I am building an Affordable Solar Radio Telescope and for it I need an amplifier. I have built the circuit described in pg 8 of this link.

I have built the exact circuit but I am using a dual supply of 12 V (because I am using op amp IC 741 and read that it works on dual supply).

But the op amp is providing negligible gain, and even though my input is positive. And I am using non inverting mode my output is negative.

What to do?

  • \$\begingroup\$ The op-amp circuit should give about zero V out for 0V in (+/- a few tens of mV), since the gain is +11. However, if you leave the non- inverting input open , the output will go to about -11V because it needs about 80nA flowing into the input to hold it at 0V, \$\endgroup\$ – Spehro Pefhany Jul 30 '14 at 3:34
  • 3
    \$\begingroup\$ Oh my. People need to stop using the '741. \$\endgroup\$ – Ignacio Vazquez-Abrams Jul 30 '14 at 3:35
  • \$\begingroup\$ Eh. At least the 741 has a nice characteristic failure mode. Some other, "better" op amps have a bias which can be of either polarity, and that makes the problem less obvious. \$\endgroup\$ – WhatRoughBeast Jul 30 '14 at 3:54
  • \$\begingroup\$ It's a very old, very expensive amp with poor parameters that people use only because that's what was in the figures in their circuits text. \$\endgroup\$ – Scott Seidman Jul 30 '14 at 13:14

Spehro's comment is almost certainly correct. Here is how you can check:


simulate this circuit – Schematic created using CircuitLab

Add R3 and R4 to your circuit, and the two voltages shown should match. Next, short R4 with a clip lead or a piece of wire, and the output should go to zero.

If it works in both cases, you're in business.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.