# Small-signal transfer function from this differential input

1. Apr 3, 2015

### CorHawk

1. The problem statement, all variables and given/known data

2. Relevant equations

Small Signal Equivalent Circuit, Kirchhoff Current Law, and BJT equations mentioned in the question.

3. The attempt at a solution

It looks like I did something wrong. Can someone help me with fixing this?

2. Apr 5, 2015

### rude man

I would not use equivalent circuits for this. You're given all the equations you need to solve the problem without resorting to equiv. ckts.
Hint: at some point use ex ≅ 1 + x for 0 ≤ x << 1.
OR you can do it with just gm.

Last edited: Apr 5, 2015
3. Apr 5, 2015

### CorHawk

I don't understand how you want to get to the answer 1/2*gm by only using gm and nothing else. And I don't see how e^0=1 would be related to this.

Last edited: Apr 5, 2015
4. Apr 5, 2015

### rude man

I'll get you started:
gm = dI1/d(Vb1 - Ve) = -dI2/dVe
where Vb2 = 0 is assumed wlog ("without loss of generality"). Vb is base voltage, Ve is emitter voltage.
That would come in handy if you choose to use I = Isexp(qVbe/kT) instead of gm directly.
I recommend using gm directly, it's shorter.

Last edited: Apr 5, 2015