Gary K4FMX Schafer wrote:
First let's define classes of operation:
Class A, plate current flows over 100% of the cycle of the input signal.
Class B, plate current flows over exactly only 50% of the cycle of the
input.
************
Class C, plate current flows over less than 50% of the cycle of the input.
Class AB1, plate current flows over greater than 50% of the cycle of the
input with no grid current ever drawn.
Class AB2, plate current flows over greater than 50% of the cycle of the
input WITH grid current drawn.
True or (almost) class B amplifiers are only found in push pull audio
amplifiers where one tube amplifies the positive half of the drive signal
and the other tube amplifies the negative half. Even then often the tubes
are biased for a small amount of idle plate current.
************
I must respectfully disagree. The: "plate current flows over exactly
only 50% of the cycle of the input" is the "book learning" explanation
(I went to a better school than that) and true (actually used) class "B"
operation is in no way restricted to: "class B amplifiers are only found
in push pull audio amplifiers" as many 250. Watts to 50,000. Watts am
radio transmitters use class "B" audio modulators and many RF amplifiers
also, including almost all solid state 100. Watt transceivers and many
MOS high power amplifiers use true class "B" final amplifiers - with an
elevated bias for a tiny 100% (degrees) current.
That does not make them class "A" or class AB."
The proper definition of class "B" amplification is not:
"exactly 180 degrees of conduction" it is:
"one device amplifies the positive half of the drive signal and the
other device amplifies the negative half" also not:
"often the devices are biased for a small amount of idle plate current."
But: "class "B" linear amplifier devices are ALWAYS biased for a small
amount of idling plate current to cancel the crossover distortion."
The only possible exception is where non linear RF generator is being
use to heat something such as plywood glue setting and such. Frequently
50 kW.
Class "B" final amplifiers are the most overall efficient form of high
power linear amplification if you take the idling power into
consideration, the power consumed over a long period of time, such as a
week or month.
Additionally they cancel out their second (the big one and all even)
harmonics.
I imagine class "B" final amplifiers are more linear than most single
ended amplifiers driven into grid current. (I have no proof.)
--
Ron W4BIN - Understanding is much better than
knowing how.
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