[minima] Re: Q6 7 8 Amp - gain?
- From: <jerry@xxxxxxxxxxxxxxxx>
- To: minima@xxxxxxxxxxxxx
- Date: Mon, 20 Jan 2014 03:24:07 -0700
I breadboarded the circuit last night and got over 18db. Jerry
-------- Original Message --------
Subject: RE: [minima] Re: Q6 7 8 Amp - gain?
From: <jerry@xxxxxxxxxxxxxxxx>
Date: Mon, January 20, 2014 4:21 am
To: minima@xxxxxxxxxxxxx
I breadboarded the circuit last night and got over 18db. Jerry
-------- Original Message --------
Subject: [minima] Re: Q6 7 8 Amp - gain?
From: "Steve VK2SJA" <vk2sja@xxxxxxxxxxx>
Date: Mon, January 20, 2014 3:45 am
To: minima@xxxxxxxxxxxxx
Hi Mark,
I should keep my mouth shut to hide my ignorance as I don't really know
what the gain should be but here are a couple of thoughts. Farhan will
chime in with the correct answer shortly no doubt.
That amp is the suggested improved Bitx bidirectional amp by W7ZOI.
There
is a technical paper discussing it in the references section of
Farhan's
Minima web page right down the bottom. As originally suggested it is
intended to be bidirectional with 50 ohm termination on both ports.
So first suggestion is have as read of that document.
Second is that if it is indeed still setup as a 50ohm in and out then
your
measurements on the 50ohm scope would be better if you ditch the 10x
probe
and just use a small length of 50 coax to the scope.
I suspect that amp should gave gain closer to 15-20dB but don't really
know as Farhan seems to have departed from the listed resistor values
suggested by Wes in the article. Have a read and you'll see what I
mean.
http://www.phonestack.com/farhan/bidiamp.pdf
There. Now I shall press send and see a one-liner from Farhan telling
us
exactly what the gain should be. :-)
Steve
VK2SJA
> HI Gang
>
> I've built half of the bi-directional amplifier to the right of the
> diode mixer in the schematic and am seeing very small gain in Q6 7 8.
> The amp comprising these three is drawing 60mA and mine has been
> constructed using MPSH10s (because I have thousands of them!).
>
> Any thoughts on what we expect from this part of the circuit - in the
> attached yellow is in and blue is out. Gain is about 4.5dB - is this
> right? The scope is 50R terminated on both channels and I am using
X10
> probes.
>
> Mark
> G0MGX
>
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