Plus Signaling

Why use many different stages of transistor amplifiers?

If I use vcc= 9v, then my output signal should swing maximum 9v-pp also. why should I use many stages of amplification, even my signal will never be more than 9v-pp. Plus what is difference between small signal amplifier and power amplifier. Do I have to used different type of transistors for small signal and another kind for power amplifier.

The 'end result' desired is to make a speaker cone vibrate, thereby producing sound. Typical speaker is 8 ohms impedence. To produce 10 watts thru 8 ohms at 9Vcc ( P=IE ) will require amplified signal of 1.1 amps.

Small signal transistors can carry currents of 50ma typically without overheating ( NOT enough for power amp stage.You'll need at least 20 times that!! ). Yet the base current of a power transistor will require at least 40ma typically...More than a microphone output can handle.

The solution? Amplify the microphone ( or whatever) input using a small signal transistor ( pre-amplifier ), then feed the collector of the pre-amp stage to the base of a power transistor, using resistors to bias as needed.

 

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