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Class C amplifier biasing.

paulmdrdo

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Hello. What is the role of RB resistor here? I know that the -Vbb biases the transistor below cutoff but I am wondering what's RB's function? Can I just remove that? Thanks.

2020-05-23 19_42_44-electronic-devices-ninth-edition-thomas-lfloyd.pdf _ - Foxit Reader.png
 

KlausST

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Hi,

Hello. What is the role of RB resistor here? I know that the -Vbb biases the transistor below cutoff but I am wondering what's RB's function? Can I just remove that? Thanks.
I recommend to use a simulation tool. Some are free, quite good and easy to use.
Then you can play around and see the obvoius.

Like here:
Wihtout a resistor then there is a constant negative voltage at the base. Independent of the input signal. What do you expect then?
Yes you can remove it, but then nothing happens anymore...
You then may even remove the BJT, Rc, Vout and VCC.... without further changes.

Klaus
 

BigBoss

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You cannot remove it because that resistor limits Base Current
 

albbg

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First of all setting the base voltage to -Vbb means the transistor starts to conduct when the input waveform reach roughly Vbb+0.7 V. In practice the input waveform is shifted down by Vbb volts. That is -Vbb will set the conduction angle.
If you remove the resistor (Rb -> infinite) the transistor will start to conduct at 0.7 V. If, instead, you short it (Rb=0) it also will short the voltage generator.
The value has to be choosen in order not to affect the amplitude of the AC voltage at the lower frequency you are interested in, since Rb toghether with C will act as a high pass filter.
The Rb resistor will play no role in setting the base current unless Vbb is positive (but this is not possible since you are speaking about a class C amplifier).
 
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FvM

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The Rb resistor will play no role in setting the base current unless Vbb is positive (but this is not possible since you are speaking about a class C amplifier).
Although Rb doesn't directly limit the peak base current, it clearly plays a role in setting it. As average current through C is zero, the average base current has to be sourced by Rb. Increased Rb must be compensated either by reduced conduction angle or peak base current, practically both.
 

albbg

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Although Rb doesn't directly limit the peak base current, it clearly plays a role in setting it. As average current through C is zero, the average base current has to be sourced by Rb. Increased Rb must be compensated either by reduced conduction angle or peak base current, practically both.
You are correct. It's my mistake.
 

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