Look at the picture, which shows a Class C power amplifier (source). What is the role of the resistance in the base of the transistor, and what happens if it's removed?
A simple answer:
The capacitor Cin fully charges through the base-emitter junction during the positive half wave of Vin and slightly discharges through R1 during the negative half wave. Thus it can be thought as of a "rechargeable battery" connected in series to Vin.
During most of the positive half wave, its voltage subtracts from Vin and it acts as an opposing voltage source that shifts down the base voltage with about 0.7 V. Only at the top the effective voltage is enough to turn on the base-emitter junction and accordingly, the transistor.
Since the input impedance is low when Vbe>0.6V <100 Ohms the base resistor should R1 pulls up more than down to 0V. R1 forces the conduction duty cycle or phase conduction angle to increase. The concept is to pump as hard as possible for the shortest % of time so that the Bandwidth of the filter can reject the modulation noise when saturated.
Removing R1 only reduces the pulse duration when there is sufficient drive as it should be. If there is insuff,drive , it turns off.
This shows with a slider for R1 on the right. You can also remove it and vary the drive level.
Class C means pumped LC resonator with practical efficiency 50% to 80% as it only conducts in saturation as a switch for low duty cycle to apply some modulation frequency to the carrier.
It also means "low fidelity" modulation so BP filtering is essential. REF
What is the role of the resistance in the base of the transistor, and what happens if it's removed?
The peak current and pulse width is then increased, no problem.
The example in question is not really useful but just a proof of concept. Whereas the above is more practical of a high efficiency RF AMP.
Cin and Vbe form a positive clamp circuit such that the Vbe = 0.6Vpk at 1mA and 0.7Vpk at >10mA and also the input impedance drops with the base current. Thus the dV/dt=Ib/Rbe for the peak only resulting in a sharp narrow pulse on the peak input if ~2Vpp Increasing the amplitude just saturates the Vce(sat) more and the current and carrier amplitude then increases.
Advantages of Class C power amplifier.
High efficiency. Excellent in RF applications. Lowest physical size for a given power output.
Disadvantages of Class C power amplifier.
Lowest linearity. Not suitable in audio applications. Creates a lot of RF interference. It is difficult to obtain ideal inductors and coupling transformers. Reduced dynamic range.
Values not intended to be a useful design, just to show some characteristics. It could be used for FM with a varicap to modulate the frequency and keep at constant envelope.
In an rf amplifier that prevents parametric oscillation.
The capacitor in series with the base is provided to keep DC off the input from rectification of the drive signal. In a lot of Cb type amps it is omitted, but that isn't proper.
That is all. The ARRL manual for the Radio Amateur discusses this in the rf amplifier section..... At least it used to.