Electrical – How to bias a class C amplifier with negative voltage

I think you are picking up AM radio interference because of the relatively high resistances you are using. High resistances make it easier for a low energy signal like a radio wave to imnpress a voltage on a circuit. I would suggest 2 remedies. One is to reduce all of your resistors by a factor of 10. Why do you need to put such a large resistor (R4) in series with your output? Another remedy is to put a small bypass capacitor across R2 to filter out the AM signal. If you reduce R2 to 10k, then a capacitor of about 200 pf should be sufficient. If you reduce R2 to 10k, then use a capacitor of about 2000 pf.

Let's start our with a single op amp and work out way toward a class AB design. Consider the voltage follower.

^{simulate this circuit – Schematic created using CircuitLab}

In this case \$V_{in}=V_{out}\$. Why? Negative feedback. Negative feedback forces the inverting pin's voltage to match the non-inverting pin. In other words, the op amp will do whatever it takes with its output to make \$V_{NI}=V_{INV}\$.

Let's take that a step further. We need more power to drive a low impedance speaker. Well, the average op amp will only have a few tens of milliamps of drive capability. That is where we add the power stage.

^{simulate this circuit}

By tying the inverting pin to the output of the power stage, we have created a voltage follower. \$V_{in}=V_{out}\$, but now the circuit has the ability to deliver much more current than the op amp output ever could. Because the op amp has negative feedback, \$V_{NI}=V_{INV}\$. The cross over distortion is eliminated by the op amp doing whatever it takes to satisfy that relationship. As an exercise, build this up on a bread board, put a sine wave into \$V_{in}\$, and observe the output of the power stage and output of the op amp. The two will not look anything alike!