Calculate BJT Amplifier amplification

amplifierbjt

I want to calculate the amplification of the following amplifier.
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I used the re model.

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I do the calculation this way..

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So, in my case:
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So according to my calculation the amplification factor is 39.4.

Here's the problem.
When the input signal has amplitude of 0.01v, the output wave swings from 3.54v to 2.01v
Thus an amplification of (3.54 – 2.01)/0.01 = 153 (not even near what I have calculated.)

What am I doing wrong?

Ps:re value is multiplied by 2, because, I think with an Ie of only 0.48mA, the junction is in the knee region, where the resistance is more.

Best Answer

What am I doing wrong?

(1) You're dividing the output peak-to-peak voltage by the input peak voltage. When you say the input has an amplitude of 0.01V, that is the peak value. So, your actual voltage gain is half of what you came up with - 76.5 versus 153

(2) I don't believe the factor of 2 for your calculation of \$r_e\$ is appropriate. If you use the typical

$$r_e = \frac{26mV}{0.48mA} = 54.2 \Omega$$

instead, the calculated gain, ignoring the capacitor impedance, becomes

$$|A_v| \approx \frac{4400}{54.2} = 81.2 $$

which is close to the 76.5 value I believe your actual gain is.

Including the capacitor impedance, the gain is

$$|A_v| = |\frac{4400}{54.2 - j3.18}| = 81.0 $$

so the capacitor impedance is almost insignificant.

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