# Determine voltage gain for the CE amplifier.

Discussion in 'Homework Help' started by BruceBly, Aug 6, 2012.

1. ### BruceBly Thread Starter New Member

Jul 26, 2012
25
0
I need to find out what the voltage gain is for this CE amplifier.

I have calculated the DC voltages and currents for this circuit but I am at a loss when it come to finding the voltage gain. Can someone tell me where to start. Below is what I have for the DC part of the circuit.

VR2=15V[4.7k/(22k+4.7k)]=2.64045V
VRE=2.64045V-.7V=1.94045V
IE=1.94045V/390=4.97551mA
IC=4.97551mA
VRC=4.97551mA(1.0k)=4.97551V
VCE=15V-(1.94045V+4.97551V)=8.08404V
VC=1.94045V+8.08404V=10.02449V
VB=2.64045V
VE=1.94045V

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2. ### absf Senior Member

Dec 29, 2010
1,462
356
You didnt include the values of C1 to C3. What type of signal at what frequency and amplitutde are you using for input?
What is the transistor you used and at what gain?

Did you try simulating the circuit using LTSpice?

Last edited: Aug 7, 2012
3. ### #12 Expert

Nov 30, 2010
16,021
6,537
Crudely put, the gain is almost Rc/Re (assuming the capacitors are large enough to not interfere)...but not quite. There is a loss due to Vbe changing a little with the change in collector current.

Gotcha pointed in the right direction?

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4. ### BruceBly Thread Starter New Member

Jul 26, 2012
25
0
I believe I was over thinking this problem. This is what I have came up with for the answer. Can someone confirm this for me?

r'e=25mV/4.97551mA=5.02461Ω
Av=1.0kΩ/5.02461Ω=199.02042

5. ### Yakima Member

Jan 23, 2012
35
2
Good work! You are correct!

6. ### Audioguru New Member

Dec 20, 2007
9,411
895
The circuit has no negative feedback so it is extremely distorted if its output level is more than moderate.
With an input of 30mV peak then its positive-going output is squashed to +3.5V peak and its negative-going output is -5.9V peak. It is impossible to measure its voltage gain.

If the input signal is only 2.3mV peak then its distortion is much lower and its voltage gain is 170.

A load will reduce its voltage gain.

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7. ### BruceBly Thread Starter New Member

Jul 26, 2012
25
0
This circuit was right out of my text book for me to analyze. I suppose I should have stated that in the first post. I'm not expected to build this circuit.

8. ### Audioguru New Member

Dec 20, 2007
9,411
895
I did not build the circuit, instead I simulated it. Soon you might be taught how to simulate circuits using simulation software like I did. If I did build it my results would be the same.

All the circuits I design and build use negative feedback to accurately set the gain lower, then the distortion is very low at any amplitude.

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