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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...?

  • \$\begingroup\$ 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? \$\endgroup\$ – 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:

bode plot of voltage gain

(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:

amplifier input impedance

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:

output impedance plot

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

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

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

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