TL071CP Experience

zorbzz · Oct 14, 2008

Hi there, Im new to the site which I must say is excellent for people like me.
I am trying to input a 0.100 to 0.950 volts into the input of a TL071 op amp to recieve an output of approximately a 17% gain that will keep at roughly that ratio from 0.100 to say 1 volt is this possible?

My test circuit has been configured for roughly a %50 gain and is as follows.
**broken link removed**

The problem I get with this is that when the input voltage drops to 0.100v the output runs at 9v
The output only corrects itself after the input voltage is more than 0.850v and above.
Any ideas?

audioguru · Oct 14, 2008

Your circuit has a voltage gain of 1.5 as shown.
With 0V input then the output is 0V.
With +0.1V input then the output is +0.15V.
With +0.85V input then the output is +1.275V.
With +0.95V input then the output is +1.425V.

If the input becomes negative then the output is negative until the input is within 3V of the negative supply then the output will suddenly go to +7.8V (phase inversion problem).
Its max output is +9V - 1.2V= +7.8V (not 9V).

zorbzz · Oct 15, 2008

I wish those figures were working for me.

To test the circuit I am using a voltage divider with a resistor and potentiometer and a 1.5 volt AAA battery as the supply but when I turn the voltage input down to anything under 0.8 volts the output is way too high.
I cant get my 1.5 gain with 0.1 volts That is my problem.

ericgibbs · Oct 15, 2008

zorbzz said:
I wish those figures were working for me.

To test the circuit I am using a voltage divider with a resistor and potentiometer and a 1.5 volt AAA battery as the supply but when I turn the voltage input down to anything under 0.8 volts the output is way too high.
I cant get my 1.5 gain with 0.1 volts That is my problem.

hi,
To be sure that the circuit is what we think it is, can you please add pin numbers to the circuit, also draw in the 1.5V battery and divider input.

Nigel Goodwin · Oct 15, 2008

zorbzz said:
I wish those figures were working for me.

To test the circuit I am using a voltage divider with a resistor and potentiometer and a 1.5 volt AAA battery as the supply but when I turn the voltage input down to anything under 0.8 volts the output is way too high.
I cant get my 1.5 gain with 0.1 volts That is my problem.

Isn't 1.5V far too low for that chip?, you also require a split supply, or a single supply with an artifical split generated from it. Try using two 9V batteries for +/- 9V.

zorbzz · Oct 15, 2008

Ive tried the split supply using 2 9v batteries but didnt have any different results.
It still worked fine untill the input voltage went under 0.8 volts.
Should it work ok without the split supply as with or without the split supply I get the same results.

If I change the voltage that powers the opamp to say 12v then when the input is under 0.8 volts it outputs around 11 volts.

I dont know whats happening.
Would it be possible for someone to draw a circuit that will input 0.1 to 1 volt and output that to 17% more? Ill build that and see what happens.
Im just about ready to give up
Thanks for your help so far.

ericgibbs · Oct 15, 2008

zorbzz said:
Ive tried the split supply using 2 9v batteries but didnt have any different results.
It still worked fine untill the input voltage went under 0.8 volts.
Should it work ok without the split supply as with or without the split supply I get the same results.

If I change the voltage that powers the opamp to say 12v then when the input is under 0.8 volts it outputs around 11 volts.

I dont know whats happening.
Would it be possible for someone to draw a circuit that will input 0.1 to 1 volt and output that to 17% more? Ill build that and see what happens.
Im just about ready to give up
Thanks for your help so far.

I'm still waiting for the circuit with pin numbers and the test battery connections that you using.

zorbzz · Oct 15, 2008

Ok here it is
**broken link removed**
Sorry its so big I just quickly done it then.

ericgibbs · Oct 15, 2008

hi,
Thanks,
A quick look says the circuit should work. I wanted to be sure that you had connected the amp correctly.

You say under 0.8Vin the output is incorrect.

If you set the pot to 0.5Vin what is the measured output on pin 6.?

EDIT:
The divider resistors 10K and 10K variable will only permit a Vin as low as 0.75V.

Perhaps the pot wiper is faulty at one end.

Swop the 10K and 10K pot around, so you adjust from 0Vin to +0.75Vin then see what you get.

zorbzz · Oct 15, 2008

at 0.50 volts input, the output is 8.64 volts hehe I dont know whats going on.
same to 0.70
then 0.75 is 8.44
then 0.80 is 1.48
0.90 is 1.44
1.00 is 1.45
1.10 is 1.47
1.5 is 1.75
pin 4 of the op amp is also grounded
??????
No clue

ericgibbs · Oct 15, 2008

zorbzz said:
at 0.50 volts input, the output is 8.64 volts hehe I dont know whats going on.
same to 0.70
then 0.75 is 8.44
then 0.80 is 1.48
0.90 is 1.44
1.00 is 1.45
1.10 is 1.47
1.5 is 1.75
pin 4 of the op amp is also grounded
??????
No clue

Look at my edit and try swopping the 10K and 10K pot around, checking the pot OK.?

zorbzz · Oct 15, 2008

ericgibbs said:
Swop the 10K and 10K pot around, so you adjust from 0Vin to +0.75Vin then see what you get.

Ok thanks for helping me, i'll try it in the morning im gonna go hit the sack for now.
thanks again

picbits · Oct 15, 2008

Add a 2k resistor between the output and ground and see if that helps. I've had a couple of opamps in the past which have been a bit weird with no load on the output.

audioguru · Oct 15, 2008

zorbzz said:
pin 4 of the op amp is also grounded
??????

Then the opamp does not have a negative supply voltage and will not work.
Also then the negative 9V battery is shorted.
The junction of the two batteries must be connected to ground instead.
Do it like this:

zorbzz · Oct 16, 2008

Thankyou so much.
I disconnected the ground from the op amp main supply and hooked 2 batteries up as you described with ground on the junction between the 2 and bingo! It works perfect. at 0.100 volts input i now recieve a perfect 0.150 volts output.
Thanks again

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