I'm studying the 4 resistors bias configuration, I'm trying to understand why we need everyone of these resistors.
If I understood correctly, Re is used to make the Q-point stable, while R1 and R2 are used to determine Vbe, but I can't understand why we need RL.
I also would like to ask why in the common base configuration we have a bypass capacitor in parallel with R2 and in the common collector we have a capacitor between the collector and the ground. Why we need to bypass these resistors in these cases?
I understood that in the common emitter we bypass Re because it reduces the gain, is it the same thing in the others configurations?
Thanks
Answer
We need RL otherwise the output voltage would always be equal to the supply voltage, this is true for the common emitter amplifier,but if we take the emitter voltage as output voltage(common collector configuration) we don't need RL anymore.
Moreover the DC component of the output voltage depends on RL
About the bypass capacitors, we use them because some resistors reduce the gain(but we need these resistors during the bias).
Thanks to eveyone.
Best Answer
RL is not required to bias the transistor collector current, however without RL you will get zero output, since the output would be connected directly to Vcc if RL is replaced by a short.
Typically you want to make RL as high as possible to maximize the voltage gain, but not so high that the output clips high or low with extremes of input voltage and taking bias tolerances into account. That means that RL is usually selected so that the collector voltage (with no input) is more than half of Vcc.