Reactive Loads
    Last month we covered how the SRPP circuit works best into an optimal load impedance, which would seem to preclude using this amplifier topology with  a reactive load, as its impedance varies with frequency. Another way of putting it is that for any given load impedance there is an optimal Rak resistor value and as the impedance of a reactive load is frequency dependent this topology is unsuited to drive a reactive load. True enough, but why would anyone want to drive a reactive load? Electrostatic speakers and headphones come immediately to mind. (In fact, one commercial SRPP power amplifier has been sold for driving electrostatic speakers.) Still, when faced with a moving target, the problem of finding an optimal Rak value remains.

    If the value of Rak were not fixed, however, then the SRPP could be used to drive a reactive load. How do we make Rak's value vary with frequency? The answer is to use a reactive element either in series or in parallel with the Rak resistor.

    The case of driving an inductive load is easy enough to accomplish. An inductor's impedance falls with a decrease in frequency. At DC its impedance is zero. Therefore, we begin by specifying the optimal value for resistor Rak when the load is zero ohms:
    Rak = rp + 2RL 
                mu - 1

Since the load is 0 ohms this formula reduces to:
    Rak = rp / (mu - 1).
This value for Rak defines one extreme of operation, DC; at the other extreme, infinite AC frequency, the optimal value for Rak would be infinity. Placing an inductor in series with resistor Rak covers both extremes. As the frequency drops, so too the impedance of this

All transformers are designed and built in the United States to high quality commercial standards, and are vacuum-impregnated with varnish to protect against humidity.

One Electron transformers
are distributed by
Antique Electronic Supply

< PREVIOUS

NEXT >

pg. 9

www.tubecad.com   Copyright © 2000 GlassWare   All Rights Reserved