Amplification of audio signals

Hi there
I am slightly confused about audio amplification (particularly using transistors).
Does audio vary in current, voltage or both?
Also, if audio does vary in current, then how does a bias voltage shift the audio above ground?

Audio is a varying voltage, when this varying voltage is applied to a resistance, it causes a variable current to flow. If the resistance is infinite, like an unplugged lead, then the voltage would still be there but no current would flow.
Because of the high value of the resistors in a bias circuit, they do have a small audio current in them, but most of the audio current would flow into the transistor.
Frank

An amplifier transistor is biased so that with no signal, its output voltage is about half of the supply voltage so it can swing symmetrically up and down for the maximum amount of output swing when it has a strong input signal.
An audio signal feeding the input (capacitor-coupled to the base of the transistor) causes the base voltage and base current to swing up and down which causes the output (capacitor-coupled from the collector of the transistor) to also swing up and down but amplified so the voltage and current swings are much higher than the input.

Ask your professor to show you the audio voltage swings on an oscilloscope.

Thanks, will do!

An audio amplifier is an electronic device that raises the strength of audio signals that pass by it.

Hi in order to amplify audio do I need to amplify both the negative and positive input voltage or just the positive voltage ?

Audio means an alternating voltage and current with audio frequency. Therefore you have positive and negative voltage alternating.

Please read some basics on electrical engineering. Without that it is a waste of time trying to explain it bit-by-bit here.

I do not know what you mean by negative input voltage or positive input voltage.

An audio signal swings up and down. It can swing positive and negative if 0V is its average, or it can swing more positive and less positive if half the supply voltage is its average.
If your circuit clips off half of the signal then it will sound extremely distorted.

Hi is this an amplification of source ? purple is source, green is output.

NSunJ said:

Hi is this an amplification of source ? purple is source, green is output.

Click to expand...

Your oscilloscope is confusing because it looks like the green is voltage but the purple is current.
The green voltage is a positive signal only but the purple current swings positive and negative.
The green and purple have a 180 degrees phase shift.

Voltage amplification is provided by an amplifier when the amount of output signal swing is more than the amount of input signal swing.

you say you are confused, but the real magic of audio is how they make sure that the audio output does not clip when it receives the input signal from the mic.
The amount of amplification given to the mic signal is magically and dynamically "tailored" so that no matter how big is the mic signal, the audio amp output does not clip....now that is clever....because your audio amp is having to vary its gain depending on the strength of the mic signal.......and at the same time it must keep the volume to the selected level......I mean, normally, you would expect a bigger mic signal to give a bigger audio amp output signal....but this doesn't happen, because if it did, then you might get clipping and distortion.........so it is an extreme secret of the audio world how they vary the amplification of the audio amp in order to avoid clipping and distortion when the mic input signal suddenly gets bigger.

Audio compression sounds awful. People's voices and live music have NO compression and they sound perfect. If your mic needs a compressor so that the audio amplifier does not clip then either the mic level is too high (simply turn it down) or the audio amplifier's maximum power is not enough.

But this discussion is not what the OP is talking about.
The people asking do not understand simple Ohm's Law where a speaker is a resistor, increasing the signal voltage to it also increases the signal current in it.
The people asking do not understand that an amplifier "amplifies" the input signal so that it is increased enough for the speaker to make as much sound as you want if the amplifier has enough power.
The people asking might not know that a common-emitter transistor increases the signal voltage amplitude but might not provide enough current to drive a low impedance speaker and that the collector signal is out-of-phase (180 degrees phase shift) with the input.

EDIT: Many people play a radio or a stereo with its output clipping like crazy. Rock groups play and their amplifiers are clipping like crazy and they even use circuits (FUZZ FACE circuit for electric geetars) to produce more clipping distortion.