You bet they could do better! Many hams have no idea what these figures mean
or that poor performance here results in more splatter. When told about it
they blithely say "QST liked it" or "it is FCC type accepted".
73
Steve wd0ct
----- Original Message -----
From: "Tom Rauch" <w8ji@contesting.com>
To: <al_lorona@agilent.com>; <w9ac@arrl.net>; <tentec@contesting.com>;
<RMcGraw@blomand.net>; "James Duffer" <dufferjames@hotmail.com>
Sent: Friday, April 19, 2002 10:04 AM
Subject: Re: [TenTec] OMNI VI Transmit IMD
> 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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