COMPENSATION:



COMPENSATION

Goal:

Construct a Two-stage CMOS opamp and learn how to compensate it by,

a) Dominant pole compensation method

b) Millers compensation method

Set Up:

Bias Details:

1) Connect (+) and (-) terminals to the power supply, vary R1 and fix up the point A near the ground.

2) Disconnect (+) and (-) terminals from the power supply.

Procedure:

1) Construct the two-stage opamp as in figure.

2) A capacitor (0.1(F) is connected as a load, because a CMOS opamp is intended mostly to drive the capacitive loads.

3) Connect the gates of T1 and T2 to the positive supply, vary R1 and set the DC operating point. [Approximately near the ground].

4) Without R and C, sweep the frequency and draw the gain-frequency plot and phase margin plot.

5) Note the phase margin at the unity gain frequency.

6) This gives the phase lag of the circuit, which you have built.

7) From the frequency response plot and the phase margin plot, derive the transfer function of the circuit.

ADDING COMPENSATION CAPACITOR

Compensation:

1) Now, add a zero to the transfer function that you derived. [Dominant pole]

2) Lets assume that you intend to reduce the phase margin, say to 15 degrees.

3) Having this as your goal, manipulate the transfer function to derive the required capacitor and the resistor you need to add.

4) Now sweep the frequency and draw the gain-frequency plot and phase margin plot. [After compensation]

5) Note the phase margin at the unity gain frequency.

6) This gives the phase lag of the circuit, which you have built.

Verify whether the observed phase margin is same as that of the observed phase margin. Why?

Now add the same capacitor in the input stage as a Miller Capacitor. Report the 3-dB point and phase response you observe.

RESULTS:

Without Compensation,

1) The 3-dB point is ___.

2) The unity gain frequency is ___.

3) The phase margin is ___.

With Compensation,

1) The 3-dB point is ___.

2) The unity gain frequency is ___.

3) The phase margin is ___.

With Millers Compensation,

1) The 3-dB point is ___.

2) The unity gain frequency is ___.

3) The phase margin is ___.

-----------------------

-10 V

+10 V

+

-

R1

0.1(F

+10 V

A

A

-10 V

-

+

R1

0.1(F

R C

0.1(F

C miller

A

+10 V

-10 V

-

+

R1

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