Continuing my hunt to gain some understanding of oscillators, I'm posting this video clip of a basic audio amplifier.
I've written down the schematic and worked example here, so please have a look https://drive.google.com/file/d/0B5wtctB1jrOnY1RHX0xtNXk0N1k/view?usp=sharing
Oscillators use a frequency-determining circuit (like an LC tank circuit) which creates the timing, an amplifier that magnifies this frequency and a positive-feedback circuit, to push energy back into the frequency-timing circuit.
The 'secret' of a one-transistor amplifier seems to be to energize the transistor to hold a collector voltage that is half the supply voltage. In my example, using 10v supply, the aim is to get the transistor collector (output) midday - this allows the amplifier to produce an output voltage with the largest range to swing up or down. This is known as a Class A amplifier arrangement.
I used an 2N3904 NPN transistor and built the circuit shown here with essentially uncalculated resistors, measuring the base and collector currents, the ratio was shown as a gain of 245.
A few more assumptions and I calculated my resistors to get that half supply collector voltage output.
The 'decoupling capacitor' is needed for the following reason: with no input signal, the amplifier and transistor is in a 'quiescent state; around 5v. When input signals are applied, the output voltage would fluctuate around 5v - this is 'varying dc', which is an ac voltage superimposed on a stable dc voltage. To simplify the output for other circuits or speakers, etc., we just want the ac signal. A capacitor removes all dc, allowing the low (audio) ac frequencies to pass through. I used a '103' 10nF cap in this circuit.
To show the amplification, I use a piezo transducer: these are high impedance (voltage/current = large ratio) devices that convert voltage into sound and vice versa, but work with tiny currents. The amplifier increases the output current to effect large 3-4-5v signals as seen on the oscilloscope.
Thanks for looking.
