recognising a constant envelope in modulation

Audioguru again

Joined Oct 21, 2019
3,208
The article says that a "constant envelope" occurs with an FM transmission that has a constant carrier output level (envelope). Therefore a class-C RF amplifier can be used.
AM modulation needs the low distortion produced by a class-AB RF amplifier because the sounds are produced by the envelope fluctuations..
 

Thread Starter

yef smith

Joined Aug 2, 2020
246
Hello Audioguru why Class C is better for constant envelope?
Why the potential of distorion is hugher in class C then in class AB?
Thanks.
 

Audioguru again

Joined Oct 21, 2019
3,208
Class-AB is linear and is symmetrical push-pull.. It has 2 transistors that are never turned off so they get hot with wasted power.
Class-C uses a pulse each carrier cycle. Since its transistor is turned off during a fairly larger part of the cycle then it is efficient.
 

crutschow

Joined Mar 14, 2008
27,243
Why the potential of distorion is hugher in class C then in class AB?
It's not potential, it's actual.
As AG noted, a Class C stage is on for only part of the sinewave cycle, so cannot generate a low-distortion amplitude-modulated signal.
The sinewave of a Class C stage is generated by an LC tank circuit.
The transistor just periodically pulses the tank to keep it oscillating.
 
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