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Old 19th November 2012, 09:27 AM   #1
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Default Question on Transistor amplification

Hi,
Please consider this as a newbie question. I have been trying to read up on how a transistor amplifies signals.
From my understanding in say a common emitter mode, the VBE is around .7V. Superimposed on this the Signal voltage effects the base current which is then amplified into the collector current producing a larger drop on the Load resistor on the collector.
The question is the Source voltages we encounter in a typical say CD line out could be couple of volts. Would this not either drive the transistor either heavily into saturation or the other way around?
I do understand there is a coupling capacitor but am not sure if this will actually reduce the voltage from a CD line out?
Hope my question is clear.
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Old 19th November 2012, 09:35 AM   #2
lineup is offline lineup  Sweden
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We differ about VDC and AC voltage.
You run the transistor at a certain Volt DC. This with no signal.
When we add AC voltage through the capacitor, the voltage becomes VDC+VAC.

A good rule of the thumb the DC voltage across emitter resistor should be at least 1 Volt.

You need to search with Google for transistor basics.
There are good tutorials out there.
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Old 19th November 2012, 10:07 AM   #3
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As you say (and lineup reaffirms), the AC signal is superimposed on the DC base voltage. Proper bias also includes the output, the collector in the common-emitter configuration - if it isn't right then yes, saturation or cutoff will occur.
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Old 19th November 2012, 10:23 AM   #4
Mooly is offline Mooly  United Kingdom
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These two threads might give you some insight,
How to derermine the optimum bias resistor value ?

Basic Common Emitter Amp Help

as lineup hints at, the emitter resistor in a common emitter amplifier plays a huge part in determining the amplifiers behaviour. You need to really understand how the stage works and how the choice of two resistors allows the designer to set the gain.
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Old 19th November 2012, 11:31 AM   #5
DF96 is offline DF96  England
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Negative feedback is the answer to the OP's actual question; either local feedback such as emitter degeneration or global feedback. NFB reduces the signal voltage which is actually applied across the base-emitter junction. Without this you would get severe distortion, as you said.
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