# BJT Common Collector Amplifier

Discussion in 'Homework Help' started by The_Cook, Nov 17, 2014.

1. ### The_Cook Thread Starter Member

May 29, 2014
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Hi guys I need some help with starting this problem. For question (a) don't I need to draw the mid-band small signal amplifier equivalent circuit given below to calculate the base current, emitter current and rPI?

2. ### The_Cook Thread Starter Member

May 29, 2014
48
0

This is what I have for (b) going off of what was given in the circuit. Is this correct?

3. ### Jony130 AAC Fanatic!

Feb 17, 2009
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1,115
Your small-signal model is wrong. In CC amplifier Vo is across emitter, and emitter is not connected to GND the collector is .

4. ### The_Cook Thread Starter Member

May 29, 2014
48
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Ok I was just basing it off of what was given with the question. So I'm guessing that's wrong?

Secondly for the first question how do I calculate the IB and IE and rPi?

5. ### Jony130 AAC Fanatic!

Feb 17, 2009
3,990
1,115
Simply use this small-signal equivalent circuit

And draw the current

As for the part A, simply use circuit theory (KVL, KCL etc) and solve for Ib, Ie

6. ### dalam Member

Aug 9, 2014
58
6
I have solved this taking some approximations.
I calculated Ib using KVL in the base emitter loop. I found it to be 18.87 micro amps.
Then to find Ic I used Ic=ßIb. It turns out to be 1.887 mili amps.
And Ie = (ß+1)Ib=1.9mili amps.
To find Rpi =ß/gm(trans conductance)= ß/(Ic/Vt)=1.377Kohms.
But my Ic does not matches with the one given in the question. Will you please tell me what am I missing here?

7. ### Jony130 AAC Fanatic!

Feb 17, 2009
3,990
1,115
You have not missed anything, Ie is 1.9mA. But is doesn't matter, for small-signal analysis we assume Ic =2mA.
And re = 26mV/2mA = 13 ohm so Rpi = 101 * 13 = 1.313k ohm. As you can see not much different from yours Rpi.

May 29, 2014
48
0
9. ### dalam Member

Aug 9, 2014
58
6
How are you calculating Ie is beyond my understanding. Check your Vbb and the equivalent resistance at base. You need to look at the pdf once again and understand what does this mean. Merely using formulas are not going to help. Clear your circuit theory concepts as well. You can use https://www.scribd.com/doc/37003448/Microelectronic-Circuits-6th-Edition .