Transistor equivalent circuit question

1. Jul 30, 2013

null void

This is the equivalent circuit to a transistor,
I am to find:
i )Short-circuit input impedance, H11
ii )Open-circuit reverse voltage gain, H12
iii)Forward current gain, H21

2. Relevant equations

Vin = (H11)(Iin) + (H12)(Vout)
Iout = (H21)(Iin) + (H22)(Vout)

3. The attempt at a solution
So first i short the output, and the Vout = 0, then i have

Vin = (H11)(Iin)
Iout = (H21)(Iin)
H11 = R1 + R2//R3 = 110 Ω
H21 = -IR3 / Iin = 2/3

Then I open Iin source,
Vin = (H12)(Vout)
Iout = (H22)(Vout)

H12 = Vin / Vout
= (VR2 / Vout)
= 2/3
1/H22 = R3 + R2
H22 = 1/90 Ω -1

Am i doing this question correctly?

Reason of editing: Vin = V1; Vout = V2
Iin = I1 ; Iout = I2

2. Jul 30, 2013

Staff: Mentor

Something funny going on with your determination of H21 though. Which resistor in your diagram is carrying Iout? And what's the labelled direction for that current?

3. Jul 30, 2013

null void

Is it R2?

I use currendivider rule to find the current flow through R2, and i put negative sign because Iout is opposing the I_R2. So this is wrong?

4. Jul 30, 2013

null void

Ah didn't notice the typing error, it should be

H21 = -IR2 / Iin = 2/3

= 60 / (30 + 60)
= 2/3

5. Jul 30, 2013

Staff: Mentor

Indeed Now, what happened to the minus sign?

6. Jul 30, 2013

null void

ah, -2/3..., thank you very much

7. Jul 30, 2013

Cheers.