My main point is that very sloppy amplifier designs can make IM3
specs close to what some modern radio's produce. Surely designers can
do better than they are!
-35dB referenced to a single tone would be acceptable, but most of
all they should watch higher-order products.
> Is zero-bias synonymous with Class C ???? I was under impression that
> operation of Class C required the amplifying device to be biased
> beyond cutoff allowing conduction for a portion less than 180 degrees
> (typically at the peaks of the input signal -- 90 degrees). Does zero
> bias do this.....maybe for some devices????. Zero bias would seem to
> be more of a Class B operation.....Please elucidate.
A transistor is cut-off when bias is zero, because collector current
is zero.
By definition, Class A is 360 degree conduction, Class B exactly
180 degrees, and class C anything less than 180 degrees.
A zero-bias conventional bipolar transistor operates class C, since
it takes about .7 volts to put it in conduction.
Class B is almost impossible to maintain, because it has **exactly**
180-degrees conduction. That's why we have class AB, which is less
than 360 and more than 180.
Class B bias would be whatever bias level sets PA idle-current
exactly to zero mA with no signal, and still allows 180-degree
conduction.
With class C, efficiency can range nearly up to 100%. With class B,
in the 75% or so range maximum, and with class A less than 50%.
Other classes with higher letters like D and E have crept in, but
most of them are actually class-C amplifiers. We can easily get 90%
efficiency with class C PA's, if we use harmonic resonators in the
drive and output circuit. RCA did that in the 1950's with a 5kW AM BC
transmitter. Typical efficiency was slightly over 90%.73, Tom W8JI
W8JI@contesting.com
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