741 Offset voltage problem

[sherbo]

Hello guys, I'm using the UA741 op-amp but theres a problem.. offset voltage ! I always get (saturated +ve output) until the inverting input (V-) is 1.8 volts greater than V+.

Now 1.8V... isn't that ALOT ?
Anyway, I tried using the offset null pins but they didn't work !
Here's what I did exactly:

Pin 2 (V-) : GND
Pin 3 (V+) : GND
Pin 6 (Vo) : DMM
Pin 1 (offset null) : 10k Pot beginning
Pin 5 (offset null) : 10k Pot end
Pot slider ---> GND
Supply pins: +13v & GND.


the opamp insists on giving +13V (sat.) regardless of the pot. setting.
Does this mean a bad IC ?? I've tried 3 of them so far.. maybe the whole lot was bad ??

[audioguru]

Hi Sherbo,
You are overlooking the 741's common-mode input voltage limit of 3V max from the positive supply or from ground in your single-supply circuit.

An LM324, LM358, MC3458, MC3307x or MC3317x can operate with their inputs at ground but not near the positive supply.
A TL07x can operate with its inputs at the positive supply but not near ground.

Maybe you need a "rail-to-rail" opamp.

[Kevin Weddle]

Everything lookks good. The supply voltage is a little low which means that it will have a narrow range of operating conditions. But beside that, do you know how to adjust the pot. In your case, since you are using 13v and ground, you will need to equally bias the inputs at 6.5 volts with a voltage divider. Then adjust the pot until it reads 6.5 volts output. This is your best bet to ensure the pot is adjusted to the opamp.

[Miles Prower]

Pin 2 (V-) : GND
Pin 3 (V+) : GND
Pin 6 (Vo) : DMM
Pin 1 (offset null) : 10k Pot beginning
Pin 5 (offset null) : 10k Pot end
Pot slider ---> GND
Supply pins: +13v & GND.

Here is your problem. If you are going to operate the 741 from a single-ended power supply, you need to maintain ¹/₂V_CC at the inputs. If you don't, then you get that lock-up problem.

Here is a solution that I used in one of my projects: