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I did a standard inverting operational amplifier (UA741), with Rin = 6k and Rf = 1k.

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I have a lab power supply, with 30V of difference for the amplifier.

I don't understand why this doesn't work, it should be Vout = -Vin/6 but I measure Vin = Vout = V-(AO).

I put Vin between 0 and 10V for instance, static, and voltage measurements are made with a standard voltmeter. I tried with several new op amps and it's the same problem.

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    \$\begingroup\$ What is Vee? and AO(?) Are you expecting a negative output voltage ? \$\endgroup\$ Jul 24, 2020 at 14:12
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    \$\begingroup\$ Where is your power supply connected? Show the full schematic. \$\endgroup\$
    – Andy aka
    Jul 24, 2020 at 14:13
  • \$\begingroup\$ V-(AO) is the point between the two resistors \$\endgroup\$
    – Jom
    Jul 24, 2020 at 14:14
  • \$\begingroup\$ Is your GND halfway between your supply rails, i.e. are they +/-15V wrt GND? \$\endgroup\$
    – user16324
    Jul 24, 2020 at 14:16
  • \$\begingroup\$ Is that a 741 op amp? What is its pinout, and how does that compare with the photo of the circuit? \$\endgroup\$
    – tim
    Jul 24, 2020 at 14:30

2 Answers 2

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It's an inverting amplifier hence with Vin at some positive value, Vout has to be a negative value but, you don't have a negative supply rail on pin 4 hence it won't work because the output cannot become a negative value.

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You can't just put a 30V power supply on the op-amp, you need a ground so that you have something like +/-15V.

If your lab supply does not share a ground with your signal source you can create a pseudo ground at DC by doing something like this:

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ V2 +/- will float up and down if V1 signal has a signal thru R2 current. So not quite OK. \$\endgroup\$ Jul 24, 2020 at 18:10
  • \$\begingroup\$ @TonyStewartSunnyskyguyEE75 Well, it's expected to float up and down, that's the whole premise. Simulation with a 10Hz 10V input: i.imgur.com/5LKx7my.png \$\endgroup\$ Jul 24, 2020 at 19:17
  • \$\begingroup\$ ground impedance is too high relative to 1k. tinyurl.com/yyayzlfe \$\endgroup\$ Jul 24, 2020 at 19:23
  • \$\begingroup\$ @TonyStewartSunnyskyguyEE75 You forgot a connection. \$\endgroup\$ Jul 24, 2020 at 19:42
  • \$\begingroup\$ Some improvement tinyurl.com/yy5qn3ou \$\endgroup\$ Jul 24, 2020 at 19:59

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