# R-C coupled amplifier (feedback)

Discussion in 'Homework Help' started by adam_246, May 6, 2009.

May 6, 2009
5
0
Hi all,

I am new on here and seeking some help please.
I am struggling to answer the following revision question.

An RC coupled amplifier has a feedback network added to modify its performance. Before the addition of the network, the amplifiers gain was 25<180(degrees).
3% of the output voltage is fed back to the input via the feedback network, which introduces a phase shift of 60(degrees) at the midband frequency of the amplifier. Determine the following:

1) the value of the feedback factor in polar form
2) the loop gain, in polar form
3) the closed loop gain in polar form
4) the gain and phase margins of the feedback amplifier.

May 6, 2009
5
0
This is what i have got so far:

B= 3% = 3/100 = 0.03

LOOP Gain

AV= 25 < 180

BAV= 0.03 x 25 / ( 180 + 60)
= 0.75/<240
BAV IN PF = 0.75 < -120 DEG

1 - BAV = 1 - ( 0.375 - j0.65) (J FORM FROM ABOVE)
= 1.375 = 0.65
Polar = 1.52 < 25 DEG

If anyone could guide me it would be much appriciated

3. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
790
Is that positive or negative feedback? i.e. Is the feedback added to the input signal or subtracted from it?

May 6, 2009
5
0
It is negative feedback.

5. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
790
Hope the attachment makes sense.

• ###### Feedback.pdf
File size:
5.9 KB
Views:
36

May 6, 2009
5
0
I see how you have worked out the loop gain and closed loop gain.

How can i work out the gain and phase margains?
Also the feedback factor?

Thanks

Last edited: May 8, 2009
7. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
790
I would use the term I denoted as 'H' for the feedback factor.

Others may disagree and hopefully you will receive other comments & views, but I don't believe there is sufficient information to work out the gain and phase margins. Primarily because the feedback definition is quite vague - we are just told it has a mid-band gain and phase. Nothing about it's cut-off frequencies and / or roll off behavior. Then we have the amplifier described as an RC coupled type - without any elaboration other than the gain & phase [presumably mid-band]. To derive gain and phase margins one usually graphs the open loop frequency [gain vs phase] response and "reads off" the margins as elaborated by said plots.

I think it's quite possible for the example's resulting closed loop system to be unstable anyway - and under those circumstance the concepts of gain and phase margin are rather hazy, to say the least.