# Strange Op Amp Circuit Operation

Thread Starter

#### danielb33

Joined Aug 20, 2012
105
The circuit below is part of a circuit I am analyzing.

The non inverting terminal has 2.5VDC always.
Note that +5VA is 5 Volts Analog - not AC.
I am confirming that my thoughts are correct.
The circuit operates as follows:
1) Left node of R37 has a voltage less than 2.5V --> op amp output is D21 voltage drop above 2.5 V --> PTRAN is OFF. Circuit is isolated from Q5 collector.
2) Left node of R37 is above 2.5V. Here the op amp will try to reduce the non inverting terminal but cant because of D21.
This causes the output to drop to about -5V causing the PTRAN to turn on - shorting the output of the non inverting terminal to the collector of Q5.

Any issues with my logic? Thanks for the support.

#### PeterCoxSmith

Joined Feb 23, 2015
148
it looks like part of a voltage to current converter

#### Dodgydave

Joined Jun 22, 2012
11,304
So what your problem?

Thread Starter

#### danielb33

Joined Aug 20, 2012
105
I don't have a problem....yet. I explained how it should operate if I had 2.5V at the non inverting terminal and varies voltage at the left node of R37. I just wanted to make sure my thoughts are correct.
It is not a voltage to current converter unless I am way off. Please explain.

#### Dodgydave

Joined Jun 22, 2012
11,304
Post the full circuit then we can see what going on.

#### dannyf

Joined Sep 13, 2015
2,197
This causes the output to drop to about -5V causing the PTRAN to turn on - shorting the output of the non inverting terminal to the collector of Q5.
Your analysis is correct except this one: the output of the opamp would be so that the current going through the transistor is the same as the current going through R37, for the 2) scenario. ie. the output current is linear with regards to (V37-2.5)/2.2K.

The diode provide a path for the current for the 1st scenario.

#### PeterCoxSmith

Joined Feb 23, 2015
148
I don't have a problem....yet. I explained how it should operate if I had 2.5V at the non inverting terminal and varies voltage at the left node of R37. I just wanted to make sure my thoughts are correct.
It is not a voltage to current converter unless I am way off. Please explain.
...it looks like this type of circuit Fig 4.1:
https://wiki.analog.com/university/courses/electronics/text/chapter-4
dannyf has explained above...

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