Inverting Amplifier Question

  • Thread starter hemant03
  • Start date
  • #1
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Hi,
I was trying to simulate a simple inverting amplifier in LTSpice. However, the results that I get don't agree with what I am supposed to get.
Please look at the PDF attached.
Based on the current law, it is easy to tell that Vout = -Vin. But I am seeing Vout = app. 0V.
Am I missing something. Please help
 

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Answers and Replies

  • #2
sophiecentaur
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Have you connected your negative power supply to 0V or -5V?
You won't get lower volts out than your negative rail.
 
  • #3
1,762
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The non-inverting input should be connected to V3/2 or 2.5 V. That way when V1 is at +5 V the output will be at 0 V and when V1 is at 0 V, the output will be at 5 V. And of course when V1 is at 2.5 V, the output will be at 2.5 V.

There seems to be a common misconception that the Vo = Av * (i(+) - i(-)). If that were true when V1 is equal to 2.5 V you would see 0 V at the output instead of 2.5 V. The correct formula is Vo = V(+) + Av * (V(+) - V(-)).
 
  • #4
sophiecentaur
Science Advisor
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The simulation is giving you the correct answer for the conditions you've given it. You just need to decide whether you want +&- power rails and to use 0V as your reference or to use +5 and 0, in which case, your reference (non-inverting input) should be connected to 2.5V
 

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