[AMPS] CLASSes of amps.

Tom Rauch (W8JI) w8jitom@worldnet.att.net
Mon, 31 Mar 1997 08:55:43 -0500


d l groves wrote:
> 
> I'd like to know about what the different classifications of amps means.
> Lately I've seem AB1, C, GG(I think) being used. Whats the differences in
> the classes? What do the different class mps get used for??

Looking at the classes commonly used in amateur service, they are
divided by grid current and conduction angle. Conduction angle is the
letter designations,and grid current is the number. Remember the tube is
really a time varying resistance that is controlled by the grid drive,
and not a "source" of power. When I say efficiency I am speaking of the
dc to ac energy conversion efficiency of the tube.

Class 1 indictes the control grid is never driven positive (into grid
current), class 2 indicates control grid current is present. That
applies to all amplifier letter classes. So you can have class A1 or
class A2, AB1 or AB2, etc.  

Class A is anything that conducts 360 degrees. That means the tube
resistance never reaches infinite resistance. Maximum efficiency (in
theory) is 50%, while typical operating efficiency ranges from zero to
about 30%, depending on signal level.

Class B is a conduction angle of exactly 180 degrees. With no drive, the
resistance of the tube is infinite, the tube is exactly at cut-off. The
theoretical maximum of efficiency is 75%, while practical circuits range
from 50 to 70% efficiency.

Class C is any conduction angle less than 180 degrees. The tube is
biased into cut-off, and spends more than half it's time as an infinite
resistance. Theoretical maximum efficiency is 100%. Practical circuits
range from 60 to 90% efficiency.

Class AB is a mixture of B and A, the conduction angle is anywhere more
than 180 and less than 360 degrees. It is the widest conduction angle
class of operation, and the most common in amateur PA. Theoretical
maximum efficiency ranges from 50 to 75%. Practical efficiencies are
60-70%. 

One of the most common misconceptions is that larger conduction angles
result in less distortion in RF amplifiers. The transfer characteristics
are actually more important than amplifier class in determining RF
amplifier distortion.

Hope this helps.

73 Tom


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