[AMPS] Re: Parasitics

[Jon Ogden]

>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.

Well, OK.  It's certainly not how I designed my amps.  While it's hard 
(damn near impossible) to measure S22 in a high power amplifier, we would 
always figure that we were pretty well matched when our output power was 
peaked.  We were certainly in the range of the impedance of the load.

Basic Smith Chart theory tells this.  I have no idea about audio amps, 
but I never realized their output impedance was a fraction of 8 Ohms.  
The output of most modern transistors is a fraction of 50 Ohms and we go 
through all sorts of contortions to match the impedance so that we do get 
max power transfer.  I would bet that while the amplifying device in an 
audio amp has low impredance, there is some sort of transformer or other 
matching network to step up the impedance to 8 Ohms.  There has to be.  
The laws of physics insist on it, otherwise you'd get all sorts of 
reflections.

73,

Jon
KE9NA


--------------------------------------------------------------------------
Jon Ogden

jono@webspun.com
www.qsl.net/ke9na

"A life lived in fear is a life half lived."


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