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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?

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

schematic

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.

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