# Need help making the small signal circuit

#### mesaman000

Joined Jun 29, 2010
16
So here is the picture... I need to Determine the DC Base and collector currents (which I already did that part wasn't bad. But the next part asks to draw the small-signal equivalent. I think I drew the small signal equivalent correctly if it looks like this: Does this look right? I feel like I'm missing something

If it is right, the last part says "Assume Vs is the rms value of a sinusoid. determine Av= Vo/Vs. Can someone give me some guidance on what this means? I don't exactly know how to evaluate this part. Any help would be great, thanks in advance!

#### mesaman000

Joined Jun 29, 2010
16
wow.. it wouldn't let me title it what i wanted it to but obviously what I meant to title it is that I need help making the small signal circuit..

I just made the realization that the 10k resistor shoudlnt be at Ve because the capacitor is shorted in the AC model. However, when I do the calculations using nodal analysis at Vb and Vc, I get Vo/Vs= .008 and I know that can't be right... By the way I assume theres no value for ro since they never give us the early voltage

#### hgmjr

Joined Jan 28, 2005
9,029
I have edited the title to be closer to what you wanted it to be.

hgmjr

• mesaman000

Joined Dec 26, 2010
2,148
wow.. it wouldn't let me title it what i wanted it to but obviously what I meant to title it is that I need help making the small signal circuit..

I just made the realization that the 10k resistor shoudlnt be at Ve because the capacitor is shorted in the AC model. However, when I do the calculations using nodal analysis at Vb and Vc, I get Vo/Vs= .008 and I know that can't be right... By the way I assume theres no value for ro since they never give us the early voltage
Yes, that answer must be a few orders of magnitude in error. You are asked to find the small-signal gain, and as you rightly state the first part is to calculate the DC working point. I won't do this for you but here are a few pointers.

1) Have you got the correct DC working point? The circuit has a few Volts supply and collector and emitter resistors of tens of kohms. What order of magnitude would be reasonable for Ic? Check your working.

2) What value have you found for gm? A few milli Siemens would seem to be in order - be careful of not making an error of 1000 times here, e.g. by assuming Vt to be in Volts instead of mV.

3) Don't worry about not having the Early voltage. This would make only a small difference, you are looking for a much bigger error.

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#### mesaman000

Joined Jun 29, 2010
16
well I drew my small signal diagram in the 2nd picture.. That should be right if you leave out the 10k resistor which allows you to do nodal analyses at Vb and Vo which gives you 2 equations

So i have -(Vs-Vb)/1k + Vb/rpi + Vb-Vc/1M= 0 and rpi= 25k

and my 2nd equation is -(Vb-Vc)/1M + Vc/10k + gm(Vo/25k) = 0

When I simplify and comibne them I get 124919Vo=1000Vs and then Vo/Vs =.008?

My value of gm is right too, it's Ic/Vt= 99uA/25mV

I don't know why I'm getting such a small gain though..

Joined Dec 26, 2010
2,148
well Ihttp://forum.allaboutcircuits.com/newreply.php?do=newreply&p=319615 drew my small signal diagram in the 2nd picture.. That should be right if you leave out the 10k resistor which allows you to do nodal analyses at Vb and Vo which gives you 2 equations

So i have -(Vs-Vb)/1k + Vb/rpi + Vb-Vc/1M= 0 and rpi= 25k

and my 2nd equation is -(Vb-Vc)/1M + Vc/10k + gm(Vo/25k) = 0

When I simplify and comibne them I get 124919Vo=1000Vs and then Vo/Vs =.008?

My value of gm is right too, it's Ic/Vt= 99uA/25mV

I don't know why I'm getting such a small gain though..
What is the controlling voltage for gm?

#### mesaman000

Joined Jun 29, 2010
16
AHHH ok I got it! thank you very much for the help!

#### DMJYwyg

Joined Jan 7, 2011
1
AHHH ok I got it! thank you very much for the help!
I can't understand it ,what the picture meaning?you have the answer.