# Different small signal equivalent circuit

Discussion in 'General Electronics Chat' started by samy555, Sep 13, 2014.

1. ### samy555 Thread Starter Active Member

May 24, 2010
116
3
Hello

"Semiconductor Devices and Basic Applications " page 435, the booktalks about:

Figure 6.63 (b) is the small signal equivalent circuit. Notice that the dependant current source directed from the collector to the emitter.

In other pdf file I found in:

http://www1.i2r.a-star.edu.sg/~ongmichael/Chapter67.pdf

On page 137 the following small signal equivalent circuit:

Here the dependant current source directed from the Base to the Collector.

My question: Is these MODELS are equivalent? If yes, how?

Thank you

2. ### t_n_k AAC Fanatic!

Mar 6, 2009
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In the first document note that $V_\pi$ is equivalent to Vbe. In the second document V is Veb. Hence the source direction reversal is consistent with the opposite polarity notations for the controlling parameter.

However, the source should presumably still be located between emitter & collector rather than base & collector.

May 24, 2010
116
3
thank you

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4. ### samy555 Thread Starter Active Member

May 24, 2010
116
3
Thank you all
I have discovered the reason for this discrepancy
The reason is that one of them using the hybrid pi model, where the other using the T model
please look at the attached file

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5. ### t_n_k AAC Fanatic!

Mar 6, 2009
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Good point.
One would also have to make the distinction that $\text{r_\pi \ and \ r_i }$ as shown in the two equivalent circuits aren't the same.

samy555 likes this.
6. ### samy555 Thread Starter Active Member

May 24, 2010
116
3
Yes, I agree entirely with you
Thank you t_n_k