Offset Null Of Op Amp

Thread Starter

BlackMelon

Joined Mar 19, 2015
170
Dear All,

I would like to check my understanding about the offset null pin of the OP AMP UA741.
Here, I use it as a non-inverting amp. The input Vin+ ranges from -400mV to 0V. So, I connected the circuit as shown.
To do the offset null, I grounded the Vin+, probed the Vout, and calibrated the potentiometer. However, I still see some +200mV at Vout.

I would like to know if my connection is correct or not. If not, could you advise?

Best Regards,
BlackMelon
 

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MrChips

Joined Oct 2, 2009
30,994
741 op amp cannot run on a single 9V battery. It is not a single supply op amp. Use two batteries, +9V and -9V supply rails.
 

dl324

Joined Mar 30, 2015
17,022
I would like to know if my connection is correct or not. If not, could you advise?
With a single supply, you're violating the common mode input voltage range and the opamp can't function correctly.
1708270451791.png
1708270480135.png
With a -9V supply, the guaranteed input voltage range would be -3V to -6V.

The output voltage range has similar restrictions:
1708270633582.png
1708270648238.png
A lightly loaded output isn't guaranteed to get within 3V of the power rails. In your case, -3V to -6V.
 

LadySpark

Joined Feb 7, 2024
194
Dear All,

I would like to check my understanding about the offset null pin of the OP AMP UA741.
Here, I use it as a non-inverting amp. The input Vin+ ranges from -400mV to 0V. So, I connected the circuit as shown.
To do the offset null, I grounded the Vin+, probed the Vout, and calibrated the potentiometer. However, I still see some +200mV at Vout.

I would like to know if my connection is correct or not. If not, could you advise?

Best Regards,
BlackMelon
Part of the op amp is not positively biased, that is why you have that offset. Try installing two diodes in series, forward biased between the V+ pin and GND. Which will force about -1V to be shown on the +V pin.

Edit: just remember that when you use a split rail power op amp with a single power, the output wave is asymmetrical, so if you are not building this for a sound effects/drum synthesizer buffer, you need to use a single supply op amp.
 
Last edited:

Ron H

Joined Apr 14, 2005
7,063
Part of the op amp is not positively biased, that is why you have that offset. Try installing two diodes in series, forward biased between the V+ pin and GND. Which will force about -1V to be shown on the +V pin.

Edit: just remember that when you use a split rail power op amp with a single power, the output wave is asymmetrical, so if you are not building this for a sound effects/drum synthesizer buffer, you need to use a single supply op amp.
That will make the problem worse. The input common mode range does not include Vcc.
 
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