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I Dont Understand

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rag's

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hello,
why do we use an emitter bypass capacitor in a CE configuration(for a small signal amplifier)??What happens if we dont use it??how will the output look for a sinusoidal input when there is no emitter bypass capacitor??
 
The AC voltage gain of a common-emitter transistor amplifier is almost the collector resistor value divided by the unbypassed emitter resistor value.
Therefore the voltage gain is much higher when there is a capacitor bypassing the emitter resistor.

Without an emitter bypass capacitor the transistor has a low voltage gain because it has high negative feedback. Then its distortion is very low.
when the emitter resistor is bypassed with a capacitor then the transistor's distortion at high output levels is very high.

Here is a simulation of a transistor with and without its emitter resistor bypassed by a capacitor.
 

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Hi audioguru

One question about your cuircuit. I want to set quiescent point to middle of load line. Which is proper base current( what R1 and R2) for achieve it?
 
Hi audioguru

One question about your cuircuit. I want to set quiescent point to middle of load line. Which is proper base current( what R1 and R2) for achieve it?
Transistors have a range of current gain so they also have a range of base current in a circuit.
The 2N3904 has a current gain from 100 to 300. I selected the divider's current to be 10 times the base current if the gain is 200.

Transistors have a range of base to emitter voltage. I selected the emitter resistor value to be 1/10th the collector resistor value so that the emitter voltage (and also the base voltage) are high enough that the actual base to emitter voltage doesn't matter.

At idle, the base voltage is 1.1V and the emitter voltage is at about 0.45V. Then the collector voltage is at about 5.5V. The signal at the collector can swing up to 10V if there is plenty of negative feedback and can swing down to 1V.
 
Transistors have a range of current gain so they also have a range of base current in a circuit.
The 2N3904 has a current gain from 100 to 300. I selected the divider's current to be 10 times the base current if the gain is 200.

Transistors have a range of base to emitter voltage. I selected the emitter resistor value to be 1/10th the collector resistor value so that the emitter voltage (and also the base voltage) are high enough that the actual base to emitter voltage doesn't matter.

At idle, the base voltage is 1.1V and the emitter voltage is at about 0.45V. Then the collector voltage is at about 5.5V. The signal at the collector can swing up to 10V if there is plenty of negative feedback and can swing down to 1V.
Thanks for the Nice treatment AudioGURU. Thanks also to Sniper007 which lead to this answer.
 
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