# Difference Amplifier Simulation PSPICE

#### emr0327

Joined Sep 22, 2016
10
Hi, I am stuck on this problem where I need to plot the input and output waves. I also do not know how to simulate the 180 degree out of phase shift of v2. I have attatched both the questions and my PSPICE circuit.

#### crutschow

Joined Mar 14, 2008
26,053
The 741 cannot drive a 50 ohm feedback resistor.
It should be at least 2k ohm.

The calculated gain of that circuit is -R2/R1 = -.05 (ignoring the problem in the next paragraph).
I assume you want a gain of 20.(?)

Why do you have the output connected to ground? With that your gain will obviously be zero.

#### emr0327

Joined Sep 22, 2016
10

#### emr0327

Joined Sep 22, 2016
10
The 741 cannot drive a 50 ohm feedback resistor.
It should be at least 2k ohm.

The calculated gain of that circuit is -R2/R1 = -.05 (ignoring the problem in the next paragraph).
I assume you want a gain of 20.(?)

Why do you have the output connected to ground? With that your gain will obviously be zero.

After careful calculations I see now that R2=R4=10k. The gain of the circuit must be 10 V/V.

I do not understand what the prompt means when it says the input voltages must have an output resistance of ohm. How can I represent that in the simulation?

When I take out the GND from the output, I do not get a 50 mV pk-pk waveform.
After careful calculations I see now that R2=R4=10k. The gain of the circuit must be 10 V/V.

#### JoeJester

Joined Apr 26, 2005
4,390
I do not understand what the prompt means when it says the input voltages must have an output resistance of ohm. How can I represent that in the simulation?
How do you represent the internal resistance of a battery?

The same way you represented Ri of a battery on paper is the same way you represent Ri of any signal source in the simulators.

The circuit drawings are the same whether you are using paper and pencil or a simulation program.

#### crutschow

Joined Mar 14, 2008
26,053
....................
When I take out the GND from the output, I do not get a 50 mV pk-pk waveform.
................
With GND on the output, the output must be zero.
How can it be otherwise?

#### emr0327

Joined Sep 22, 2016
10
How do you represent the internal resistance of a battery?

The same way you represented Ri of a battery on paper is the same way you represent Ri of any signal source in the simulators.

The circuit drawings are the same whether you are using paper and pencil or a simulation program.
I believe this is the correct way to represent the output resistance. Can you please confirm or deny this for me? I cannot assume that the resistance would go in series with R3 or R1. So the only other place is in the output node.

#### crutschow

Joined Mar 14, 2008
26,053
A 741 cannot drive a 50 ohm load.

#### emr0327

Joined Sep 22, 2016
10
With GND on the output, the output must be zero.
How can it be otherwise?
I agree. The drawing sketches make it seem that I must connect the output to ground, but I understand now why that cannot be done. I was thinking of it more as an open circuit, but I know that that is virtual and should not be schematically represented that way.

Can you please help me in how I can simulate the 180 degree out of phase shift of v2.

#### emr0327

Joined Sep 22, 2016
10
A 741 cannot drive a 50 ohm load.
I do not understand then where to put the 50 ohm resistor.

#### JoeJester

Joined Apr 26, 2005
4,390
I do not understand then where to put the 50 ohm resistor.

#### emr0327

Joined Sep 22, 2016
10

#### JoeJester

Joined Apr 26, 2005
4,390
I've read that. I want you to tell me what it says. I know what it says.

• emr0327

#### crutschow

Joined Mar 14, 2008
26,053
We'll help you do the simulation you first need to understand the circuit.
It makes no sense to try to simulate a circuit you don't.
Learning is not turning a crank to get an answer.

Do you know what "input voltage source" means?

#### emr0327

Joined Sep 22, 2016
10
We'll help you do the simulation you first need to understand the circuit.
It makes no sense to try to simulate a circuit you don't.
Learning is not turning a crank to get an answer.

Do you know what "input voltage source" means?
Yes, the input voltage sources are labeled V1 and V2 on my schematic. It represents the voltage that is going into the circuit.

#### emr0327

Joined Sep 22, 2016
10
I've read that. I want you to tell me what it says. I know what it says.
It states that the output resistance for VI1 and VI2 is 50 ohm. Meaning, when a voltage is inputted into the circuit, the output resistance will read 50 ohm. Reading this I am confused if it is asking to change the resistors right after the input voltage sources to 50 ohm or if it's asking to put resistors near the output.

#### JoeJester

Joined Apr 26, 2005
4,390
After you designed your circuit, then hook up the sources and their load.

#### emr0327

Joined Sep 22, 2016
10
After you designed your circuit, then hook up the sources and their load.
Ohhh okay I understand it now. The resistor is connected with the input voltage sources as depicted in the attached illustration.

#### JoeJester

Joined Apr 26, 2005
4,390
either side of the source would be correct.