To Analyse The Effect Of Shunt-Series Feedback And Shunt-Shunt Feedback On A Single Ended Op-Amp

LAB 7 FEEDBACK ANALYSIS

Construct a single-stage single-ended opamp and analyze the effect of

a)Shunt-Shunt Feedback on frequency response

b)Series-Shunt Feedback on frequency response

Set up:

Remember that R1 V in Vout

Is an INVERTING AMPLIFIER and uses Shunt-Shunt Feedback with gain = (-R2/R1)

Vin Vout

Is non – inverting amplifier and uses Series - Shunt Feedback with gain = [1 + (R2/R1)].

Now for both the cases, we will employ a lgainl of 10. i.e we will use R1 = 1k and R2 = 10k. For ideal cases we should get a gain of approximately 10 in both the cases. Sometimes (not always!) we wont be obtaining a gain of 10 since the opamp we construct is a very simple one and does not have a high enough open loop gain to render the closed loop gain dependant only on feedback. We will be getting a gain less than 10. The simple differential stage: +10V

x y (-) (+) A B

-10V -10V

Biasing:

1) Connect the A and B terminals to the ground.

2) Now adjust the Rb (10 k) pot so that X and Y are approximately near 0 volts. Make sure X is closer to 0V rather than Y. Because X is where we are going to tap the output for this stage.

3) Now disconnect A and B from the ground.

Now you have constructed a simple differential amplifier and it is in open loop mode. Now lets assume that we tap the output at X w.r.t. Ground.

Now we move on to the Shunt – Shunt Feedback

+10V 1k 1k R2=10k Input (-) x y (+)

A R1=1k B

10k R2

Rb Vin V -10V -10V out

Procedure:

1. After biasing the output point, we need to connect the feedback resistors R1 and R2 as in the figure. 2. Now at the input, give a sine wave of 100Hz - 100mVpp – zero V DC offset. You will be getting the waveforms as in the following figure.

Input 100 mVpp

Output 0.7 Vpp

Note: At any stage of the experiment, if you see the output waveform appearing clipped either on positive or negative side, you can always adjust the bias resistor Rb to get the unclipped waveform. Now sweep the input frequency from 100Hz to 10 MHz and draw the

frequency response plot and the phase response plot and find the F3dB, Bandwidth,

Funity, Phase lag at Funity and the Phase margin. [Same as what you did last week]

Next we move to the Series-Shunt Feedback circuit. +10V

output

1k 1k R2=10k

(-) x y (+) Input A R1=1k B

Rb Vin V -10V out -10V R2

Procedure:

1. After biasing the output point, we need to connect the feedback resistors R1 and R2 as in the figure. 2. Now at the input B, give a sine wave of 100Hz - 100mVpp – zero V DC offset. You will be getting the waveforms as in the following figure.

Input 100 mVpp

Output 0.61 Vpp Note: At any stage of the experiment, if you see the output waveform appearing clipped either on positive or negative side, you can always adjust the bias resistor Rb to get the unclipped waveform.

Now sweep the input frequency from 100Hz to 10 MHz and draw the frequency response plot and the phase response plot and find the F3dB, Bandwidth,

Funity, Phase lag at Funity and the Phase margin. [Same as what you did last week]

RESULTS:

Turn in the Frequency response plot and the Phase response plots for both the feedback circuits and fill in the following.

With Shunt-Series Feedback,

1) The 3-dB point is ___. 2) The unity gain frequency is ___. 3) The phase lag at unity gain frequency is ___. 4) The phase margin is_____

With Shunt-Shunt Feedback,

1) The 3-dB point is ___. 2) The unity gain frequency is ___. 3) The phase lag at unity gain frequency is ___. 4) The phase margin is_____ 5) Compare these answers to those you got in Lab 5 for the open loop diff pair.