I'm trying to build a simple class a amplifier using a mosfet. I have read few articles about how to do it, but I'm just confused now. I'm not sure how to select resistor values. The articles I've read show how to calculate this values, but the requirement is conduction parameter "k" which is not in documentation for transistor I have(IRFZ44N). I tried calculating this value from equation k = Id/(Vgs-Vth)^2 after measuring current and voltages for different gate voltage, but the "k" I get isn't constant.
I also tried selecting resistor values without calculations and with that I have the most success, however the output signal is not amplified, it is diminished.
I chose Rd so that Vd is Vcc/2. R1 and R2 set gate voltage to 4.8V, because gate threshold voltage is 3V and sawtooth amplitude is 500mV(I can set Vg lower, but it doesn't change much). I'm not sure about Rs. The 8.2 Ohm resistor is a "speaker".
Why is the signal not amplified? I realise that the answer might be complex, so I would be also grateful for some good study material books/tutorials etc..