Question

a. Why are amplifiers needed in electronic circuits?
b. Will amplifiers make the S/N increase, decrease of stay the same?

Answer 1

a. Why are amplifiers needed in electronic circuits?

The simplest “current” amplifier is a step-down transformer. It does not amplify power, but it can exchange voltage for current. A step-up transformer does the opposite, it swaps current for voltage - but, neglecting losses, the power in, equals the power out,

Most amplifiers boost power, ie voltage and/or current. A simple common collector (common drain) amplifier is a basic example of an electronic circuit that amplifies current, but not voltage. A common-emitter circuit is, by contrast, can be both a voltage and current amplifier.

A radio receiver extracts a narrow band signal out of a sea of wide band signals and noise, and amplifies the power of what is received by as much as 150+dbm. Ie it can get a radio signal of -120dbm (a millionth of a millionth of a milliwatt), and generate maybe a watt (+30dBm) of power to drive a small speaker with the signal modulated on that radio signal.

That requires amplification of both voltage and power.

Prime example: an audio signal from a CD player or MP3 player, needs to be amplified significantly to drive a speaker.

A common CD player level audio signal is about a volt into 10,000 ohms or more, that is about 0.1 mA from the CD player.

An amplifier in a stereo receiver or amplifier will take that signal and increase it to typically 1 to 10 watts in a typical home situation (sometimes much more for loud stereos in big areas - auditoriums and stadiums). Since speakers are 8 ohms, the current drive is required to be around 0.35 to 1.1 Amps. A current gain of roughly 3,500 to 11,000.

b. Will amplifiers make the S/N increase, decrease of stay the same?

Increase