From: gary@ke4zv.uucp (Gary Coffman)
Newsgroups: rec.radio.cb
Subject: Re: SWR Tuning and PLL
Date: 1 Jan 93 16:24:57 GMT
Reply-To: gary@ke4zv.UUCP (Gary Coffman)
Organization: Destructive Testing Systems

In article <92Dec28.210622.19648@acs.ucalgary.ca> ming@enel.ucalgary.ca (Yang Ming) writes:
>In article <1992Dec28.153913.25266@ke4zv.uucp> gary@ke4zv.UUCP (Gary Coffman) writes:
>
>>mirror. The only effect of the reverse wave on the transmitter is to
>>shift the operating point, which under certain conditions can reduce
>>the *efficiency* with which the stage turns DC power to RF power.
>>*That* can cause the output stage to get hot, but it has *absorbed*
>>none of the reverse power. It is not a load.
>>
>
>This explanation is questionable. As well understood, the operating point is
>determined by the DC bias. Of course when working in nonlinear region, the
>operating point also moves with the magnitude of the power, but a) this is
>only secondary effect, and b) most amplifiers work in linear region.
>
>It's not true that output stage is not a load. There is a parameter called
>*output impedance* for every circuit, which has both real and imaginary
>parts. Physically speaking, the output stage is nothing elso but a piece
>of semiconductor, which has bulk resistance. This absorbs power.

I went over this already, but you deleted that part. 
Let's take it step by step again.

The operating point, or load line, of an amplifier is the plot of
dynamic plate voltage versus plate current, or collector voltage
and current for solid state devices. This load line establishes
the *output* impedance of the device for matching purposes. By
Ohm's Law, the output impedance is the ratio of E/I where E is
dynamic plate voltage and I is dynamic plate current. This is
the *slope* of the load line. THIS IS NOT A DISSIPATIVE LOAD, 
it's a mathematical expression useful for matching to a real 
genuine honest to God load attached to the output impedance matching 
network. Note again that this is an *output* impedance, and is
NOT an *input* impedance for a signal attempting to enter the
*output* port. Operating amplifiers are *not* reciprocal devices.
This load line *cannot* dissipate any power, it's a mathematical
fiction and not a physical load.

Where amplifier linearity enters this picture is in the *shape*
of the load line. If the load line has a constant slope, the
amplifier is linear. If the load line curves, the amplifier
is non-linear in the curved region. Static bias is used to place 
the static, no signal, amplifier in the linear region of it's load 
line. Under drive, the amplification occurs around this static
point, as the dynamic voltages chase up and down the load line.

The reflected wave can combine vectorially with the dynamic voltage
present at the output of the amplifier. This changes the *slope*
of the load line which changes the *output* impedance of the 
amplifier since the output impedance *is* the slope of the load
line. This changes the operating characteristic of the amplifier,
usually shifting it off most efficient operation, *and* changes
the *virtual* impedance seen looking into the output port through the
matching network to a virtual short circuit. This is similar in
principle to the "virtual" ground at the input of an opamp. This 
*conjugate mirror* causes the reverse wave to be re-reflected 
totally from the transmitter output because it appears as a 
completely unmatched termination to the line.

See chapter four of Walt Maxwell's classic "Reflections" for
a through mathematical treatment of this subject. Walt was
chief designer for RCA for many years and is *the* recognized
expert authority on this subject.

Gary
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