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What is the function of this opamp circuit with two bits in PIC programmer?

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matrixofdynamism

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What is the function of this opamp circuit with two bjts in PIC programmer?

The Microchip document DS31028A-page 28-11 gives schematic for a "Example Driver Board Schematic" for PIC programmer. My question is related to the function of the second op-amp. It is clear that the first op-amp is being used as a current buffer in unity feedback.

The second opamp has these two bjt transistors at the output and is receiving feedback in way I have not seen before. Besides this, initially I thought that the complementary bjt pair must be forming a push-pull pair but I think that is not true as they are not connected together like a push pull pair. If this was a push pull pair its purpose would have been to provide more current gain. The way it is, I am not sure what its purpose is.

circuit.jpg
 

Re: What is the function of this opamp circuit with two bjts in PIC programmer?

The purpose of the second op-amp is to make a closed loop to keep Vpp constant. What any op-amp do, by definition, is to make all effort possible to keep both inputs with equal value.

If Vpp rises, the inveting input will be higher than the non-inverting input (Vpp reference). In this case, the op-amp output will decrease, making the PNP transistor turn on and decrease Vpp value.

If Vpp goes low, the non-inverting input goes higher than the Vpp reference, so the op-amp output will increase, making the NPN turn-on injecting +15V to Vpp bus.
 

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