# BJT Common Emitter Amplifier

Discussion in 'Homework Help' started by The_Cook, Oct 20, 2014.

1. ### The_Cook Thread Starter Member

May 29, 2014
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0

The values are :
• Vs = 10 mV
• Rb = 100 Kohms
• Rc = 2 Kohms
• Rl = 4 kohms
• gm = 170 mA/V
• r(pi) = 580 ohms
• Rs = 50 ohms
For this circuit I have to draw (a) draw a small signal amplifier equivalent circuit and (b) find the mid-band voltage gain for given Vs

I would appreciate if someone helps me get started with the equivalent circuit.

Thanks!

2. ### crutschow Expert

Mar 14, 2008
12,535
3,070
That circuit will not amplify AC as it is not biased. The output will basically be a half-wave rectified signal. Here's an example of a transistor biased to amplify an AC signal.

3. ### The_Cook Thread Starter Member

May 29, 2014
48
0
Okay but can a small signal equivalent circuit be drawn for it? I am kind of lost on where to begin

4. ### b1u3sf4n09 Member

May 23, 2014
115
14
Last edited: Oct 21, 2014
5. ### The_Cook Thread Starter Member

May 29, 2014
48
0

This is what I came up with, could any tell me if it is right or if I need to fix anything?

6. ### Jony130 AAC Fanatic!

Feb 17, 2009
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Everything looks ok except Rs resistor, it should be connected in series with Vs

7. ### MikeML AAC Fanatic!

Oct 2, 2009
5,450
1,066
However, as Zapper pointed out, the hybrid PI model cannot be used to model the original schematic because the hybrid pi model assumes that the biasing has already been done.

8. ### crutschow Expert

Mar 14, 2008
12,535
3,070
The problem is that you will have to use a value of zero for the model gain (gm) since the transistor is biased off.

9. ### The_Cook Thread Starter Member

May 29, 2014
48
0
Yeah I understand that the Hybrid Pi model cannot be used. But if we assumed that it is biased how would you go about solving the problem? I don't know if my professor made a mistake when he wrote it or not?

10. ### Jony130 AAC Fanatic!

Feb 17, 2009
3,936
1,088
Well, simply solve this small-signal circuit as a ordinary DC circuit.

11. ### The_Cook Thread Starter Member

May 29, 2014
48
0

Hey guys could you check my work real quick, appreciate it!

12. ### b1u3sf4n09 Member

May 23, 2014
115
14
With the given information, vbe = 0, therefore gain = 0. Not only is the amplifier not on, but Vs is a DC voltage, which will have some issues bypassing the coupling capacitor. Is there a given frequency for Vs?

13. ### The_Cook Thread Starter Member

May 29, 2014
48
0
Nope just the voltage

14. ### crutschow Expert

Mar 14, 2008
12,535
3,070
He made a mistake if he intended the amplifier to have any gain above zero.