Subtracting AC Voltages for Audio Amp Simulation

[ntetlow]

TL;DR Summary

Subtracting input voltages input to long tail pair

I am studying the calculations made for small signal amplification with in the web article http://www.ecircuitcenter.com/Circuits_Audio_Amp/Basic_Amplifier/Basic_Audio_Amplifier.htm.
I have created the simulation for the basic amplifier in Ltspice (attached)
I'm having a problem finding the calculating the difference in the input voltages, V2 and V3 to verify the equation

ic1 = vin · 1/2 · gm1 = vin · 1/2 · 0.0192 = vin · 0.0096 (A/V)

My calculations so far give a value for IC1 as 3.5 microamps which would point to a value of V(2,3) as 0.000365 amps.
Please help me to prove this value correct?

 

[ntetlow]

PS how do i attach a Ltspice asc file, the forum won't allow me?

 

[berkeman]

PS how do i attach a Ltspice asc file, the forum won't allow me?

Rename it .txt and mention in the text of your post that it needs to be changed back to .asc to run it in LTSpice.

 

[ntetlow]

Rename it .txt and mention in the text of your post that it needs to be changed back to .asc to run it in LTSpice.

this is an asc file

 

[Baluncore]

That works.
Label your nodes 2 and 3.
To measure differential voltage from 2 to 3, run, then click and drag node 2 to node 3.
Zoom the plot window, that will give you; V(2,3) = 42.5 uV Vpp.

My calculations so far give a value for IC1 as 3.5 microamps which would point to a value of V(2,3) as 0.000365 amps.
Please help me to prove this value correct?

A voltage V(2,3) cannot have the unit amps.

 

[ntetlow]

Thank you. I didn't know you could do that to get the voltage difference.
According to the above equation stated (ic1 = vin · 1/2 · gm1 = vin · 1/2 · 0.0192 = vin · 0.0096 (A/V))
then ic1 should equal 42.5uV pp X 0.0096 which gives 4.08E-7V.
Looking at ltspice it is showing as 7.2uA pp, what do you think?

 

[ntetlow]

Thank you. I didn't know you could do that to get the voltage difference. According to the above equation stated (ic1 = vin · 1/2 · gm1 = vin · 1/2 · 0.0192 = vin · 0.0096 (A/V)) then ic1 should equal 42.5uV pp X 0.0096 which gives 4.08E-7V. Looking at ltspice it is showing as 7.2uA pp, what do you think?

Reference: https://www.physicsforums.com/threads/subtract-ac-voltages.1015880/