[Amps] MOSFET amp filtering - was: auto-tune

[Manfred Mornhinweg]

I also shudder somewhat when I see a Morse code transmission referred to 
as "continuous wave".

 > That amateurs (and the entire commercial radio industry) called
 > on/off keyed telegraphy "continuous wave"....

Only the radio industry does. The semiconductor industry doesn't. When 
you look into the datasheet of an RF MOSFET and you see information 
given for "CW" operation, they indeed usually mean a continuous wave 
without any amplitude modulation, rather than an interrupted one. When 
they _don't_ say CW, they mean pulsed service, usually with rather short 
pulses and low duty cycles. The small print usually tells about the 
pulse duration and duty cycle.

A similar confusion arises with "class AB". Hams think of class AB as 
being reasonably linear, and used for almost all "linear" amplifiers. 
Instead the LDMOSFET datasheets usually don't use it that way! When you 
see an LDMOSFET data sheet stating an efficiency like 80% in class AB 
service, they mean saturated class AB, which gives up linearity in favor 
of efficiency and maximal output. After all, the only thing "class AB" 
means is that the active device conducts for more than 180 but less than 
360 degrees. "Class AB" doesn't tell whether or not the device is driven 
into saturation. Neither does it tell how much linearizing feedback is 
used, if any.

LDMOSFETS are run in class AB rather than class C, even when linearity 
isn't required. The reason is that their gates can't take much negative 
voltage. But for nonlinear, high efficiency amplifiers, saturated class 
AB with minimal or no negative feedback is used. This is simply called 
"class AB" in the datasheets, and does _not_ tell at what ratings to use 
that LDMOSFET in linear service!

Life is full of pitfalls...

Manfred

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