Hi,

When designing an amplifier or oscillator in common-emitter configuration,
at the collector leg, sometimes the resistor is replaced with inductor?

What is the purpose of replacing the resistor with inductor?

 

Hi,

When designing an amplifier or oscillator in common-emitter configuration,
at the collector leg, sometimes the resistor is replaced with inductor?

What is the purpose of replacing the resistor with inductor?

Higher AC gain. The inductor can be selected to have a higher impedance at the signal frequency than a resistor which gives higher AC gain. The value of the resistor is limited by the required transistor bias current and supply voltage.

 

Also, when using the (NPN) transistor in an RF application, the inductor (RF choke) will assist in keeping RF from straying off an intended path.

 

Higher AC gain. The inductor can be selected to have a higher impedance at the signal frequency than a resistor which gives higher AC gain. The value of the resistor is limited by the required transistor bias current and supply voltage.

Why cant we just put a resistor which its value is the same as the impedance of the inductor at the specific frequency?

 

Why cant we just put a resistor which its value is the same as the impedance of the inductor at the specific frequency?

Because the impedance of the resistor would be too high to carry the desired bias current for the transistor using a normal low amplifier supply voltage.

 

Because the impedance of the resistor would be too high to carry the desired bias current for the transistor using a normal low amplifier supply voltage.

Can you explain in more detail?

 

Hello,

Have a look at this PDF:
RF-prototyping.pdf

From page 8 and on there is an explanation on the RF chokes.

Bertus