# What is this circuit for?

1. May 13, 2016

### ENE

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Hello,
What is this circuit working for?

2. May 13, 2016

### willem2

I'm pretty sure this circuit has no practical use whatever. There will be no feedback if the output ever gets positive

3. May 13, 2016

### ENE

I have used op amp and comprator like 741 and L339.
I don't understand the circuit Q npn.

4. May 13, 2016

### f95toli

It is a transistor. But I agree with willem2 that the circuit looks very odd and does not serve a practical purpose, I can't see why you would put a transistor in the feedback loop of an op-amp; especially not with the base grounded(!)
Hence, I don't think the circuit will do anything useful but that does not mean that you can't answer the question.

5. May 13, 2016

### Averagesupernova

For you naysayers it is most likely a log amplifier.

6. May 13, 2016

### lychette

If a load is connected between Vout and -15V then the circuit acts as a constant current (5mA) source for resistive loads of up to 3kΩ.
This configuration is the basis of constant current sources using op amps.

7. May 13, 2016

### Staff: Mentor

It looks like it could serve a number of functions.
(a) Over a range of positive input, the output voltage will be a logarithmic function of the input voltage [at fixed temperature].

(b) Or if you maintain the input voltage fixed, the output will be a linear function of temperature of the transistor probe.

(c) Or it could simply be an analog comparator, with output of -0.7V or +15V.

8. May 14, 2016

### ENE

So, what is the answer?

9. May 14, 2016

### ENE

How to calculate it?

10. May 14, 2016

### Staff: Mentor

I didn't realise that you are asking for help with the question in your post, I thought you were asking something else, viz., what might this circuit be used for.

In operation, either the transistor will be functioning as a transistor, or it won't be. If it is not functioning as a transistor then it is "off". For the transistor to be functioning as a transistor, describe the voltages around the circuit you will definitely expect to see.

11. May 14, 2016

### ENE

let the transistor be off so open circuit for feed back.
so, +ve pin to 0V
and -Ve pin to 5V
that mean it should be -15V?
how to do?

12. May 14, 2016

### Staff: Mentor

Now using the result that Vout = -15V does this confirm your premise that Q is "off"?

13. May 14, 2016

### ENE

NO, the question same as above i am guessing that it will be -15V when BJT is open.
no idea when it will work.

14. May 14, 2016

### Staff: Mentor

If Vout = -15V will this turn the transistor "off"? Look at the voltages that this type of transistor requires for it to be on or off.

15. May 14, 2016

### ENE

The BJT is npn it will get more -ve with respect to base for turing on it want 0.7V

16. May 14, 2016

### Staff: Mentor

We need a yes or no answer to the question: Is it possible in this circuit for Vout = -15V to be keeping the transistor off?

If these 2 conditions can coexist then that is how it must be. If these conditions cannot coexist, then you need to make a different initial assumption.

17. May 15, 2016

### lychette

If you look at the input side of the amplifier the 5V DC with 1kΩ resistor will make a current of 1mA flow towards the inverting input. The current cannot flow into the op-amp and will therefore flow by the feedback link towards the output. It will be the collector current of the transistor.
Can you see what the output voltage will be?

18. May 15, 2016

### Staff: Mentor

This is not correct.

19. May 15, 2016

### rude man

Yes, but not a good one.
Vout = VT ln{(Vin/Rin)/is} assuming high beta of transistor. But is = saturation current which varies with temperature & device to device within the same family. VT = kT/q, k = bolzmann constant, T = kelvin temp., q = electronic charge. It's a poor indicator of temperature also since again is = is(T).