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A question about an Op Amp

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isaacnewton

isaacnewton

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The attached is the circuit schematic of an op amp. Can somebody explain the Functions of Q7, Q11 and R1? Why do we need them?

Thank you in advance.
 

I would say that R1 and Q11 provide current-limit protection for Q13, while Q7 is the overemperature protection. As you can see, if either Q11 or Q7 turn on, they tend to turn off Q13.
 

VVV said:
I would say that R1 and Q11 provide current-limit protection for Q13, while Q7 is the overemperature protection. As you can see, if either Q11 or Q7 turn on, they tend to turn off Q13.
Click to expand...

Just express my personal idea.

I do agree with VVV that the R1 and Q11 is for over current protection.
For Q7, I guess it is used to provide a push-pull ability to the output stage. If Q7 is omitted, the output stage can only dynamically sink current, but not sourcing current.

Scottie
 

I think Q7 is used to improve output rail-to-rail capability. When outputing High, Q7 is cut-off, Q12 can provide max current to load. When outputing LOW, Q7 sinks much current coming from Q6, eventually minimizes Q12 current which generates a voltage drop on R1.
 

then what is about Q8's function?
 

Q8 & Q10 are current mirror
 

it is true that Q10 mirror Q8, but Q8 mirror what? why not only use Q7's common source output drive Q10?
 

Iq8 = Iq6 - Iq7

Iq8 is not fixed, it vary by signal.
 

so what is Q8's function, enlarge Q7's output valtage?
 

mists said:
it is true that Q10 mirror Q8, but Q8 mirror what? why not only use Q7's common source output drive Q10?
Click to expand...

In case you use Q7 as a common souorce amplifier to drive Q10, the whole amplifier will become a 3-stages rather than 2-stages. C1 and R5 alone cannot stabilize a 3-stage amplifiers.

Q8 is there to make sure thwo whole amplifier is still 2-stages, and provides push-pull output driving capibility.

Scottie
 

it is a good question that i think.

Added after 51 seconds:

please notice the bipolar transistor
 

zenisle said:
it is a good question that i think.
please notice the bipolar transistor
Click to expand...

Where is the Bipolar Transistor? You mean the Diode D1? Thank you.
 

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