On 5 mayo, 17:06, "J. C. Mc Laughlin" wrote:
Dear Wimpie: *The content of the paragraph below may well rise above the
noise of this thread.
I expect to learn something of value from your comments about why twice the
(apparent) output Z of an amplifier was of importance and what you did to
have the amplifiers conform.

73, * *Mac * N8TT
-------------------"Wimpie" *wrote in message

...

Hello Dave and John,

snip

Regarding the "academic discussion" I also agree. In my professional
career where I designed several RF PA's, only 2 times the output
impedance of the amplifier was of importance. *In one of these cases I
couldn't meet the specs and had to insert attenuation (some waste of
power…).

snip

With kind regards,

Wim
PA3DJSwww.tetech.nl
Remove abc first before setting free the pigeon.
--------------

J. C. Mc Laughlin
Michigan U.S.A.
Home:

Hello,

Regarding the defined output impedance.

The first time was during my thesis (third harmonic peaking PA). The
stability margin was not very large (expected), and it could be
improved by keeping the impedance seen from the base within certain
limits. My teacher said, be careful, you only have two devices
(BLW76). I tried to make an exciter (based on 2SC1307 BJT) with
defined output impedance, but without success. So in the end I
increased the output power of the exciter and inserted a 3 dB
attenuator, not elegant, but it did the job.

The second time was for H-field generation where wider bandwidth was
achieved by adding a second resonator. When driving from a 50 Ohms
source, it had a nice Chebyshev type pass band. Because of the ripple,
it shows reflection in the pass band (this happens with Chebyshev
response). However when driving from a PA (that was flat within the
pass band when loaded with 50 Ohms), everything went wrong.

I redesigned the filter/coil combination, designed a switching PA
(half bridge in class DE operation) and skipped the 50 Ohms (I
designed around 8 Ohms). The PA drives the filter directly in such
away that most of the time the PA sees a nice (mismatched) load (that
is inductive for harmonics). This resulted in the desired pass band
with significantly increased efficiency. The strength of the H-field
is controlled by varying the supply voltage (PWM circuit).

Leaving out the 50 Ohms in between, saved several capacitors and
inductors.

With kind regards,


Wim
PA3DJS
www.tetech.nl