# 2 stage transistor amplifier

Discussion in 'Homework Help' started by mcgregor.1, Dec 7, 2009.

1. ### mcgregor.1 Thread Starter New Member

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Nov 22, 2009
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I need help designing a two stage voltage transistor amplifier with a gain of 30. I already have the first stage worked out for a gain of 5, is it right that I just use the same calculations to calculate the 2nd stage for a gain of 6. ??

When I have tried it the gain on the 1st stage drops, is there a way I can calculate the voltage drop. ?

2. ### mik3 Senior Member

Joined:
Feb 4, 2008
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Post the circuit and your calculations.
Which gain are you talking about, voltage or current gain?

What is this amplifier for?

3. ### mcgregor.1 Thread Starter New Member

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Nov 22, 2009
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It's a voltage amplifier, it's a project for uni.

Calculations
Chose Ic 1mV
Ve= 10% of VCC
R4= Ve/Ic =900Ω
R3= Vcc/2*Ic= 4.5K
R2= 10*Re=9K
R1=Vcc-(Ve+0.7)/Ve+0.7 * R2 =9KΩ

Used the same calculations for 2nd stage but to calculate R8 I used R3/R4 to get the gain of 6

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4. ### mik3 Senior Member

Joined:
Feb 4, 2008
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When you derive the AC model of the circuit to calculate the gain of the first stage you have to consider the total input impedance of the second stage in your calculations. Thus, in this case consider R3//R5//R6//β*R8 to calculate the gain of the first stage.

β is the beta of U2

You have to consider the total input impedance of the second stage because it loads the first stage and voltage is dropped across the output impedance of the first stage and thus the output voltage of the first stage drops.