"....Pills ran at class C have their bases tied to ground........"
yeah and the base-emitter junction is reverse-biased as a result, so what?
that's what makes them so efficient when they're used with the proper
modulation schemes. they're switches. no collector current flows until
the reverse bias is overcome by sufficient levels of rf input. when that
input level is reached the base-emitter is now forward biased and (rf output
is produced) collector current flows. that why by the time you get 1 watt
of drive into it you've got 100W of output as in the case of a palomar
elite 250 mentioned in another forum here earlier. that's an indication that
the amplifier is biased class c.
simply by virtue of the manner in which a class c amplifier operates there's
no headroom to accomodate the proper peak-to-carrier levels required
to produce adequate modulation levels once sufficient input drive has
been applied to create enough forward bias to "turn on" the collector
outputs.
on the other hand, in a class ab amplifier the output is always in proportion
to the input. consonant sounds in human speech have less energy than vowel
sounds. class ab is required to faithfully reproduce those nuances in energy
levels that attend human speech. that's why class ab amplifiers are forward biased
from the get-go.
that's why ab is used with signals that are modulated with asymmetrical human
speech and we save the class c switches for modulation modes where the
asymmetry of human speech is not involved.
class c switches cannot faithfully reproduce the variations in human speech
without excessive levels of distortion because they're either ON or OFF.
that's why class c amplifiers are used in these services:
and FM transmitter is either ON or OFF, there's no in between.
a CW transmitter is either ON or OFF, there's no in between.
an RTTY transmitter is either ON or OFF, there's no in between.
that's why class ab amplifiers are used in these services:
an AM transmitter produces a signal that constantly varies in amplitude.
an SSB transmitter produces a signal that constantly varies in amplitude.
a DSB transmitter produces a signal that constantly varies in amplitude.
and just so there's no confusion, ab can be used in all these services
while c doesn't enjoy the same all-round operating flexibility.
if you want higher efficiency then you must sacrifice fidelity.
if you want more fidelity then you must sacrifice efficiency.
i just don't happen to think that a signal increase of .16db.
is worth the additional distortion and out of band products
and there are many here who would agree.
a LINEAR amplifier is required equipment when using asymmetrically
(amplitude) modulated signal sources for faithful reproduction of the
original signal as well as to produce output power in direct proportion
to the input signal. distortion in this case is defined as any signal or
signals not present in the original inputted signal.
as for your cb radio transmitter producing 100% negative modulation
levels for extended periods of time, you can eliminate that with
proper adjustment of the amc. they don't come from the factory
that way. i have yet to see a single cb radio that didn't produce
4W+ of carrier and anywhere from 12 - 18WPEP right out of the box.
now that you have helped make the case for the DIFFERENCE between
ab & c, where do you want to go next?
