I've copied Joe Rocci's original post below.
TL'DR:
Change the 220 Ohm collector resistor to 1000 Ohms to decrease
the current consumption. Increase the emitter degeneration
resistor to 20 Ohms to bump the gain up to 20 dB.
73,
Mac AE5PH
Original message posted by Joe Rocci:
Hi Steve
Here are some results from modeling the Tx section in LTSpice:
1) Gain is about 17db @ 20 mhz
2) Input match (return loss) is about 16db and pretty flat up to 50
Mhz
3) Output match is quite good, at the expense of about 6db gain loss,
due to the series output matching resistor
4) Current draw is about 55ma
Some comments:
1) The voltage-amp stage is biased at almost 11 volts on the
collector,
leaving very little headroom for large voltage swings.
2) The emitter follower buffer stages draw the lion's share of the DC
current, but are non-optimum for driving large voltage swings into
non-resistive loads at high frequencies; that's why so much static DC
current is required.
3) If you increase the 220 ohm collector resistor to 1000 ohms, the
collector drops closer to 5.5 V, and the overall current consumption
drops to about 28 ma. If you then increase the emitter degeneration
resistor to 20 ohms, you end up with about 20 dB gain (@20 Mhz), 22 dB
input match, and presumably much better output voltage swing headroom.
I
discovered all this while trying to goose the amp up for a little
better
performance at 30 Mhz, anticipating a new crystal filter.
Of course, all of this is 'on paper' and needs to be verified.
Joe
W3JDR
On 10/14/2014 08:22 PM, Steve VK2SJA wrote:
Thanks Joe,
So ~16 to 20dB in both directions.
I had a quick look around the archive this morning. If there is a mod
for
reducing current in the RX side of the bidirectional amp I've managed
to
miss it.
Anyone know of such a mod?
The original W7ZOI bidirectional amp article quotes a noise figure of
5.8dB when the amp was setup for about 16dB of gain. So 4.5dB seems
an
improvement on that and around the same ball park. So the LTSpice
simulation is probably on the money.
Last night I was reading EMRFD and learnt how to go about measuring
noise
properly. I've ordered a LNA and when it arrives I'll try measure it
in
the real-world and report back.
I've been reading that at HF frequencies there is so much atmospheric
noise that a 3kHz wide SSB rig will typically see about 14dB of
noise.
And
that as such any amp with a noise factor (NF) of of a bit less than
~14dB
is probably adequate for this application. Because you won't see the
locally generated amp noise over the atmospheric noise anyway. So
while
we
should always design and build as best we can. From a practical stand
point is there anything to be gained by reducing the NF to a figure
much
below ~6dB?
73, Steve.
Steve
LTSpice simulation indicates about 16db gain at 20 mhz with the
component
values shown on the web page. The standing current is also very
high,
at
over 40ma. I think there was a mod to bring the current down? Also,
the
simulated noise figure is about 4.5db, pretty high. However, I've
never
checked to see how accurate LTSpice is in this regard.
Joe
----- Original Message -----
From: "Steve VK2SJA" <vk2sja@xxxxxxxxxxx>
To: <minima@xxxxxxxxxxxxx>
Sent: Tuesday, October 14, 2014 6:40 AM
Subject: [minima] RX gain of Bidirectional Amp?
Hi Guys,
The gain of the Minima Bidirectional Amp in the TX direction is ~16
to
20dB.
Can someone tell me what the gain is supposed to be for the other
half
of
the duel Amp, in the RX direction? Same?
73, Steve.
