# Feedback amplifier design

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#### vinhpq

##### Newbie level 6
I am doing an experiment about this topic (using bjt and op-amp). Can you give me some experience or any document? Or if you already studied this in school (Electronic Lab course or other similiar courses), could you be so kind send me reports? I'm new to this, plz help me, thanks so much

#### ambreesh

##### Full Member level 5
Dear vinhpq,
I would suggest u read the OPAMP book by Sergio Franco
It would help u a lot

#### byom

##### Newbie level 5
vinhpq said:
I am doing an experiment about this topic (using bjt and op-amp). Can you give me some experience or any document? Or if you already studied this in school (Electronic Lab course or other similiar courses), could you be so kind send me reports? I'm new to this, plz help me, thanks so much
if you are using opamp, this IC itself is a amplifier , the one and only one this is you have to design a feedback path to control the gain . generally we do go for negative feedback to have better control over the ckt. to design a feedback ckt you can use a resistor between the output terminal and the inverting terminal. another registor in between the the inverting terminal and ground. now the input can be given in either inverting ar noninverting terminal.
the gain of the circuit depends on the terminal in which you have given the input.
i do like to suggest to go thorugh some book, of opamp on of the best book i found for beginer is "Op-Amps and linear integrated circuits" - by Ramakant A. Gayakwad.
hope you will enjoy this
if neded mail me
byom

#### wwfieee

##### Member level 3
sergio franco's book is good. Design with Operational Amplifiers and Analog Integrated Circuits, Sergio Franco, McGraw-Hill, 3rd Edition. ISBN: 0-07-232084-2

however for beginning studies, almost any introductory electronic books gives a good intro to op-amps which basically can be modelled by dependent sources.

franco's electric circuit fundamentals is also good.

#### vinhpq

##### Newbie level 6
I have a bad skill in circuit analysis. I uploaded the figure of the fdbk amplifier circuit. I have some questions which need your help:
- This is the open-loop case since the fdbk voltage will run through C1 (so the voltage to the base of Q2 is 0). Which chages (in amplitude-freq characteristic) if we remove Rp,Cp? What is the purpose of Cp,Rp and transistors?
- If I remove C1, it is the close-loop circuit. The gain is just 10 (while in the open-loop case 10000). What is the formula for calculating the gain?
- Which changes in phase margin in the two case above?
If you have time, can you tell me a bit about the principle of this circuit and some rerated formula? I appreciate this!

#### ambreesh

##### Full Member level 5
It is a representation of your circuit.
Correct me If I am wrong.
It is aa closed loop system.
Now you could find out relation between Vout and Vin , or put the circuit in LT spice with various R and C value and observe the results

#### vinhpq

##### Newbie level 6
Hi ambreesh,
Yeah, it's representation of my circuit. The feedback voltage from output to the base of Q2 is zero because of C1. So it's open-loop circuit. The Vout measured is 10000 times than Vin. When C1 is removed, it's closed-loop. The gain is just 10 and the circuit is more stable. Am I right?

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