# H parameter analysis of transistor amplifiers.

1. Nov 12, 2013

### 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:

Vin = h11Iin + h12Vout ___(1)
Iout = h21Iin + h22Vout ___(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 Vin is decided by us. I would say Vin 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?

2. Nov 12, 2013

### Staff: Mentor

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

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

3. Nov 12, 2013

### ShreyasR

OMG! Silly me... thank you!

4. Nov 17, 2013

### 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 Vin, and the h parameters. So how do i theoretically calculate 3 unknowns (Vout, Iin and Iout) when i have only two equations from the h parameters?

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

5. Nov 17, 2013

### 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 Vin, and the h parameters. So how do i theoretically calculate 3 unknowns (Vout, Iin and Iout) when i have only two equations from the h parameters?

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

6. Nov 17, 2013

### Staff: Mentor

What is the relation between Iout and Vout?

7. Nov 17, 2013

### ShreyasR

Iout = h21Iin + h22Vout this is the relation between Iout and Vout (Equation 2)

8. Nov 17, 2013

### The Electrician

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

9. Nov 17, 2013

### Staff: Mentor

The answer I was looking for ......

is that Iout = - Vout / RL

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

10. Nov 17, 2013

### ShreyasR

Oh my! Thank you NascentOxygen and The Electrician... I made a mistake in marking Iout... It is the current flowing through the load and not through hoe... 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 {Vin, Vout, Iin and Iout} 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!

11. Nov 19, 2013

### ShreyasR

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

12. Nov 20, 2013

### Staff: Mentor

That sounds okay.

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