Designing a basic Common Emitter amp

Thread Starter

meddyliol

Joined May 5, 2014
8
I seem to remember back in the day when I vaguely knew what I was doing :) that the very basic design could be done by 'the rule of 10'. All I can remember was that Rc was ten times Re and (I think) the base current was a tenth of the emitter current. I am sure I am not making this up but if anyone can enlighten me I would be very grateful.

Thanks

Brian
 

wayneh

Joined Sep 9, 2010
17,505
The currents need to add up!
All I can add is that, if the base current is 10% of the collector-emitter current, then the transistor is fully saturated.
 

Wendy

Joined Mar 24, 2008
23,483
There are several configurations that are referred to as common emitter, DC stabilization is a pretty important design criteria.

The X10 gain rule you are referring to only applies when it is in saturation (think digital). One of the reasons DC stabilization is so important is even within the same batch of transistors the base gain (β, called beta) is all over the place, 50-100 is typical.

Tell use what you are wanting to do (learn the circuits?) and we can be of more help.
 

Thread Starter

meddyliol

Joined May 5, 2014
8
Thank you for your replies. I am getting back into this sort of stuff after some time out. I want it to be a class A amp so Vc needs to be about half Vcc in the quiescent state. I am not building it to actually work so to speak, just as an exercise. I have just bought a Raspberry Pi and have breadboards, a 9V battery power supply and loads of bits. I am a bit annoyed that I have forgotten so much.

Cheers

Brian
 

THE_RB

Joined Feb 11, 2008
5,438
The ten times rule I remember was that the total gain of a one transistor CE amp should be 10 or less.

That was from the days when CE stages were used in audio amps as a standard design (before opamps dominated audio) so you wanted to make a CE amp with low distortion.

And so that rule of 10:1 will also apply to Rc and Re because of their relationship's effect on the gain.
 

ian field

Joined Oct 27, 2012
6,536
I seem to remember back in the day when I vaguely knew what I was doing :) that the very basic design could be done by 'the rule of 10'. All I can remember was that Rc was ten times Re and (I think) the base current was a tenth of the emitter current. I am sure I am not making this up but if anyone can enlighten me I would be very grateful.

Thanks

Brian
This is included in a set of concise tutorials on one of the ham radio sites.

N1HFX IIRC.
 
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