Engineering Transistor equivalent circuit question

[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
iv)Open-circuit output admittance, H₂₂

Homework Equations

V_in = (H₁₁)(I_in) + (H₁₂)(V_out)
I_out = (H₂₁)(I_in) + (H₂₂)(V_out)

The Attempt at a Solution

So first i short the output, and the V_out = 0, then i have

V_in = (H₁₁)(I_in)
I_out = (H₂₁)(I_in)
H₁₁ = R₁ + R₂//R₃ = 110 Ω
H₂₁ = -I_R3 / I_in = 2/3

Then I open I_in source,
V_in = (H₁₂)(V_out)
I_out = (H₂₂)(V_out)

H₁₂ = V_in / V_out
= (V_R2 / V_out)
= 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

 

[gneill]

Your approach is fine.

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

 

[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?

 

[null void]

H21 = -IR3 / Iin = 2/3

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

H₂₁ = -I_R2 / I_in = 2/3

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

 

[gneill]

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

H₂₁ = -I_R2 / I_in = 2/3

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

Indeed Now, what happened to the minus sign?

 

[null void]

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

 

[gneill]

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

Cheers.