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I built this small circuit to test if the op amp is working; or if I understand how they should work.

I don't have a bench power supply so I take the 7 voltage that I have, I used a resistor divider to create 1 volt max; around 900mV and send that into the op amp's inverting input. I ground the non-inverting input.

I also run the +7 volt to the v+ of the op amp chip and v- is set to ground.

I tried switching the inverting and non inverting wires as well while using a multimeter to measure the output pin and ground.

I am getting 32mV from the output.

The op amp is OPA2604; dual op amps in one package.

Do I need resistors or something on this op amp? How come I am getting a max of 32mV at the output and zero if I switch the inverting/ non-inverting inputs?

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ What did you expect to happen? \$\endgroup\$ – pipe Jan 4 at 16:24
  • \$\begingroup\$ What's your voltage source exactly? \$\endgroup\$ – pipe Jan 4 at 16:25
  • \$\begingroup\$ I'd like to see the opamp output voltage rise/ fall based on if the polarity of the inverting and non inverting inputs. I am powering this with a 110v - 8v iron core step down transformer with on the output winding. \$\endgroup\$ – wenchemist Jan 4 at 17:49
  • \$\begingroup\$ V+in should always be tied to Vcc/2 unless it has PNP inputs that sense to Vee. Your Op Amp is not one of them the CM Input loses +/-3 out of +/-15V spec meaning 6V dead zone so you might get a 1Vpp operating range on input. If you are lucky \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jan 4 at 18:30
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I don't have a bench power supply so I take the 7 voltage that I have, I used a resistor divider to create 1 volt max; around 900 mV and send that into the op amp's inverting input. I ground the non-inverting input.

That won't work. As soon as the inverting input goes higher than the non-inverting input the output is driven as far negative as it can go. This will be close to 0 V as you found out.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. Reconfiguration as a voltage follower.

  • We use negative feedback so that the output will follow the voltage on the non-inverting input.
  • Your 100 Ω test load is too low for most op-amps. I've increased it to 10k. -Your R1 and R2 values could be increased by a factor of 100. The op-amp has a very high input impedance so it doesn't need to be fed from such low value source.
  • Once you have this working have a look at non-inverting op-amp circuits and see if you can modify it for a gain of two.

enter image description here

Figure 2. Extract from the datasheet.

  1. The op-amp characteristics are measured (unless specified otherwise) at ±15 V.
  2. The op-amp can operate from ±5 V to ±24 V so your 7 V supply is too low. You'd need 10 V.
  3. With a ±15 V supply the inputs can only be allowed to go to ±12 V. In other words, it can't operate and closer than 3 V from the supply rail. Your 900 mV is way to low.
  4. The output is similarly restricted. You won't get closer than 3 V from the negative supply rail.

You need a better power supply for your purposes. Two 9 V batteries would do.

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  • \$\begingroup\$ Additional note: The datasheet for OPA2604 claims ±4.5V as the minimum operating voltage. 7V is too low. \$\endgroup\$ – pipe Jan 4 at 16:28
  • \$\begingroup\$ Thanks, @pipe. I hadn't checked the datasheet. I've added in some more information. \$\endgroup\$ – Transistor Jan 4 at 16:48

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