Summer+inverter op amp[multisim diagram]

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Discussion Overview

The discussion revolves around a circuit designed to convert input voltages of 100mV to 400mV into output voltages of -10V to 10V using an inverter operational amplifier, as simulated in Multisim. Participants are verifying the circuit's correctness and addressing issues encountered during simulation.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant reports that the simulation yields 14.50V for DC and 0V for AC, questioning the correctness of the circuit design.
  • Another participant identifies potential issues, noting that there may be an unintentional short circuit in the connections to channel A on the oscilloscope.
  • Concerns are raised about the absence of negative power supplies, with a suggestion that connecting a negative DC supply could lead to confusion regarding polarity.
  • Participants discuss the grounding of the function generator, with one suggesting that the negative terminal should not be grounded.
  • There is confusion about the connections for the oscilloscope and function generator, with a request for clarification on where to connect the negative terminals.
  • One participant realizes that the circuit diagram was difficult to interpret and initially thought the oscilloscope had single-ended inputs, leading to a misunderstanding of the connections.
  • Another participant advises against connecting the oscilloscope's negative terminal to the op-amp's negative input, suggesting it should be connected to a zero-volt point instead to avoid potential issues.

Areas of Agreement / Disagreement

Participants express various concerns and suggestions regarding the circuit design and connections, but there is no consensus on the correct configuration or resolution of the issues raised.

Contextual Notes

Participants mention potential limitations related to grounding and circuit connections, as well as the implications of using differential inputs on the oscilloscope. However, these points remain unresolved.

bos1234
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I have the circuit below which is supposed to take input of 100mV to 400mV and output a voltage of -10v and 10V respectively. I have designed the circuit below through multisim and would like to verify if its correct before trying to connect on breadboard. When I simulate the circuit, I get 14.50V for dc and for AC i get 0V ?

29eqgp3.png
 
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Without analyzing it in detail two big problems stand out.

1. I don't use "multisim" but it looks to me that you're unintentionally short circuiting those two points connected to channel A on the scope.

2. You have no negative power supplies! If you take a DC supply and connect the nominal positive side to ground but then give the supply a negative value (-15), then it's actually a positive supply (by virtue of double negative).
 
uart said:
1. I don't use "multisim" but it looks to me that you're unintentionally short circuiting those two points connected to channel A on the scope.
An oscilloscope has high input impedance.

In addition to what uart said about the supplies, don't ground the negative end of the function generator.
 
Jiggy-Ninja said:
An oscilloscope has high input impedance.

In addition to what uart said about the supplies, don't ground the negative end of the function generator.

I'm getting confused here. Where should the - terminal of oscilloscope and function generator be connected to?

The results of the simulation are shown below. Keep in mind I am supposed to get -10V and 10V

dc
vgsgtd.png


AC
11uasew.png
 
Last edited:
bos1234 said:
I'm getting confused here. Where should the - terminal of oscilloscope and function generator be connected to?

The results of the simulation are shown below. Keep in mind I am supposed to get -10V and 10V
The scope is fine. The - terminla of the function generator shouldn't be connected to anything.
 
Jiggy-Ninja said:
An oscilloscope has high input impedance.

Ha I know that. :smile:
That circuit diagram was a bit small on my monitor and I thought the scope had single ended inputs (with respect to ground). So that the two input wires would have been connected together to a common node at the oscilloscope input instead of to separate "+" and "-" nodes. I had to look a bit closer to notice that it's a differential input (+,-).
 
Last edited:
bos1234 said:
I'm getting confused here. Where should the - terminal of oscilloscope and function generator be connected to?
You've currently got the scope negative connected to the opamp negative input, which should be a "virtual earth" (if your circuit was working properly). Whether or not it will make a difference in multisim, in real life you shouldn't be adding extra capacitance and perhaps extra noise pickup here. You should connect it directly to a zero volt point (earth) instead.
 

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