H parameter analysis of transistor amplifiers.

[ShreyasR]

In h parameter analysis, we take the input current and output voltage as independent variables. and the Input voltage and output current are dependent on the input current and output voltage. They are related as:

V_in = h₁₁I_in + h₁₂V_out ___(1)
I_out = h₂₁I_in + h₂₂V_out ___(2)

In case of a transistor amplifier (say, common emitter RC coupled amplifier), practically say if we are feeding an input voltage (sine) to the amplifier using a signal generator, then V_in is decided by us. I would say V_in is not dependent on the output voltage of the amplifier.

If this is the case, if we are doing h parameter analysis, I feel the use of Equation (1) becomes invalid as it suggests that the input voltage is dependent on the output.

Because of this, I am not convinced with h parameter analysis of transistor circuits. Someone please help me with this?

 

[NascentOxygen]

Hi ShreyasR. These are linear equations, so you can rearrange them to make any term the subject of the forumula. If you hold V_in fixed, then you are determining I_in and V_out.

You can connect one signal generator to set V_in and another to set V_out if you wish to fix both!

 

[ShreyasR]

OMG! Silly me... thank you!

 

[ShreyasR]

Sorry. I thought that made sense but then i landed up in this situation.
When i connect a signal generator to the input, all that i know is V_in, and the h parameters. So how do i theoretically calculate 3 unknowns (V_out, I_in and I_out) when i have only two equations from the h parameters?

I wish i had I_in... But when i draw the H parameter equivalent model, I am not able to obtain another equation to get I_in.

 

[ShreyasR]

Sorry. I thought that made sense but then i landed up in this situation.
When i connect a signal generator to the input, all that i know is V_in, and the h parameters. So how do i theoretically calculate 3 unknowns (V_out, I_in and I_out) when i have only two equations from the h parameters?

I wish i had I_in... But when i draw the H parameter equivalent model, I am not able to obtain another equation to get I_in.

 

[NascentOxygen]

What is the relation between I_out and V_out?

 

[ShreyasR]

I_out = h₂₁I_in + h₂₂V_out this is the relation between Iout and Vout (Equation 2)

 

[The Electrician]

So how do i theoretically calculate 3 unknowns (V_out, I_in and I_out) when i have only two equations from the h parameters?

Without a load, Iout is zero; that gives you two equations in two unknowns.

 

[NascentOxygen]

I_out = h₂₁I_in + h₂₂V_out this is the relation between Iout and Vout (Equation 2)

The answer I was looking for ...

is that I_out = - V_out / R_L

So there is your third equation. (3 equations, 3 unknowns)

 

[ShreyasR]

Oh my! Thank you NascentOxygen and The Electrician... I made a mistake in marking I_out... It is the current flowing through the load and not through h_oe... Well now i get it all...

Now i guess i get a clear picture of two port networks. This is how I have understood it:

For any linear two port network, we can consider any two parameters out of the set {V_in, V_out, I_in and I_out} to be fixed and write them down as a linear function of the other two parameters.

And when we consider two parameters to be fixed, they need not actually be independent, but this is just to obtain an equivalent model (either Z parameter model, Y parameter model or hybrid parameter model). And any of these models can be converted to some other model.

Can i say this is the basic principle used in two port analysis? I have just typed what I have understood. If there is anything wrong in the way i have understood this, please correct me. Thanks!

 

[ShreyasR]

Someone please correct me if I am wrong... Is my understanding of this concept just fine? This will be the last question...

 

[NascentOxygen]

That sounds okay.