Re: Norton noiseless feedback amp calculation

On 6 Jul 2006 02:40:18 -0700, Winfield Hill <Winfield_member@xxxxxxxxxxx>
wrote:

Jake wrote...
On 3 Jul 2006, Winfield Hill wrote:
Jake wrote...
Winfield wrote,

You may want to add an emitter follower.

For Zo = 50 ohm with a follower (BJT) implies a IC of 1 mA or less.
Is that a good idea with an amplifier that needs an IP3 of +20..+30dBm?

However in this application I required a Zo = 250 ohms and gain
in the region of 20dB from a FET.

It's not a good idea to use the output impedance of an emitter
follower as a matching source impedance for a filter, etc.,

Not something I would usually do. I was trying to establish a Zo of
560 ohms in the drain circuit via the feeback winding in the source.

Understood. But not the best idea, IMHO.

Like most things in electronic design there are trade offs. I wanted to be
able to more fully understand the realtionship between Zo, the feedback
ratio and operating current.

My thought was that it had to be be better than a 560 ohm resister in the
drain circuit and amplifier stage with no feed back at all. The objective
was to improve the signal handling capability and dynamic range without
messing up the match by to much.

For the parameters with IC and N I need an equation that provides al least
a ball park figure for Zo. Try as I may I can't see how to solve this.

I was in fact trying to avoid the resistive loss <GB> which would have
to be added to the filter insertion loss. Since it is the RF front end,
noise figure was also important as well as large signal handling
capability over 0.5 to 30Mhz.

because the output impedance is a small-signal value that can
be highly nonlinear with signal swing and transistor current.
Instead establish a nice low Ro value, with 2 to 4mA, etc.,
and then add a series resistor to drive your filter.

It's certainly laudable to be careful with every dB of gain while
the signal is still weak, or the signal impedance is high, etc.,
but once you've gone through a healthy gain stage, e.g. 20dB,
you no longer need to save every dB (or even every 6dB) and other
considerations take precedence, such as low distortion, accurate
filter properties, etc. By adding an emitter follower (EF), you
lower the impedance, so you can then precisely control it with a
series resistor. The loss of signal is of no consequence. Make
the stage gain 26dB if it worries you that much. :-)

Gain has its own problems of dynamic range with high Zo with a Vs of 10Volt
as well as IP3 for the stage. Feedback has the advantage of added
linearity reducing other undesired products of ajacent and large signals.

N turns are the drain winding but Zo must be determined to a large
degree by the source winding which sees via feedback the transformed
source impedance.

Understood.

I wanted a insight to the calculation of the drain Zo with source
feedback transformer so I could see how well established it was
and what would have the greatest influence on it.

Yes. But not a very good approach, unless you're a high volume
manufacturer desperate to save every part. Here, more is better.

For the FET I got 560 ohms or close enough. Device was a
BF966 @ 7mA and N was 35:3 after several iterations and a check
for low frequency response.

Lucky break.

I still have no idea how well established Zo = 560 ohms was for small
and large signals. The original circuit used a 560 ohm drain resister
and no feedback.

Exactly, thereby precisely setting Zo = 560 ohms to insure the
filter would have its designer-intended frequency response.

Sure but I'll bet IP3 and overall linearity are considerably worse.

Use the EF with say 2mA for Zo = 12 ohms (and with over 500mV
filter-drive capability), and follow it with a 560-ohm resistor.
That's 572 ohms, close enough! Or follow it with 549 ohms 1%
to be right on the money.

1% resisters of such values are like hens teeth. ;-)

I would be more tempted to run at 10mA ;-) and 2.5 ohms with RE of 470E.
Then Rseries match of 560E. Should Zin/hFE not be added to this?

Sorry I have never been much of a fan of resistive matching for RF unless
absolutely no other better way exists. One needs to achieve a good design
balance for the full spectrum of requirements most times.

Retro fitting to existing circuits has it own problem of never enough space
or remote mounting adding even more problems. But if the transformer
feedback proved inferior I'll give it a try.

Unfortunately I don't have equipment to check the filter response
with both amplifier circuits or I would have done this. The
possible influence of the filter reflected impedance changes on
the amplifier drain are also an unknown and my thinking was feedback
may improve this.

Improve, yes, but get you where you want to be, no.

A perfect match is not the only requirement but one of a set involving
noise, dynamic range, large signal handling, linearity....

It takes a considerable amount of time and often one has to think real hard
to find a way of evaluating what you need to know. More frustrating is
perhaps better because I know how but lack the equipment. For messing about
there is no chance of recovering a large outlay on seldom used equipment.

Loading the amplifier's high Zo directly with the filter is a
dangerous game, creating distortion as you unwisely rob it of
the excess loop gain it should be using to keep distortion down.

But a sage with no feedback has no such protection or is the IC current
drive to the resister match drain load sufficient to overcome the reflected
impedences?

I am happy to reduce the gain down to 6 dB or whatever the noise figure is
at the filter input plus 1 if needed because it is far easier to add a good
low noise 50 ohm in/out stage at the antenna input ahead of the preselector
or the preselector input which is 50E and 1st RF amp.

Add the EF, you'll be happy, then the exact Zo of the amplifier
stage won't matter.

What does EF refer to? Sorry I am not with you on this.

Emitter follower. Cathode follower (CF) without a filament. :-)

Ok but out of interest and because it has now become a mission I would
still like to know how to calculate the approx Zo for the emitter feedback
circuit given.

I am busy putting the final touches to ALC on my cheap and cheerful Chinese
signal generator and that will allow me to more easily check the filter
under both conditions. I gave up trying to control the tubes large output
with junction fets or cathode degeneration and instead regulated the plate
voltage. Pair of signal diodes + TL431 + 4N35 optocoupler and IRF830 as
the series pass element feeding a 150E resister ahead of the smoothing
capacitor. Rectified and regulated heater voltage was used to supply the
TL431 and opto diode. The 4N35 CE shunts the gate and 15 volt protection
zener driven by a 100K resister by the plate voltage connected to the
drain. Crude but works well once the TL431 is stabilised. Then to modify
my old AVO signal generator with 6J5 tube and figure out how to mount the
stuff. ;-) A BF495 and J310 cascode works fine as a 6J5 replacement.

Regards and thanks for your adviced and insight to the problem.

Peter

.

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