Hi Jan,

Perhaps you are right, but the book from which I learned the most about
class C and D amps (Radio Frequency Electronics by Jon Hagen, not a
really great book, although I don't know it to be flat out wrong
anywhere) describes class C amps as saturated switches and class D amps
as totem-pole saturated switches (two transistors, like a digital inverter).

Sean


On Tue, 10 Jul 2001, SkinTech wrote:

> Sean,
>
> I think you mix up class C with class D. Class D is where the active device
> is either fully on or off. This is sometimes referred as 'digital amps'; in
> fact it is some form of pulse modulation (pulse width modulation, duty cycle
> modulation, whatever). Think of it as a power ADC. You need a filter at the
> output to reconstitute the original (analog) signal.
>
> Class C is when the active device conduct for less than 180 degrees of the
> signal cycle (assuming a sinewave signal). The output will be the top (or
> bottom) of the sinewave, but for less than half a cycle. This is of course a
> grossly distorted replica of the input signal. Therefor, a tuned filter
> (also called a 'tank', normally a parallel LC circuit) at the signal
> frequency is added at the onput. The filter starts to 'oscillate' at the
> signal frequency, thus providing an amplified version of the input signal.
> One consequence is that class c has very narrow bandwidth, unless the
> reconstitution filter is tunable or has low Q; in the latter case the
> efficiency suffers. In fact, one can argue that the output filter takes the
> power of the output 'pulse' and transforms it into a sinewave. The effective
> output power is the area under the output 'pulse' minus the losses in the
> output filter.
>
> As for the transistor to use, it must a) be a power device, b) have good
> high-frequency specs (Fc, low capacitances, low rise/fall times, the usual
> stuff). By nature it does not have to be particularly linear.
>
> Cheers, Jan Didden
>
>

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