M

2020

4040

6060

8080

100db100db

Muhammad Amir Yousaf4

1010 100100 1K1K 10K10K 100K100K 1M1M 10M10M

-180-180

-135-135

-90-90

-45-45

00

-225-225

-270-270

Muhammad Amir Yousaf5

1010 100100 1K1K 10K10K 100K100K 1M1M 10M10M

2020

4040

6060

8080

100db100db

1M1M

-180-180

-135-135

-90-90

-45-45

00

-225-225

-270-270

A feedback factor of -80db or Acl of 80 db can make this circuit stable.

Muhammad Amir Yousaf6

1010 100100 1K1K 10K10K 100K100K 1M1M 10M10M

2020

4040

6060

8080

100db100db

1M1M

-180-180

-135-135

-90-90

-45-45

00

-225-225

-270-270

A feedback factor of -80db or Acl of 80 db can make this circuit stable.

(AolB)db

(Acl)db

Muhammad Amir Yousaf7

1010 100100 1K1K 10K10K 100K100K 1M1M 10M10M

2020

4040

6060

8080

100db100db

1M1M

-180-180

-135-135

-90-90

-45-45

00

-225-225

-270-270

b) Highest bandwidth 10K

without stabilization

(Acl)db

Muhammad Amir Yousaf8

1010 100100 1K1K 10K10K 100K100K 1M1M 10M10M

2020

4040

6060

8080

100db100db

1M1M

-180-180

-135-135

-90-90

-45-45

00

-225-225

-270-270

A feedback factor of -80db or Acl of 80 db can make this circuit stable.

Muhammad Amir Yousaf9

Lead Compensation

Muhammad Amir Yousaf10

Lead Compensation

Muhammad Amir Yousaf11

1010 100100 1K1K 10K10K 100K100K 1M1M 10M10M

2020

4040

6060

8080

100db100db

1M1M

-180-180

-135-135

-90-90

-45-45

00

-225-225

-270-270

A Zero is placed at 10KHz to eliminate the pole at this frequency

Aol

Phase Response

Gain Response ’Lead

Compensation Circuit’

Muhammad Amir Yousaf12

1010 100100 1K1K 10K10K 100K100K 1M1M 10M10M

2020

4040

6060

8080

100db100db

1M1M

-180-180

-135-135

-90-90

-45-45

00

-225-225

-270-270

A Zero gives +45 degree phase shift at 10KHz

Aol

Phase Response

Phase Response ’Lead

Compensation Circuit’ 00

4545

9090

Muhammad Amir Yousaf13

1010 100100 1K1K 10K10K 100K100K 1M1M 10M10M

2020

4040

6060

8080

100db100db

1M1M

-180-180

-135-135

-90-90

-45-45

00

-225-225

-270-270

AolAdding Aol and

Lead compensation

circuit’s Gain.

Gain Response ’Lead

Compensation Circuit’

Resultant Aol after

compensation

Muhammad Amir Yousaf14

1010 100100 1K1K 10K10K 100K100K 1M1M 10M10M

00

4545

1M1M

-180-180

-135-135

-90-90

-45-45

00

-225-225

-270-270

A Zero gives +45 degree phase shift at 10KHzAmplifier

Phase Response

Phase Response ’Lead

Compensation Circuit’

9090

Resultant phase

response

Muhammad Amir Yousaf15

1010 100100 1K1K 10K10K 100K100K 1M1M 10M10M

2020

4040

6060

8080

100db100db

1M1M

-180-180

-135-135

-90-90

-45-45

00

-225-225

-270-270

AolAol and phase response after compensation

After compensation, we get -135degree

phase shift at 1MHz

Gain at 1MHz is 40dB. This means that the feedback factor must be atleast -40dB to get Loop Gain(AolB) = 0db a@ 1MHzSo amplifier is stable for Acl

>= 40dB

Muhammad Amir Yousaf16

But our requirement was to make it stable for 20dB amplification.

Still not achieved……

Muhammad Amir Yousaf17

Lead Compensation

Here is the solution.

This factor can be

adjusted for lower gain in

Lead compensation

ciruits

Muhammad Amir Yousaf18

1010 100100 1K1K 10K10K 100K100K 1M1M 10M10M1M1M

-180-180

-135-135

-90-90

-45-45

00

-225-225

-270-270

Aol

Amplifier should be stabilized with 20dB

gain.

Compensation circuit Gain Response is adjusted by

selecting proper values of R1,R2-20-20

2020

4040

6060

8080

100db100db

00

Muhammad Amir Yousaf19

1010 100100 1K1K 10K10K 100K100K 1M1M 10M10M1M1M

-180-180

-135-135

-90-90

-45-45

00

-225-225

-270-270

Aol

Resultant Gian Response

Compensation circuit Gain Response is adjusted by

selecting proper values of R1,R2-20-20

2020

4040

6060

8080

100db100db

00

Muhammad Amir Yousaf20

1010 100100 1K1K 10K10K 100K100K 1M1M 10M10M1M1M

-180-180

-135-135

-90-90

-45-45

00

-225-225

-270-270

Aol

Resultant Gian Response

Now we have 20 dB at 1MHz @ -135 degree phaseshift

-20-20

2020

4040

6060

8080

100db100db

00Feedback factor should be -20

and Acl = 20dB….Achieved