FREQUENCY RESPONSE OF OPAMP
Goal:
Construct a two stage opamp and find its,
a)
b)
c)
d)
Frequency response,
Open-loop Bandwidth,
3-dB point and
Unity gain frequency.
Set up:
TWO STAGE OPAMP
+10 V
R1
10k
R2
A
+
T1
10k
-
T2
o/p
R3
0.01F
10k
-10 V
Bias Details:
1) First ground (+) and (–) terminals. Now adjust the R1 so that the voltage at A
is centered.
2) Similarly, adjust R2 and R3 to center the output.
Procedure:
1) Now input a sine wave [1Vpp - 5 Hz – zero DC offset] between (+) and (-).
2) Observe the output using an oscilloscope.
3) Now sweep the frequency from 5 Hz to 5 MHz.
4) Draw the frequency response and phase margin graphs in logarithmic sheets.
Frequency response plot
Gain in dB
Hz
Phase response plot
Phase in -
Hz
Analysis:
a) Find the 3-db point and the unity gain frequency.
b) What is the phase lag at the point of unity gain?
Now lets analyze Miller Effect for this opamp.
So add a Capacitor of 8 pF across the gate and the drain of T1.
Repeat the same procedure 1 through 4.
Draw the frequency response and phase response graphs.
a) Find the 3-db point and the unity gain frequency.
b) What is the phase lag at the point of unity gain?
RESULTS:
Without miller effect,
1) The 3-dB frequency is ___
2) The Unity gain frequency is ___
3) The phase lag at unity gain point is___
With miller effect,
1) The 3-dB frequency is ___
2) The Unity gain frequency is ___
3) The phase lag at unity gain point is___