# differential amplifier

Discussion in 'Homework Help' started by ihaveaquestion, Dec 12, 2010.

1. ### ihaveaquestion Thread Starter Active Member

May 1, 2009
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hi guys,

need a little help with the attached circuit as im not sure why im able to get three different values of transconductance:

here's my evaluation so far:

- va = 1/λ, therefore λ=1/va leads to 1/10 = λ=0.1
- since we are given that Iss = 0.32, we know that since the transistors are matched evenly the current splits evenly, so Id = 0.16mA.
- ro = 1/λId = 62.5kΩ
- Id = 1/2μCoxW/L(Vgs-Vt)^2(1+Vds*λ)... we are given or know everything except Vgs and so we solve for that and get 0.879 V.
- Vov = Vgs - Vt = 0.179V
- The trouble starts when finding the transconductance, gm.
gm = μCoxW/L*Vov = sqrt(2*μCoxW/L*Id) = 2*Id/Vov

However, i'm getting three different values when i use those three equations -- any ideas? thanks in advance....

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2. ### ihaveaquestion Thread Starter Active Member

May 1, 2009
314
0
any thoughts?

3. ### ihaveaquestion Thread Starter Active Member

May 1, 2009
314
0
hmmm, also, to find the other parameters:

i think i can use the same Id = 1/2μCoxW/L(Vgs-Vt)^2(1+Vds*λ) equation for the pmos on the top.... i know everything except Vgs and Vds... BUT i do Vg = Vd and I know Vs = 2.5.... so I can turn that into an equation with only one unknown... but that gives me the following equation which I don't know how to solve:

01.Vg^3 + 0.75Vg^2 - 2.736Vg + 2.43 = 0.04

any thoughts?

thanks guys

4. ### ihaveaquestion Thread Starter Active Member

May 1, 2009
314
0
no one? really?

5. ### Jony130 AAC Fanatic!

Feb 17, 2009
3,957
1,097
Well I try to look on your problem tomorrow.

6. ### ihaveaquestion Thread Starter Active Member

May 1, 2009
314
0
thanks but its due in 2 hours

7. ### ihaveaquestion Thread Starter Active Member

May 1, 2009
314
0
for the record, that post i made earlier about getting a third order equation is how you do it....