[AMPS] Re: Parasitics

[Steve Thompson]

In message <B0000287832@emerald.axnet.com>, Jon Ogden <jono@webspun.com>
writes

snip

>>The tubes anode system, so far as that time-varying resistance, does 
>>not come into play. We are only looking at a termination for that 
>>system, so source impedance does not concern us greatly.
>
>Source impedance does not concern us???  Tom, I think you need to go back 
>and re-read your initial RF transmission line theory textbooks.  If 
>source vs. load impedance doesn't concern us then why don't we just run 
>PAs and transmitters into any SWR?  Source impedance is of extreme 
>importance when we are talking about terminating a network.  Perfect 
>power transfer to a load occurs when the load is the complex conjugate of 
>the source.  In other words if the source has an impedance of 1700-j35 
>Ohms the load needs to have an impedance of 1700+j35 Ohms impedance.  So 
>it DOES concern us.

It is true that maximum power transfer occurs when source and load
impedances are equal and conjugate, but with power devices it is normal
to work by looking at what they do when presented with a particular load
impedance (at any given frequency, voltage etc.), rather than the device
output impedance. This is analogous to audio amps which are designed to
deliver into, say, 8 ohms while their output impedance is a tiny
fraction of this.

Steve

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