# BJT Amplifier

Discussion in 'General Electronics Chat' started by powzoom, Jan 18, 2009.

1. ### powzoom Thread Starter Active Member

Jan 18, 2009
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I've been trying to learn how to design amplifiers with BJTs but I cant seem to figure this out. I have a BJT biased with a voltage divider. I know Vcc = 9V and Vin = sin wave with 1V peak. How can I achieve a voltage gain of about 5? Beta min = 100. Can someone take me through the calculation steps because I've looked everywhere and can't seem to figure it out.

Thanks.

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2. ### mik3 Senior Member

Feb 4, 2008
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Are you familiar with the pi model of the BJT?

3. ### powzoom Thread Starter Active Member

Jan 18, 2009
41
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I've heard about it but I am not to familiar with it. I'll try to look it up.

4. ### PRS Well-Known Member

Aug 24, 2008
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The maximum input to a common emitter amplifier as you have shown is about 10 mV peak, or 20 mV peak to peak. Larger values of input voltage distort due to the nonlinear relationship between the output current and the input signal.

But, if you insist on calculating the gain for a typical 2N2222 or similar transistor used as a CE amp as per your drawing, bias Ic for about 1mA, then the gain is approximately -.04*Rc

5. ### mik3 Senior Member

Feb 4, 2008
4,846
69
I agree that if the bias collector current is 1mA then the voltage gain will be about -0.04*Rc. If you use a 125 ohm resistor for Rc then you will get a gain of -5 approximately. However, this gain is valid for a range of frequencies, at low and very high frequencies the gain will decrease due to capacitances in the circuit.

6. ### Wendy Moderator

Mar 24, 2008
21,528
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A general rule of thumb for gain of a amp is Rc/Re. Rc must include the load figured in parallel however for this to have a hope. C3 can be eliminated. It's probably time I redrew this picture to reflect these questions (you're not the first, and won't be the last).

Want to go through the biasing math?

7. ### powzoom Thread Starter Active Member

Jan 18, 2009
41
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This is intended to amplify an audio signal. If over 10mV gets distorted, then what would be a better design?

8. ### powzoom Thread Starter Active Member

Jan 18, 2009
41
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Yea Bill, any math would be helpful. I'm sure this won't be my only time designing an amplifier.

9. ### Wendy Moderator

Mar 24, 2008
21,528
2,973
It depends, there are amps out there that have no voltage gain, but lots of drive. Technically you would say their amplifying current.

The thing of it is you should know what the impedance or resistance your amp is feeding, and work from there. The Common Emitter I drew is medium impedance in and out, and will do a lot of jobs. The key is to have it in the middle of its linear region, the distortion you refer to happens on cutoff or saturation. Basically get the biasing right, and it all falls out.

Rereading your goals it can't be done. You want a power supply of 9VDC, no problem. You want an input of 1V Peak, this translates to a peak to peak value of 2V. 2VP-P X 5 (gain) is 10VP-P, with 9VDC this won't work. See the problem?

Feb 4, 2008
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11. ### powzoom Thread Starter Active Member

Jan 18, 2009
41
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I kinda follow what everyone is saying. I'll see if I can work it out. Thanks for the help.