Question about common emitter configuration


The configuration of the common emitter amplifier is one resistor at the emitter, on resistor at the collector, two bias resistors at the base as shown in this picture :

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I saw this new configuration, My questions are : What is the benefit from this new configuration ? or Why is it necessary to change the previous configuration ?
What will happen if I used the previous one instead of the new one ?
Which one is more useful for RF applications ?

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Best Answer

First, the two schemes presented, showing common emitter configurations. The name of this configuration, because the emitter terminal is shared by the input circuit and the output circuit.
The main difference between the two, is polarization. While the first uses a resistor divider to set the base current at rest, the second uses the resistive circuit, associated to the collector. In principle, by using the resistor divider, can select resistance values ‚Äč‚Äčthat give good bias stability against variations own transistor parameters.

As for the application of both circuits in RF, I think it's more a matter of working with class C amplifiers and tuned output circuits. Perhaps you have generated some confusion the impedance characteristics of the three basic configurations:

  • A common-base amplifier is a good voltage amplifier with a high output impedance.
  • A common collector amplifier is a good current amplifier with a low output impedance.

There is a mixed configuration, called "cascode" which is widely used in RF circuits, but as far as I know, should be implemented with two transistors.