RFC at the collector leg of transistor

Thread Starter

xylon89del

Joined Dec 28, 2011
17
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?
 

crutschow

Joined Mar 14, 2008
24,078
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.
 

radiohead

Joined May 28, 2009
508
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.
 

Thread Starter

xylon89del

Joined Dec 28, 2011
17
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?
 

crutschow

Joined Mar 14, 2008
24,078
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.
 
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