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# need help about operational amplifier

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#### hanson_uestc

##### Newbie level 4
I analyse a operation amplifier now,there are two loops in this operational amplifier, how could i analyse the AC characteristic about this operational amplifer with two feedback loops ? could someone give me some advice? thanks alot!
the operation amplifier see the picture I pasted below.

#### sixth

##### Member level 4
hanson_uestc said:
I analyse a operation amplifier now,there are two loops in this operational amplifier, how could i analyse the AC characteristic about this operational amplifer with two feedback loops ? could someone give me some advice? thanks alot!
the operation amplifier see the picture I pasted below.

You can treat this circuit as a simple two stage opamp which is conneted as a unit gain close loop form.
First step, you should ensure the stability of the second opamp, that is , making the phase margin larger that 60 degree.
Second step, the close loop form of the second opamp act as the second stage of the whole circuit. The gain of the sencond stage is unit and its bandwidth is the GBW of the second opamp. To ensure the stability of the whole circuit, the bandwidth of second stage should be larger than 10 times of the first stage.

sixth

### hanson_uestc

Points: 2

#### hanson_uestc

##### Newbie level 4
I want to know how to analyse this two loops,should I analyse two loops individual or analyse the unity amp first and then analyse the two loops together? and where I shoud break?
thanks alot!

#### sixth

##### Member level 4
hanson_uestc said:
I want to know how to analyse this two loops,should I analyse two loops individual or analyse the unity amp first and then analyse the two loops together? and where I shoud break?
thanks alot!

Step1 is used to analyze the stability of the second opamp
Step2 is used to analyze the stability of the whole circuit
The big capactors and resistors is used to stable the DC operating point

sixth

### hanson_uestc

Points: 2

#### hanson_uestc

##### Newbie level 4
thank you very much! you give me a big help!

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