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[AMPS] Setting the record straight--Dick Ehrhorn

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Subject: [AMPS] Setting the record straight--Dick Ehrhorn
From: (Jon Ogden)
Date: Sat, 21 Jul 2001 10:34:01 -0500
on 7/18/01 12:03 PM, 2 at wrote:

> /\  Designing an unconditionally-stable HF/MF amplifier is more difficult
> than designing a stable amplifier for VHF.   Although I have heard
> reports thereof, I have never seen parasitic damage in a VHF or UHF
> amplifier.  

I gotta throw my 2 cents in on this one because this statement is just not
completely accurate.

Building a stable amplifier that has high gain becomes increasingly
difficult the higher ones goes in frequency.  Things such as layout issues,
bypassing and genuine parasitics become critical.  When I say "genuine
parasitics" I don't mean the kind Rich talks about.  I am talking about the
inherent, real world, stray inductances and capacitances in components.

Rich's statement could argued to be correct only because generally we hams
build our HF/MF amplifiers to be pretty broad banded in their coverage.
Sure we may have different tuned circuits for different bands, but the
amplifier in general covers a very large percentage bandwidth.  For
simplicity's sake let's say the HF band is from 1 to 30 MHz, centered at 15
MHz (I know it starts higher than 1 MHz).  An amp designed to cover this
basically is designed for nearly 200% bandwidth! (Percentage bandwidth is
defined as the total bandwidth covered divided by the center frequency *
100).  Our bandwidth is 30MHz - 1 MHZ = 29 MHz. %BW = (29/15)*100 which is
approximately 200%.

That's significant and that's what makes designing plate chokes, suppressor
chokes, etc. a real art.

Now take a UHF amplifier design at 435 MHz.  Even if we wanted to cover the
entire 70cm band from 420 to 450 MHz, we only have a percentage bandwidth of
6.8%!  Yet no one designs the UHF amplifiers to have even that much
bandwidth.  It would be rather difficult.  Amazing, how we are covering
approximately the same total bandwidth - 30 MHz.

A UHF amplifier with the same bandwidth as our HF amp would literally cover
from nearly DC to 900 MHz!

My point is we have to compare apples to apples.  An HF amp with a bandwidth
of 6.8% would be MUCH easier to stabilize than a UHF amplifier with 6.8%

Rich's statement as blanket fact is just not true.


Jon Ogden
NA9D (ex: KE9NA)


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

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