# Exam problem

#### PsySc0rpi0n

Joined Mar 4, 2014
1,729
In post #51 and in your original post #1, you show a single-ended input. Now, in post #59 you have changed it to differential input.

Those two different methods of input drive give the same outputs only if Rs = ∞, but that's not the case in the circuit of post #1.

It's true that the outputs with Rs = 200k are nearly the same as if Rs = ∞, but you wouldn't know that unless you performed an analysis for both cases, would you?

Yes, but in post #59 I already converted the original circuit into it's hybrid model... That's why those 2 voltage supplies show up with half of the value and with the polarity inverted!

#### The Electrician

Joined Oct 9, 2007
2,968
Yes, but in post #59 I already converted the original circuit into it's hybrid model...
I can see what you have done. What I want you to understand is that the model of #59 will not give the same answer as the circuit in post #1. A differential input as in post #59 gives a different result compared to the single ended input of post #1, if Rs is 200k, which it is in post #1.

Your problem is to solve the circuit in post #1, isn't it? You shouldn't use a model that will give a different result.

#### PsySc0rpi0n

Joined Mar 4, 2014
1,729
Hum, maybe yes, but from what "I think" I have learnt is that the model I used in post #59 would be equivalent to the circuit in post #1 when performing a small signal analysis!

But it is also true that there is still some lack of understanding in this matters of small signal analysis, AC analysis, DC analysis, Q-points analysis and so on! I admit that! So, probably I'm still a bit lost here! and this is like this for almost 2 years! Last year I also studied amplifiers (not differential but the regular ones) and I had the same troubles!

This is highly discouraging to see guys that can do this in a blink of an eye with any struggle at all!

#### The Electrician

Joined Oct 9, 2007
2,968
Hum, maybe yes, but from what "I think" I have learnt is that the model I used in post #59 would be equivalent to the circuit in post #1 when performing a small signal analysis!

But it is also true that there is still some lack of understanding in this matters of small signal analysis, AC analysis, DC analysis, Q-points analysis and so on! I admit that! So, probably I'm still a bit lost here! and this is like this for almost 2 years! Last year I also studied amplifiers (not differential but the regular ones) and I had the same troubles!

This is highly discouraging to see guys that can do this in a blink of an eye with any struggle at all!
It's easy enough to perform a small signal analysis on the model of post #52 (with Rs = 200k) both with single ended input and with differential input to see if you get a different result.

#### PsySc0rpi0n

Joined Mar 4, 2014
1,729
It's easy enough to perform a small signal analysis on the model of post #52 (with Rs = 200k) both with single ended input and with differential input to see if you get a different result.
Ok, but supposedly that Rs is virtually grounded so it's as if it's not there, I guess!

#### The Electrician

Joined Oct 9, 2007
2,968
Ok, but supposedly that Rs is virtually grounded so it's as if it's not there, I guess!
If the circuit is driven differentially as you have it in post #59, then no signal appears at the top of Rs, but that doesn't make that node a virtual ground even though the value of Rs doesn't matter for this case (small signal and differential drive). However, the problem circuit that you show in post #1 is not differentially driven. With single ended drive as in post #1, signal does appear at the top of Rs, and its presence and its value matter.

You determined that the resistance of the current mirror feeding the two sources is hoe (200k). To solve the circuit in post #1 you will need to have Rs in place and with a value of 200k. Instead of trying to use a shortcut that would allow you to treat Rs as if it's not there (which you can't do with single ended drive), just go ahead and solve the circuit of post #1 with Rs in place.

If you were to do the analysis of the complete circuit, including Rs, with differential drive, the final result for the signal at R1 and R2 won't involve Rs even if it's in place when you carry out the analysis. So don't waste time trying to find a short cut way of eliminating Rs; just leave it in place and proceed with the analysis, either with differential drive or with single ended drive according to what the problem requires.