# Voltage gain, input/output impedance using multisim.. How? [closed]

This is the picture of my amplifier circuit using multisim... But I'm not very familiar with this software and thus... how do I do the AC analysis to get the voltage gain, input/output impedance...?

• Is that one of those $\pm 2\:\text{ppm}$ resistors up there as $R_2$? (And I don't mean $\pm 2\:\frac{\text{ppm}}{^\circ\text{C}}$.) Please do let me know the source. I've been looking for one or two. I don't use Multisim, though. So not much help there with the menu system. $C_3$ does look like an odd load. Intended? – jonk Apr 5 at 1:08

I don't use Multisim, but it's easy to find an amplifier's voltage gain and input and output impedances with the CircuitLab circuit simulator.

## Voltage Gain

I've quickly drawn your circuit and set it up to get the Bode plot of voltage gain, setting up a frequency domain simulation with V1 as the source and DB(MAG(V(out))) and PHDEG(V(out)) as the plot outputs:

simulate this circuit – Schematic created using CircuitLab

If you open the circuit above and run the simulation, you'll get the following Bode Plot showing approximately +30 dB of gain in the pass band:

(If you need to recreate this on your own, there's a step-by-step tutorial on how to draw a circuit and get the voltage gain Bode Plot here.)

## Input Impedance

Input impedance is easy: we can just take the same circuit above and instead plot the expression MAG(V(V1.nA)/I(V1.nA)), which looks at the small-signal current relative to the small-signal voltage on source V1 and plots their ratio, an impedance.

In this case, by mousing over the plot, you'll find that it's about $$\2 \ \text{k}\Omega\$$ in the pass band:

## Output Impedance

For output impedance, we need to modify the circuit slightly, just by adding a test current source connected to the output. We'll set this current source to have 0 current at DC so that it's only being used as a small-signal current source, pushing and pulling current from the output node. And then we'll plot MAG(V(I1.nA)/I(I1.nA)) to see the output impedance:

simulate this circuit

If you run this simulation, you'll get this plot for output impedance:

The output impedance is about $$\800 \ \Omega\$$.

(There's a step-by-step tutorial on how to get input and output impedances here.)

• Thanks! I had just found out on how to simulate it using Multisim and it is verified with your simulation! Very much appreciated – Nazfull Apr 15 at 16:51