# Two-Stage Negative Feedback

Discussion in 'Homework Help' started by newbie2019, May 31, 2019.

1. ### The Electrician AAC Fanatic!

Oct 9, 2007
2,682
474
newbie2019, are you still working on this?

Apr 5, 2019
95
2
Yes dude.

3. ### The Electrician AAC Fanatic!

Oct 9, 2007
2,682
474
Where are you in the process? What are you trying to do now?

4. ### newbie2019 Thread Starter Member

Apr 5, 2019
95
2
The last part of my study involves computing the ac voltages and currents for
the configuration with negative feedback. The feedback loop obviously
complicates things and I cannot figure it out.

File size:
3.2 KB
Views:
6
File size:
3.6 KB
Views:
3
File size:
40.1 KB
Views:
4
• ###### AC Analysis - Two-Stage Common-Emitter Amlifier - Negative Feedback.png
File size:
43.4 KB
Views:
3
Last edited: Jun 13, 2019
5. ### newbie2019 Thread Starter Member

Apr 5, 2019
95
2
I think that I have the dc analysis of the two-stage amplifier with and without negative feedback
figured out. I have created a new post with regard to the ac analysis of same.

Last edited by a moderator: Jun 13, 2019

Apr 17, 2019
304
10

7. ### Jony130 AAC Fanatic!

Feb 17, 2009
4,848
1,360

Do you know any of a circuit/network analysis techniques?

Also, what is the point of doing all those analyze and find AOL and ACL. If now you want to solve the circuit with the negative feedback directly? Without involving the "feedback theory" (without AOL ACL and feedback factor)?

We use a feedback theory to get approximation result without the need for direct analysis of a whole circuit.

8. ### newbie2019 Thread Starter Member

Apr 5, 2019
95
2
First I had the circuit with no feedback and I was able to analyze the ac current and voltages.

Now that I have the same circuit but with negative feedback, the ac currents and voltages are different.
If I didn't have LTSpice how would I predict these new currents and voltages?

9. ### The Electrician AAC Fanatic!

Oct 9, 2007
2,682
474
jony130, I sent you a PM.

10. ### The Electrician AAC Fanatic!

Oct 9, 2007
2,682
474
You and newbie2019 are apparently using the same text book. Have you learned standard network analysis techniques, such as nodal, mesh, loop?

11. ### Jony130 AAC Fanatic!

Feb 17, 2009
4,848
1,360
You have to analyze the whole circuit one more time from the beginning. And it will be a hardcore job to do if you want the symbolic solution.

Good luck with that.

Last edited: Jun 13, 2019
12. ### Zeeus Member

Apr 17, 2019
304
10
nope..don't have his textbook

yeah learned little of them but did not learn you and Jony's advanced method or advanced members I agree

but think this your circuit, RFB is positive feedback...Emitter of Q2 is negative with respect to Vbe...RFB is reducing emitter voltage and therefore increasing Vbe

if not then guess need re-read my book

Edit : oh sorry, see you said it is positive feedback

• ###### 2StageBJT_wFB.png
File size:
7.8 KB
Views:
3
Last edited: Jun 13, 2019
13. ### The Electrician AAC Fanatic!

Oct 9, 2007
2,682
474
Are required to use some particular technique to analyze your circuit, such as with concepts like loop gain, factors like Av / (1 + (Av * β)), etc., or can you use any technique you choose?

Apr 17, 2019
304
10
15. ### The Electrician AAC Fanatic!

Oct 9, 2007
2,682
474
Assuming that newbie2019 will be back here eventually, and that he can use any technique, my next question for him will be this: do you have access to any mathematical software that can do symbolic linear algebra such as Matlab, Mathcad, Mathematica, Maple, or Macsyma?

If that answer to that question is yes, then the next thing to do is to watch this video: