Transistor amplification related doubts

[shawrix]

I understand the basic concept. A small change in the base current will lead to large change in the collector current and will lead to amplification.

But the question is, the small change in base current is brought by input signal or the biasing? Also the coupling capacitors allow only the ac signal to be amplified, so what's the use of dc biasing? It will be wasted.

Also what's the effect of ac changing its polarity on the transistor?

Some doubts...But i will love to have them cleared. Thank you :]

Edit: Small not large change in base current.

 

[willem2]

I understand the basic concept. A small change in the base current will lead to large change in the collector current and will lead to amplification.

But the question is, the large change in base current is brought by input signal or the biasing?

I don't understand this. What large change in the base current?

Also the coupling capacitors allow only the ac signal to be amplified, so what's the use of dc biasing? It will be wasted.
[/QUOTE[

You would get an ac signal without DC component after the coupling capacitor if there was no bias network. The input of an emitter follower needs to be about half the supply voltage with no input signal, the input of a common emitter amplifier, often needs to be set so that the output will be at half the supply voltage with no input signal. It certainly will need to be positive.
The biasing does this.

Also what's the effect of ac changing its polarity on the transistor?

You mean the input signal? The bias network takes care of that. An amplifier will work with voltages that are between the supply voltages, so negative voltages shouldn't occur if you have a single supply and the negative side of the supply is at 0 volts.

 

[shawrix]

I don't understand this. What large change in the base current?

Sorry, I meant a small change in base current leading to a large change in collector current is the basis of amplification. This small change is the change in input signal right? The biasing is not changed throughout the amplification process?

For example, in a speaker while adjusting its volume do we change the input signal or the biasing? Quite confusing :S

 

[Jony130]

Try read this
https://www.physicsforums.com/showthread.php?t=709791#post4499103
http://pcbheaven.com/wikipages/Transistor_theory/?p=6

 

[meBigGuy]

None of this is constant for all applications, but generally the bias sets the DC operating point such that the small signal is being amplified by the transistors at an optimum point in their characteristics. The gain is generally changed by a resistor divider or resistor ratios somewhere in the amplifier. Generally the bias remains constant.