Discussion in 'Homework Help' started by Titanium, Nov 8, 2009.

1. ### Titanium Thread Starter New Member

Nov 8, 2009
7
0

In figure 3a) there is shown an amplifier circuit based on a npn-transistor and figure 3b) illustrates its small-signal model.

(a) Explain what is the small-signal model and what is it used for. What is
descriped by the symbols (r_π, C_π, etc.) in figure 3b)? Is there anything
missing?

(b) Determine the gain of the system A = Vo/Vs at low frequencies (C_C1
and C_C2 more significant than the other capacitances) assuming RB》r_π. Explain qualitatively what happens at very high frequencies and at
middle frequencies.

Figure：http://i645.photobucket.com/albums/uu173/lxazy/ji.jpg?t=1257527280

Thanks a lot for looking and helping!

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Last edited: Nov 8, 2009
2. ### Titanium Thread Starter New Member

Nov 8, 2009
7
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Thank you!

Apr 5, 2008
15,806
2,389
Hello,

You can post the picture here by using the manage attachment button:

Greetings,
Bertus

4. ### Titanium Thread Starter New Member

Nov 8, 2009
7
0
Here is the figure:

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5. ### Titanium Thread Starter New Member

Nov 8, 2009
7
0

Apr 5, 2008
15,806
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Hello,

I am not familair with this kind of network representation of the amplifier.
I have made notes as I see the parts in the network.

Greetings,
Bertus

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7. ### Titanium Thread Starter New Member

Nov 8, 2009
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I see, thanks anyway!

8. ### hobbyist Distinguished Member

Aug 10, 2008
774
62
The small signal model, represents the signal (AC) voltages work at, it does not consider the DC bias voltages, but only what refers to AC currents only.

The resistances have been thevenized and such, if you look into the input this will be what the input signal see's, it sees R1 and R2 as a single resistance, in parrallell with the series resistor RE and re, denoted as R pi.

The output current B*ib see's Rc and Rl in parrallel, depending on the frequency of this input signal, would determine the phase shift the signals see with the capacitors, as well as the reactances these capacitors offer to the signals, if the signal frequency is high enough, then sometimes the capacitors can be represented as shorts across them as the signal is not Aaffected by them, but that again deals with the frequencies involved.

This setup is useful when doing signal analysis on an amp stage.

9. ### hobbyist Distinguished Member

Aug 10, 2008
774
62
Titanium:
Because the forum is for all to benefit from in learning by questions asked.

Also If I give a wrong answer to a question someone more knowledgebale would have an opurtunity to interject and correct any wrong advise I may give.

With that being said: