lm741 schematic diagram

[Kevin Weddle]

What I want to know is about the input stage. It seems there is a constant current in there. You know how you can have +7 and +7 on the inputs and have a 0 voltage out. I think there is a transistor there that sets the initial current at the base of the two connected transistors. If you have an increase on one input, and then increase on the other input, the current will not change. So explain to me about the base current of the two base connected transistors. I want to be sure that I can get a zero ouput with +7 +7 inputs.

 

[ante1]

Kevin,

On the first page of this datasheet http://www.ensc.sfu.ca/reference/data-sheets/LM741.PDF
you can see exactly how the 741 is built inside.

 

[audioguru2]

Kevin,
You probably won't get zero volts output with the same input voltages. Like most old opamps, the 741 has a high input offset voltage of up to 10mV (and a high input offset current, too). Without feedback, the voltage gain of 100,000 or more will slam the output hard against a power supply rail. You will probably need to add an offset voltage adjustment pot.

You also must make your inputs within the limited common-mode-voltage rating of the old 741.

Today you can get opamps with the offset voltage laser-trimmed so that it is nearly nothing. You can also get opamps whose inputs operate up to the power supply voltages.