In a class C amplifier, how does RB biases the base of transistor?
What the resistor do is applying a negative bias. This is called self biased.
So imagine the load line, the bias point is below X axis. This means that the pulse duration is shorter above 0.7 V -> more efficiency. Also RC constant should be bigger than input pulse.
On the negative alternation the diode is reversed biased, and the circuit acts as a negative clamper.
On positive half wave, Rb shifts down the base voltage (subtracts from Vin).