Re: Analyzing a PA


Generally Ohms law is the rule.  Add RF to that and its true
but the numbers are now real and imaginary.   Generally back
of the envelope numbers set the ballpark and from there it is
dealing with all the small numbers that result.

However for push pull you simply factor times two
for load impedance and total voltage swing..

Google is your friend and try to find old ANs from Motorola
(Granberg mostly).

The magic is in the design and construction of the transformers needed.

A drain loaded to 1.6ohms would be 3.2 drain to drain for push pull.
In push pull voltage swing doubled, current is kept the same, and
load impedance is twice that of single ended.  Sometimes it helps to
thing of gett from 50 ohms down to what is needed.

Generally the big issue is load impedance (resistive) need to be lower
than whats needed to deliver power without clipping (hitting the rails).
However in real systems that's even lower as the device near cutoff and
saturation will be non-linear.

Also gate drive (if not bipolar) is voltage across a large reactive
(mostly capacitive) load.  So for example a device with a GM
of 1 Seimens (some devices higher)  would need 2V on the
gate to swing the drain 2A.  Doable but transformers need to
be correct.

For a bipolar its current so you need enough current with a
.65V standing BIAS that is voltage stiff.  That usually means
a 1-3ohm input impedance for 100W devices.  Also means
the bias source needs to have a impedance of under .1ohms.
So with a large signal Hfe of say 10 for 10 amps out, you need
1A of drive.  Typical 12V Bipolar power device has an Hfe in the 
range of 15-30.   Effective gain is usually about ~11db.

Common case is IRF510,  Run at peak current of 4a and standing
current of 100ma we can call that 4A PP current [an upper limit]. 
At nominal 13.8V (socalled 12V system) that is a peak power
of less than 52W, note the case can dissipate 43W to a massive
heat sink.  However holding to a more moderate power we have
a few things to consider.  With lower current say 2A and push pull
we have 2A PP at roughly 20V PP or about 40W (peak)  or about
28W rms and that is in the easy zone.  However to get 2A at 20W
we need 10 ohms load per device or 20 push pull and translate
that to 50 ohms.  Transformer is 50/20 or impedance translation of
2.5 which is non integer ratio of 1:1.58 or about 3/2 which is how
most get the 2 turn primary with 3 turns output.  Too many turns
and inductance kills bandwidth.  That assumes the linearity is
adequate to have each device go to cutoff and swing to near 10V. 
That is often not true.  If we wanted the same result with 24V supply
its easier as same 2a and nearly 40V PP only needs a 25 ohm load,
transformer is still a pain and the worst case peak of 48V is near half
the device breakdown voltage (IRF510 is 100V).

In both cases transformers to get into the low ohms range from
50 and reverse that need to be built for current in the 10-20A range
and inductances fairly low (10 ohms at 1.8mhz).   Those are tough
to meet.  Bottom line is for 100W (RMS out) you need at least
182W DC input assuming 55% efficient.  That 14A so I^2R is now
an enemy requiring all of the traces to be good for over 15A  with
low resistance, both signal trances, transformers, and ground plane.

FYI most of those china amps are at best 100W input DC.  A few will
do 100W but at 28, 36 or even 50V.  I have a few that were given to
me.  Lets call the numbers very optimistic same for useful bandwidth.

College text was wrong by 1976  as Silconix and others were delivering
150W devices called VMOS, DMOS, and the LDmos is the later
day follow on.  By time you got to read it it was technically out of date
on those devices.  I know as I used a Silconix VN60 in TO3 case that
did a very nice 60W at 13.5mhz for a commercial use.

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