Re: How come its OK to mix impedances in a radio system??
- From: "Reg Edwards" <g4fgq.regp@xxxxxxxxxxxxxxxxx>
- Date: Sun, 17 Jul 2005 18:58:17 +0000 (UTC)
Hi Joerg and Fred,
I can see you are both on the right track.
It's surprising how many engineers and radio amateurs assume, after
tuning up for maximum output, that a conjugate match exists between
the power amplifying device and its external load.
Actually the circuit designer doesn't need to know what the internal
impedance of a high power HF amplifier actually is. The device
manufacturer doesn't even bother to mention it in the data book.
Nobody wants to know.
You are wrong on one account, Joerg. The internal impedance of a
power amplifier is hardly ever lower than its correct load regardless
of whether it is operating under Clsss A, B or C conditions. It is
usually much higher than the external load and there is a 'poor'
impedance mismatch.
Both tubes and transistors are operated as more like high impedance
current sources than very low impedance voltage generators like power
stations.
Typical RF PA devices are Beam Tetrodes which may have an internal
anode impedance as high as several hundred thousand ohms. It's a long
way down to the usual RF load of 50 ohms. With audio ampifiers the
loud speaker load impedance can be 8 ohms or less.
At the lower frequencies transistors can have internal resistances
approaching a megohm.
Actually, the output transformer turns ratio depends on peak voltage
and peak current ratings of the device, on the DC supply volts, and on
the power to be developed in a load of given resistance. The internal
resistance of the device has nothing to do with it.
Circumstances may occur at extremely high frequencies in which
knowledge of a transistor's internal impedance is useful to obtain
optimum operation. There is often feedback via the relatively low
internal impedance. Circuit designers then take the easy way out and
copy the complete amplifier circuits conveniently provided by
manufacturers following a lot of experimentation. But they still
don't know what the internal resistance is.
Thanks for allowing me in.
----
Reg.
=====================================
"Joerg" <notthisjoergsch@xxxxxxxxxxxxxxxxxxxxx> wrote in message
news:xevCe.29$NU2.12@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> Hello Reg,
>
> Fred had already explained it nicely. Just to illustrate it in a
more
> simple term if you ever have to explain it to a person without RF
> background: In a class C power transmitter the final stage would
ideally
> be a lossless and infinitely fast switch. Then 100% of the power
that
> this stage takes from the DC rail would be transferred to the
antenna.
> Less tube heater power, filter losses etc.
>
> If this final amp were to have a true output impedance equal to the
load
> impedance it would constantly burn 50% or more of the input power,
> basically dissipate it. It would slowly melt down because they
usually
> aren't built to stomach that.
>
> If an RF power amp is spec'd 50 Ohms that means it is built for that
> load. If you load it with less, you may exceed the current limit of
the
> active element in there. A transistor would get hot, a tube would
have
> the plate glowing pretty soon. I have actually seen the glass become
> soft and being sucked inwards. If you load it with much more than 50
> Ohms and there are LC filters in there (class C kind of has to have
> these) chances are that the transistors die from overvoltage. In the
> case of a tube you might see some St.Elmo's fire inside or more
serious
> arcing.
>
> It's similar in the power world. A generator at Hoover Dam has an
> impedance that is a tiny fraction of the impedance of its load.
Ideally
> its impedance would be zero Ohms but that isn't prossible because it
> uses regular copper wire and not a superconductor for the windings.
>
> Regards, Joerg
>
> http://www.analogconsultants.com
.
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