# Feedback amplifier

1. Oct 2, 2014

### ranju

1. The problem statement, all variables and given/known data
An amplifier without feedback provides an output signal of 15 V with 10% second harmonic distortions when input signal signal is 15 mV. If 1.5% of the output is feedback to the input in a negative series shunt amplifier, what is output voltage?

2. Relevant equations
input signal (Vs) = 15mV , V0=15V , Af(gain with feedback) = V0/Vs

3. The attempt at a solution
I have got Af =1000 & feedback factor beta =0.015

2. Oct 2, 2014

### Staff: Mentor

So, you have some working and an answer showing what is the output voltage with feedback?

3. Oct 2, 2014

Af=1000 is the open loop gain right. If that is so just calculate closed loop gain and multiply with the input .

4. Oct 2, 2014

### ranju

Af is the gain with feedback .. we can find A by Af=A/1+beta*A
but I am not getting how to find the output voltage with feedback..??
we have to find the output voltage with feedback..!!

5. Oct 2, 2014

### Staff: Mentor

Can you explain what this means?

6. Oct 2, 2014

### ranju

15 V is the output voltage without feedback and 10% is the distortion D without feedback..!!

7. Oct 2, 2014

### Staff: Mentor

The gain without feedback is 1000. So if you are using "Af" to denote gain with feedback, then Af is not 1000. We don't know what is the value of Af, you are required to determine Af.

8. Oct 2, 2014

### ranju

Not again.>!! I am really sorry..Its my mistake once again..now I am not getting how to do it..!!!

9. Oct 2, 2014

Now that you know that A=15/15mV , A=1000 and you have beta=0.015 just calculate gain with feedback "Af" . Since input voltage has also been known (15 V), calculate output voltage by the equation you have given.

10. Oct 2, 2014

### ranju

This cannot be solved in this manner..!! A is found when we are knowing the input voltage Vi but that is not given.>!!

11. Oct 2, 2014

Input signal is 15 mV and output you get without feedback is 15 V.

12. Oct 2, 2014

### ranju

This terminology difference is killing me..!! ohkk..so finally its clear.. we are having without feedback: V0=15V and Vi=15mV..& hence A= 1000beta=0.015 , so we can find Af.. Now how we are going to find output voltage with feedback..??

13. Oct 3, 2014

### Staff: Mentor

Output is 1000x the input, the same as before, there is no change there. But now the input to that amplifier, instead of being Vi, is made to be (Vi - βVo). So the amplifier amplifies this by 1000x