# Simple Op-Amp Circuit

#### lfgrdwill

Joined Dec 9, 2011
27
Hello guys,

I am trying to build a simple circuit using a stand alone educational op-amp (desktop type) with a DC power supply, couple resistors and a few lead wires. The op-amp is to maintain constant voltage across a potentiometer as the resistance of the this pot is varied using a T-type feedback network.

The op-amp model is this one https://www.thesciencesource.com/store/index.php?product_id=596&type=&category=

Our model lacks the inverting/non-inverting switch. If i make the proper connections to the positive and negative inputs of the op-amp, will it act in the inverting mode that i need it to?

The max gain is 100 and the circuit parameters i measure with this thing hooked up and running are not even remotely matching up to the ideal op-amp analysis. All the ground connections are connected to the negative sign connection on the power supply, and my circuit begins with a source, corresponding to the positive sign on the power supply.

Thank you for any suggestions. Is there a simple way i could verify that the op-amp is even working?

#### Brownout

Joined Jan 10, 2012
2,390
Make sure you aren't saturating the amplifier. For example, if you have +/-15V internal supplies, and the gain set to 100, then the biggest signal you can input will be +/-15/100 = +/-.15V or 150mV. any more than they will saturate the output.

#### lfgrdwill

Joined Dec 9, 2011
27
Would the adjustment knob on this stand-alone op-amp be the open loop gain or closed loop gain?

#### Brownout

Joined Jan 10, 2012
2,390
As far as I can tell, the amplifier should be closed-loop. The adjustment knob is one of the closed-loop gain determining resistors.

#### crutschow

Joined Mar 14, 2008
33,326
.................

The max gain is 100 and the circuit parameters i measure with this thing hooked up and running are not even remotely matching up to the ideal op-amp analysis. All the ground connections are connected to the negative sign connection on the power supply, and my circuit begins with a source, corresponding to the positive sign on the power supply.

.................
All external ground connections for external signals should go to the common point for the ±6V power supplies, not the negative connection. The op amp zero point is a 0V between the two supplies. If you reference your signals to the minus connection all the signals will be offset by -6V which likely saturate the op amp.

#### lfgrdwill

Joined Dec 9, 2011
27
When i set the DC power supply to 10 Volts, i measure this set voltage across the - and + power supply connections. If i measure across the common ground and the + sign on the power supply connections, i get 0 ish volts.

One 5000 ohm (R1) resistor is hooked up to the + connection on the power supply and a resistor. The other end of this resistor goes into the negative input of the op-amp. Out from the negative op-amp input is a 1000 ohm resistor (Rf), then a potentiometer (Rw, opposite end of the pot goes to negative power supply input), and out from the Rw input to a 200 ohm resistor (R2), then R2 is connected to the op-amp output.

Still can't get this pot to maintain constant voltage and i am going in circles.

I have included an image.

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#### Brownout

Joined Jan 10, 2012
2,390
I don't think you should be connecting external feedback resistors. Those should already be inside the box???