# OP-AMP 741 Application

1. Sep 20, 2013

### darkxponent

1. The problem statement, all variables and given/known data

How will you measure a voltage of range 0.0001 volts using the 741 OP-AMP?

2. Relevant equations

3. The attempt at a solution

This a viva question and i couldn't answer. I don't know how to begin. Should i use Sample and Hold circuit or something else?

2. Sep 20, 2013

### Staff: Mentor

There are a number of things to consider What is a viva question? What kind of things are they looking for you to know and illustrate in your answer?

What can you say about the LM741 opamp? What are its input terminal errors (voltage, currents, etc)? And how would those affect amplifying a 1mV signal? What things do you need to know about the signal and signal source to help you answer this question?

Talk to us about how you would approach this question...

3. Sep 20, 2013

### darkxponent

We were two students for the VIVA voice which is a part of practical exam. The converation went this way.

Prof: How will you measure a voltage range of 0.00001V using 741. Think and then answer. And show me the circuit.

Me: I will amplify it using inverting amplifier with Rf = 10^6 Ri. (the other guy gave similar answer)

Prof: Its a very low voltage. If some current passes, the voltage will drop. Current shouldn't pass.

Me: We can take higher value of Rf.

Prof: Even a small value of current will cause the voltage to drop.It is very low Voltage. Current shouldn't pass.

Me: I will use a capacitor instead of Rf. make it an integrator circuit. For DC no current through capacitor. Vo(t) = - (1/RC)*0.00001*t.

Prof: No. Remove one zero. Measure Voltage range of 0.0001V.

Other Guy: Sir we can use sample and hold circuit.

Prof: Show me how you will you use it.

He showed his work. He drew sample and hold circuit with digital sensor at output. The prof didn't say whether it was correct or incorrect.

Prof: Okay i remove one more zero. Measure Voltage range of 0.001V. Tell me

..silence

Prof: You have studied Control Systems.

We: yes.

Prof: Start paying attention to studies.

And the conversation ended.

I couldn't understand what he was trying to ask. I think he was more interested in amplification of Voltage.

4. Sep 20, 2013

### darkxponent

I don't know any other approach except for those mentioned above. I also couldn't understand why he asked about Control Systems.

5. Sep 20, 2013

### Staff: Mentor

I'm with DXP on this. None of that exchange makes sense. Can you escalate it to the professor's supervisor? That exchange would earn your professor an infraction if he/she posted it here on the PF.

6. Sep 20, 2013

### darkxponent

He was an external professor otherwise i would have asked him after the exam. And he asked this question to every group(at least 40 students). I don't know what were the exchanges with other guys. Maybe we were too dumb so our conversation turned out this way. He is a respected Professor.

I more interested in finding the answer to how can we measure low voltage using 741 rather than the meaning of this conversation.

7. Sep 22, 2013

### CWatters

It's been 30 years since I did this so I'm very rusty but I'm thinking perhaps build a chopper amplifier? You add a square wave to the input voltage, amplify it, then demodulate it...

http://electronicdesign.com/analog/chopper-stabilized-op-amps

Perhaps this is what he was hinting at when he said

"Even a small value of current will cause the voltage to drop.It is very low Voltage. Current shouldn't pass."

Perhaps by adding a square wave you can alternate the direction of the current through the source so on average it's zero?

Like I said I'm very very rusty!

8. Sep 22, 2013

### CWatters

9. Sep 23, 2013

### Staff: Mentor

Sure, but a clue to what he had in mind probably lies in being able to interpret the gist of that conversation.

Maybe you could use the open loop gain, applying your small signal to one of the inputs. An adjustment for offset trim would be necessary. (Possibly taking the form of a feedback network passing frequencies << those contained in the signal to be measured.)

10. Sep 25, 2013

### darkxponent

Thank you guys for the reply.

I tried understanding this Chopper thing but i just couldn't. I haven't studied this chopper amplifiers in my course work.

I did gave this answer also but the prof said-'We never use the Op-Amp in open loop configuration as it will start acting as comparator.

11. Sep 25, 2013

### darkxponent

I googled 'low voltage measurement techniques' and look what i found. I didn't include the word 'op-amp' in the search. The circuit seems to agree with the prof. Can't imagine the answer to be this simple. Do you guys agree with this paper.

View attachment Low level measurement.pdf

12. Sep 26, 2013

### Staff: Mentor

That's a book. Is there an appropriate circuit that you have in mind?

13. Sep 26, 2013

### darkxponent

The circuit is in the book on the page 24.