First of all, we have to understand what is a two-port network to understand the transistor as a 2-port network.

**Two-port Network**:-

Any network with 4 nodes or 4 terminals is considered as a two-port network only if it satisfies the port condition

**Port Condition**:-

If current entering from one terminal is leaving from some other terminal with the opposite direction but with equal magnitude then these two terminals combine to form a single port. So for a two-port network, we require 4 such terminals that satisfy the port condition.

As you can see above the transistor obeys port condition and therefore we can say that the transistor is like a two-port network.

There are total of 6 parameters for a two-port network but for analysis of the transistor, we will be dealing with only one parameter that is an h-parameter. You can also go with g-parameter

Since,

**h=1/g**

Now the question arises why we are only using the h-parameter for the analysis of transistors?

So here is the answer to it.

**Z**

**-parameter:-**

V1

**=**Z11*I1**+**Z12*I2V2

**=**Z21*I1**+**Z22*I2By using Z-parameters we can only get to know input and output impedances and we not at all interested in transfer impedances for the analysis of transistors.

**Y-parameter:-**

I1

**=**Y11*V1**+**Y12*V2I2

**=**Y21*V1**+**Y22*V2Similarly, by using Y-parameters we will only get input and output admittance and we not at all interested in transfer admittance

**ABCD parameter:**-

Vs

**=**A*V**+**B*IIs

**=**C*V**+**D*IBy using ABCD parameters we can just find the source voltage and source current, which is not at all required for the analysis of the transistor.

But now let's talk about

**h-parameter**V1

**=**h11*I1**+**h12*V2.....(1)I2

**=**h21*I1**+**h22*V2.....(2)Now,

**From equation (1)**

**if V2=0 (short-circuited)**

h11

**=**V1**/**I1And as we know voltage by the current is equal to the resistance. Therefore

**h11 is also known as input impedance (Zi)****if I1=0 (open circuited)**

h12

**=**V1**/**V2So

**h12 provides reverse voltage gain(1\Av)**.So if we took the reciprocal of it then we will get the (Av)voltage gain (V2 / V1) of an amplifier.

Now,

**From equation (2)**

**If V2=0 (short circuited)**

h21

**=**I2**/**I1So

**h21 parameter provides the current gain of the amplifier**.**If I1=0 (open circuited)**

h22

**=**I2**/**V2So

**h22 provide output admittance of the amplifier**But if we take the reciprocal of it then we will get (Zo)output impedance (V2 / I2) of the amplifier.

__So to study amplifier these 4 parameters plays an important role__i.e.1)

**Input impedance****(Zi).**2)

**Output impedance****(Zo)**.3)

**Voltage gain****(Av)**. &4)

**Current gain****(Ai)**.Just see the video given below it will give you a great idea about the parameters to be used while designing any amplifier.

So in ac analysis or for small-signal analysis, the transistor model can be drawn as shown below.

**hie:-represents input impedance of transistor.**

**hre:-represents reverse voltage gain of the transistor.**

**hfe:-represents forward current gain of transistor it is also known as (b).**

**hoe:-represents open circuit or admittance of transistor.**

**(e) which is used in the above parameters represents that we are doing ac analysis of common emitter configuration.**

Now,

V1=Vin of the transistor which is in few millivolts while V2=Vce or output voltage which might be in volts but greater than Vin

Therefore

**h12=hre=V1/V2 is almost equals to zero (h12=hre=0).Thus it can be neglected**.Also,

I2=Ic or the output current of the transistor is in a few milli-amperes while the V2=Vce or output voltage which might be in volts

therefore h22=hoe=I2/I1 is almost equals to zero (h22=hoe=0).

So

**1/hoe becomes very large. Thus it can be replaced by an open circuit**.So the

**approximate model**of the BJT transistor can be drawn as shown below.I hope you understood it. If you like it then please let me know in the comment section.

Very nice article. I understood the concept very well explained.

ReplyDeleteThanks dear student

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