# Question Re: 2 FET follower use

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I have a FET follower RF amp. It is a high input impedance ~330k? and a
100 ? output impedance. I have it on an antenna that has a 330 ? output
impedance. The antenna works fine NOT being terminated with 330 ohm,
i.e. seeing a 330k ? load of the amp input.
The amp does two things, it doesn't load the antenna so I get about 2
times the voltage output. Second, it does an impedance conversion, in
this case a 1.8 voltage gain over the step down of a matching
transformer. That all conspires to be ~ 9.8db of gain.
My question with the caveat, that over driving the input of an amp will
cause unwanted problems.(but how far can I push it)
Now, say I put a step up transformer on the antenna to feed the amp
input, say 1 to 3, i.e. 330 ? to 2,970 ?, a 3 X voltage step up.
Now I have 3 times the voltage on the input of the amp. and another
~10db of gain. That seems to come at no cost, unless I start to over
drive the amp.
My question: The antenna impedance is 330 ohms,  I wind a transformer
that is 330 ? to 2,970, that would be driving a 330k ? circuit.
That's seems to be a problem, but maybe not, the antenna would just
see the transformer as a much higher impedance, just as it originally
saw the amp, when it was connected directly to the antenna.
I would think the antenna sees a inductive reactance load instead of a
resistive load. The obvious solution would be to put a 2,970 ? resistor
on the amp input.
Do I calculate the inductance of the primary any different than usual
Here's a schematic of the amp.

Mikek

Re: Question Re: 2 FET follower use
On 5/20/2020 8:09 AM, amdx wrote:

Hard question? I ask it wrong? Nobody cares? ;-)

Mikek

Re: Question Re: 2 FET follower use
On 2020-05-20 21:21, amdx wrote:

I think there are so many misunderstandings and loose ends
that setting everything straight is just too much work.

Jeroen Belleman

Re: Question Re: 2 FET follower use
On Wed, 20 May 2020 22:28:53 +0200, Jeroen Belleman

Yup.  That's about what I was thinking.  Before I could start, I had
to redraw the schematic and get a few things clarified.  I cleaned up
the schematic so I could read it:
<
http://www.learnbydestroying.com/jeffl/crud/2%20FET%20BCB%20amp.jpg

and started on a list of dumb question:
2.  The bifilar winding instructions in the upper right are for T3.
There is no T3, but there is a T2 that's missing the instructions.
3.  How did you manage to get -6dBm loss?  It's a source follower,
which doesn't have any voltage gain, but should have some power gain.
4.  All the voltages in red are wrong.  For example, if you power the
amp from 12V, the J271 gate voltage should be 1/2 the 12V power supply
voltage or 6V, not 5.2V.  If you measured 5.2V on the gate, then the
J271 gate is drawing current which suggests it might be fried.
5.  Why a pot for adjusting the drain current on the J310, but no pot
for doing the same for J271?

Only then did I re-read the question and discover that there are too
many questions and oddities.  I don't have the time.

Since this thing works only at 1MHz, it should be easy enough to build
an LTspice model in order to see what the input and output impedances

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Re: Question Re: 2 FET follower use
On 5/20/2020 4:35 PM, Jeff Liebermann wrote:

1 uf.

T1 and T2 are the same. T3 is not relevant to this circuit.

I don't see where I said -6dbm. what I said was in the circuit I have
the amp adds "9.8db of gain" I did use ~ maybe you thought it was a
negative sign.
Below, I clarify, Gain over the use of a transformer.

Ya, sorry about that, I recorded those voltages when I was
troubleshooting the circuit.

The original design did have two pots, the author had the equipment to
adjust for maximum IPs. I think he simplified it as most people can't
set IPs any way.

The question is about the transformer,
Do I calculate the inductance of the primary any different than usual
since the secondary is unloaded? Should I just add the resistor on the
secondary?

I already know and said, the input is about 330k? and the output os
Loading the input with 330? works fine and has a measured 9.8db gain,
over the use of a matching transformer.
I just realized I added that "over the use of a matching transformer",
that should have been added previously, may have helped.
To clarify, if I'm using the matching transformer on the antenna and
have 0.1v output and then I swap the transformer for the amp, I then
have 0.312V output. Thus my 9.8db gain.
Thanks Guys,   Mikek

Re: Question Re: 2 FET follower use

<https://www.dropbox.com/s/x6z2vpbseri7ysu/2%20FET%20Troubleshoot.jpg?dl=0
shows "-6 dBm preamp gain" in the box at the lower right.

You can blow up a JFET with a gate voltage pot that goes the full
range from +12V to 0V.  Did you actually build the circuit per the
schematic?

I still don't understand the question (or what problem you're trying
to solve).  Too much is unknown or ambiguous.  What do you mean by
"usual way"?  Add what resistor on the secondary?  Series or parallel?
For what purpose?

9.8dB voltage or power gain?  The input and output are two radically
different impedances, neither of which is 50 ohms.  Did you measure
this with an oscilloscope or spectrum analyzer?  If so, how did you
arrive at the 9.8dB gain?

Source followers just love to oscillate.  Did you look for
oscillations when measuring the output?  The J310 is a UHF device that
works nicely up to about 1GHz.  If you build your BCB 1MHz amplifier
as if it were a broadband amplifier (i.e. no 1MHz tuning), it's going
to oscillate at some frequency because of the inductances of the long
lead lengths.  The ferrite beads might help, but in my never humble
opinion, if you're going to use UHF devices at 1MHz, you'll need to
bypass everything from 1MHz to 1GHz and probably do something to limit
the gain above about a dozen MHz.

--
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Re: Question Re: 2 FET follower use
On 5/20/2020 5:51 PM, amdx wrote:

Sorry Jeff,
for a copy and paste.

<https://www.dropbox.com/s/x6z2vpbseri7ysu/2%20FET%20Troubleshoot.jpg?dl=0
shows "-6 dBm preamp gain" in the box at the lower right.

>> 5.  Why a pot for adjusting the drain current on the J310, but no pot
>> for doing the same for J271?
>
>  The original design did have two pots, the author had the equipment to
>adjust for maximum IPs. I think he simplified it as most people can't
>set IPs any way.

<You can blow up a JFET with a gate voltage pot that goes the full
>range from +12V to 0V.  Did you actually build the circuit per the
>schematic?

*******************************************************
I built it per the schematic.
********************************************************

>The question is about the transformer,
>Do I calculate the inductance of the primary any different than usual
>since the secondary is unloaded? Should I just add the resistor on the
>secondary?

>I still don't understand the question (or what problem you're trying
>to solve).  Too much is unknown or ambiguous.  What do you mean by
>"usual way"?  Add what resistor on the secondary?  Series or parallel?
>"For what purpose?

********************************************************************
I have a transformer that is seeing a 330k? load, I can't put enough
turns on to match that. So my primary will not see any load reflected
back. That's my conundrum.

I just wound a transformer, it has a 4.3 to 1 voltage step up, 10 turns
to 44 turns, it would match 330 ? to 6,250 ?. But I'm connecting it to a
330k? amp input.
A detail: I have the amp and transformers setup with relays so I can
switch from one to the other.
I ran a test, first with only a 330? to 100? transformer and a 100?
to 50? transformer to match the RF voltmeter. (there's a good reason for
two transformers)
I adjusted the signal generator to have a 330? output impedance, (to
simulate my antenna). I set the voltage on the signal generator so the
output of the 100? to 50? transformer had an output of 0.01V at 1MHz.
Now I switch to the amp, the voltage increases to 0.058V. That is a
voltage ratio of 5.8. I plug that into this calculator and it says I
have a 15.2db gain. (Remember this is the ratio the output voltage using
the transformers vs the amp)

I have not played it on the radio at this gain, that's for another day,
I don't think it has any problems, but it might.
************************************************************************

>  To clarify, if I'm using the matching transformer on the antenna and
>have 0.1v output and then I swap the transformer for the amp, I then
>have 0.312V output. Thus my 9.8db gain.

>9.8dB voltage or power gain?  The input and output are two radically
>different impedances, neither of which is 50 ohms.  Did you measure
>this with an oscilloscope or spectrum analyzer?  If so, how did you
>arrive at the 9.8dB gain?

******************************************************************
As above, I measured the voltage ratio and used the online converter.
Voltage ratio was 3.2.

********************************************************************

Source followers just love to oscillate.  Did you look for
oscillations when measuring the output?  The J310 is a UHF device that
works nicely up to about 1GHz.  If you build your BCB 1MHz amplifier
as if it were a broadband amplifier (i.e. no 1MHz tuning), it's going
to oscillate at some frequency because of the inductances of the long
lead lengths.  The ferrite beads might help, but in my never humble
opinion, if you're going to use UHF devices at 1MHz, you'll need to
bypass everything from 1MHz to 1GHz and probably do something to limit
the gain above about a dozen MHz.

*******************************************************************
Noted: I'll be looking for that.
With the amp energized, when I disconnect the signal generator from
the input, the RF voltmeter drops to 0V, even on the 1 mV scale. That's
a hopeful sign, but the meter is only good to 10MHz.

Thanks for engaging with me on this, this project has taken a long time,
I finally got signal from the antenna to the radio 3 or 4 days ago.
My first RF was mostly from the feed line, then I added the 100? to 50?
transformer at the radio. In my haste to hear it play, I just connected
the CAT6 wires to the connector on the radio. That let all the common
made signal through. I still have some feed line pick up even with the
isolation transformer, I have some ideas to reduce it further.
Here's a schematic drawing of the switching box. I added the voltmeter,
might help understanding.

And a picture of the layout.

Thank, Mikek

Re: Question Re: 2 FET follower use
On Wednesday, May 20, 2020 at 11:18:40 PM UTC-4, amdx wrote:

There's a problem with that.  You said you got a 1.8 voltage gain from matching a high impedance with the antenna low impedance.  The voltage follower has 0 dB of gain.  How could you get 3.2 voltage gain from using this amp???

Are you familiar with the theory of simple transistor amps?  Did I misunderstand what you are doing?  There is some sort of disconnect somewhere.

--

Rick C.

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Re: Question Re: 2 FET follower use
On 5/20/2020 10:30 PM, Ricketty C wrote:

The antenna is a voltage source with a 330 ohm internal impedance, if
I match that with a 330? to 50? transformer and connect it to a 50 ohm
radio, I loss 1/2 of my voltage across the internal impedance, vs
feeding the antenna into the high input impedance amp.
Second, a 330? to 100? impedance transformer is a voltage  step down
of 1.8 to 1. The amp does the impedance conversion without any voltage
step down.
The amp gain is relative to the transformer step down.

Mikek

Re: Question Re: 2 FET follower use
300 kohm is a ridiculously large impedance on a RF frequency.

Mike, how much capacitance corresponds to 300 kohm at 1 MHz?

It is practically impossible to get less than around 10 pF
or more of stray capacitances.

--

-TV

On 21.5.20 06:18, amdx wrote:
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Re: Question Re: 2 FET follower use
On 5/21/2020 3:25 AM, Tauno Voipio wrote:

I don't know what that havoc conspires to do.
Resonance, but...

Mikek

Re: Question Re: 2 FET follower use
On 21.5.20 13:47, amdx wrote:

It spoils your impedance level by a factor of at least a decade.

--

-TV

Re: Question Re: 2 FET follower use
On 20/05/2020 22:35, Jeff Liebermann wrote:

Are those phasing dots right?

The 5.2V reading could be influenced by the voltmeter the OP used?

piglet

Re: Question Re: 2 FET follower use
On 5/21/2020 2:12 AM, piglet wrote:

I recall calculating the meter as a 10M? resistor in the circuit.
But please, I posted that schematic with those numbers when I was
trouble shooting the amp. I should have removed those voltages
before I posted it.
Mikek

Re: Question Re: 2 FET follower use
wrote:

Oops.  You're right.  The phasing dots should be on the same end of
the transformer.
<

Oops 2.0.  You're right again.

The voltage is set by a resistor divider made of two 1M resistors.  In
order to produce 5.2v instead of the theoretical 6.0V, the voltmeter
would need to have an input resistance of:
5.2/12 = 0.433
To get this ratio, the grounded 1M resistor should be:
0.433 * 2M = 0.866M
To obtain 0.866M, a meter resistance in parallel with 1M would be:
0.866M = (R * 1M) / (R + 1M)
Therefore R (meter resistance) = 6.5M

I couldn't find any ohms/volt specs for my Extech/RS 22-816
multimeter.  So, I measured it.  I put a 10V power supply and a pile
of high value resistors in series with the DVM input.  When the meter
read 5.0V, the meter resistance is equal to the resistor value.  I
strung 10ea 1M 1/4w 5% carbon film resistors in series and ended up
with 4.95V.  Therefore the meter resistance is about 10M.  It's not my
predicted 6.5M but with all the probable errors involved, is close
enough.

Since I had the 1M resistors handy, I put two in series to form a
divider similar to the RF amp circuit.  With the DVM across the
grounded resistor and 12V applied, I got 5.48V, which is close enough
to the measured 5.2V.

So, you're right.  It was meter loading that caused the voltage error
and not a blown J271.

My background is in marine radio design, where equipment is expected
to operate normally when somewhat wet.  Under some conditions, G10/FR4
is a can simulate a sponge.  Therefore, low impedance and resistance
design is the norm.  Every time we used resistors higher than about
10K, we got into trouble with board leakage and water related
problems.  I think 100K was deemed the largest value considered
acceptable, although we did use larger values when desperate.  I would
never have used 1M resistors in anything.  With low resistances, meter
circuits is a consideration).  My apologies, but after years of living
in a low impedance world, I just didn't think about the possibility
that the meter would affect the measurement.

In this design, the antenna impedance is about 400 ohms.  10 times
that is considered sufficient to prevent the JFET bias resistors from
could have been as low as 80K and not had an effect.  Optimum NF
(noise figure) is not an issue in the BCB (broadcast band) where the
atmospheric noise is so much higher than front end receiver noise[1].

[1]Except if one is designing a MiniWhip RF amp, where the antenna
impedance is so high that 1M to 10M bias resistors are common:
<http://www.pa3fwm.nl/technotes/tn07.html
<http://dl1dbc.net/SAQ/miniwhip.html

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Re: Question Re: 2 FET follower use
On 20/05/2020 10:35 pm, Jeff Liebermann wrote:

Perhaps there is no need since one pot will set the Id for both FETs?

piglet

Re: Question Re: 2 FET follower use
wrote:

You might be right.  I didn't notice that.

The source voltages for both JFET's on the original schematic
<https://www.dropbox.com/s/x6z2vpbseri7ysu/2%20FET%20Troubleshoot.jpg?dl=0
show 7.62V and 5.35V, which is roughly what I would expect for maximum
output swing from the source followers.  The average is:
(7.62 + 5.35) / 2 = 6.49V
which is close enough to 6.0V to work properly.  It would be tempting
to split the 220 ohm resistor in two with two 100 or 120 ohm
resistors, and adjust the potentiometer for 6.0V at the junction of
the two resistors.  However, that assumes that the two JFET's are
reasonably well matched.  If that's not the case, then a 2nd
potentiometer might be a useful addition.

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Re: Question Re: 2 FET follower use
On Wednesday, 20 May 2020 09:09:16 UTC-4, amdx  wrote:

d a

? output

.

tor

=0

No details specs, an answer. BW ATI, NF, Cable type , etc etc

Re: Question Re: 2 FET follower use
On 2020-05-20 09:09, amdx wrote:

I don't believe that a pair of mismatched discrete FETs running at 10 mA
has an IP2 of +88 dBm, or an IP3 of +41 dBm.  +10 dBm / 0 dBm would be
more like it.

Where in the world did you get that circuit?

Cheers

Phil Hobbs

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Principal Consultant
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Re: Question Re: 2 FET follower use
On 5/20/2020 5:10 PM, Phil Hobbs wrote:

Hi Phil,
I can't vouch for any of the numbers, the designer is Dallas Lankford.
He was well aware of of problems caused by intercepts, especially in the
AMBCB, because there is a signal every 10kHz to cause havoc. I think he
started building and experimenting with Norton noiseless feedback amps,
in 93' or 94'. I have only started testing the antenna system, So far
the amp acted as expected. In the little use I gave it, I didn't notice
any intermod. I have other bugs I'm working on, Mainly feed line
ingress. I'm making all the mistakes, so learning a lot.

Dallas was well respected in the AM DXing community and had dozens of MW
antenna designs. The BCB is difficult because you have many stations on
the same frequency and because of the wavelength, it is more difficult
to make a directional antenna.  He had a few designs with 4 antennas
phased together. He had most of them on a yahoo group and then 6 or 8
years ago, he said, if you want anything on the site get now, I'm
shutting down the site, he did and and I haven't heard anything about
him since. I suspect the amp is better than you think, he was pretty
meticulous about everything he did. Also a bit cantankerous.
btw, here's the two trimpot version of the schematic.

Mikek