question about transistor design

The simple bias uses fewer components and has the advantage that there is no negative feedback. It needs to be set up accurately in the audio circuit, taking into account the gain of the transistor.

In the transmitter circuit, the smallest feedback is best and the highest supply voltage is needed for efficiency.

Sometimes the collector - base resistor is split and the junction bybassed to reduce feedback and so increase gain.

 

On the second circuit, for the T3, the transistor needs a small dc current on base, this is an class C amplifier, that has much less than 180 degree conduction periods.

This circuit isn't linear like the other on the first picture, the only advantage is to have a greater efficiency than the class A amplifier (T1), the output of this circuit has a high harmonic distortion, usually on commercial projects this output is filtered by a low pass filter.

As said above, to put two resistors on the base terminal causes to lower the ac resistance and lowers the gain too, one advantage of doing this is to cause a lower dependancy of the bias by the temperature (Vbe decreases with a higher temperature), causing a more stable bias, this is one great concern on RF small signal amplifiers and oscilator circuits.