# common emitter amplifier

Discussion in 'Homework Help' started by metal_militia7, Jun 13, 2010.

1. ### metal_militia7 Thread Starter New Member

Jun 13, 2010
13
0
Hi, this is my first post .
I have a project to do where i have to design a common emitter amplifier without bypassing capacitor with the gain of 12. I have been asked to make the input resistance 500ohms or greater.
I am asked to get the load resistance of 1k, so i decided to put Rc as 1K also in order to transfer max power.
the gain of must be given by Av=- Rc||RL/(RE+re') is it ? or not!
input resistance 500ohms or greater. Does that mean R1||R2 or not?
also confused in selection of Vcc
Any hints would be highly appreciated.

2. ### hobbyist Distinguished Member

Aug 10, 2008
774
65
You pretty much got the formulas right for first order approximations, of component values.

Av = (RC // RL) / RE (would suffice for a first order approx.)

VCC is chosen on a lot of different approaches as well.

With your design asked for, you can arbitrarily choose a value for VCC and work with it.

If a specific input or output voltage signal swing is required, than you would need to calculate a value for VCC to meet the requirements.

If you need more refined values, than input resistance will include the Beta of the transistor, as well as 're' and RE.

Zin= (R1//R2) // {(B+1) x (re + RE)}

Last edited: Jun 13, 2010
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3. ### metal_militia7 Thread Starter New Member

Jun 13, 2010
13
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I have figured out most of the parts values but stuck in selecting the values for the coupling capacitors.
Thank you hobbyist and one more help about selecting capacitors and i am done. waveforms are distorted,clipping and i think it is because of wrong capacitor values. Xc=1/(2pi f C)
thanks

4. ### Jony130 AAC Fanatic!

Feb 17, 2009
3,993
1,116
Post the diagram and show the values of a components.
Capacitor don't have much effect on clipping.
And the size of a capacitors is selected that Xc is much smaller then resistance of a cooperating resistor at the lowest frequency of operation Xc=0.1*R.
Xc=1/(2*PI*F*C)=0.16/(F*C)
C=0.16/(F*R)
For example;
Cin = 0.16/(Zin*Fc)
Ce = 0.16/(re *Fc)

And if you have problem with clipping then you need to increase Vcc or set Ic close to

$Ic_{opt}=\frac{Vcc-Vce(sat)}{2(Rc+Re)}$

Or reduce the input voltage or the gain.

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5. ### metal_militia7 Thread Starter New Member

Jun 13, 2010
13
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it was of great help the clipping was due to high input voltage thank you!

6. ### metal_militia7 Thread Starter New Member

Jun 13, 2010
13
0
I recently bought this usb oscilloscope and i build my common amplifier circuit in ET 3100 and did the measurement for VCEQ and VBE, it seems fine but when i Put signal to the input, and try to get the out amplified, it would not result right ...if any one has used this oscilloscope, please help me with using this oscilloscope... thanks