Biasing Common base vs Common emitter

Thread Starter

Feenz

Joined Apr 9, 2022
15
Hello everyone
In the image below ( RF amplifier circuit ) how would the resistors / capacitors be changed when changing the circuit from CE into CB ?

* I've swapped input/output capacitors in place of each other

TRF bias.jpg
 
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Audioguru again

Joined Oct 21, 2019
5,426
Biasing is done with the resistors, not with the capacitors. The resistors in each circuit are the same.
The base in each circuit has a high impedance and the emitters have a low impedance that needs the larger capacitor in both circuits.
 

Thread Starter

Feenz

Joined Apr 9, 2022
15
Biasing is done with the resistors, not with the capacitors. The resistors in each circuit are the same.
The base in each circuit has a high impedance and the emitters have a low impedance that needs the larger capacitor in both circuits.
Thanks for clarification
Sure , capacitors aren't a part of the biasing , I meant to say , is it necessary to changing the capacitors with changing the circuit?
For the MW band , are capacitors mentioned in the CB circuit accurate ?
 

Audioguru again

Joined Oct 21, 2019
5,426
Your biasing caused the severe clipping at the top of the waveform. I changed the biasing a little and reduced the input signal level to reduce the clipping. It is still distorted because it is missing some negative feedback.

The capacitors work fine in the 1MHz MW band.
 

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Thread Starter

Feenz

Joined Apr 9, 2022
15
Your biasing caused the severe clipping at the top of the waveform. I changed the biasing a little and reduced the input signal level to reduce the clipping. It is still distorted because it is missing some negative feedback.

The capacitors work fine in the 1MHz MW band.
In your attachment , both of the circuits were a common emitter !

Figure 2 is a linear common base amplifier.
I was wondering , is there are a differences in biasing between CB & CE circuits
I thought of , as long as the signal will be input into the emitter ,then its resistor should be as high as possible
to avoid weakening the signal , which will be pass in the emitter's resistor to the battery (-) line as well .
 
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Audioguru again

Joined Oct 21, 2019
5,426
A common base transistor amplifier transistor has a very low input impedance then stray capacitance at the input does not attenuate very high frequency signals.

If you increase the resistance of the emitter resistor then you must also increase the resistance of the collector resistor so that the biasing is not completely messed up.

Here is the frequency response of the common emitter amplifier I corrected: I deleted the incorrect amount of gain. See my next post.
 
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Thread Starter

Feenz

Joined Apr 9, 2022
15
EDIT.
My error with the gain. The input is at -28dB and the output is at +6dB then the max gain is +34dB which is 50 times.
thanks for your concern
That's OK , but the circuit is common emitter , my goal is the common base circuit
plus , no resistor at the collector , there is a RF transformer .
 

Audioguru again

Joined Oct 21, 2019
5,426
I have never designed or simulated a common base transistor with a transformer at the collector.
I noticed that the emitter input capacitor value must be about double the value of a common emitter capacitor value for the same low frequency cutoff.
 

panic mode

Joined Oct 10, 2011
2,364
Please, can you explain that in case of common-base?
(By the way - this is correct, but it deserves some explanation...)
well spotted and that was incorrect or at best incomplete.

instead of "base is always an input" should be "base cannot be an output" since only input current ever flows through it. so when base is 'fixed' (as in common base case), then the only way to affect or control the input is to change biasing at emitter. since current in emitter circuit is large (much larger than input current), impedance at that point is low.
 
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LvW

Joined Jun 13, 2013
1,586
..... the only way to affect or control the input is to change biasing at emitter. ...
Yes - and to be more clear - "biasing at emitter" should mean: Set resp. modify the bias VOLTAGE at the emitter (when the base is DC grounded) because it is the base-emitter voltage that matters. The currents are the result of the voltages Vbe resp. Vce.
 

DickCappels

Joined Aug 21, 2008
9,520
There is no requirement for the base to be at any particular DC voltage.

The point of injecting an input signal through a resistor or an impedance is to inject signal current into the emitter.

Back to the discussion in progress.
 
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