Re: Norton noiseless feedback amp calculation

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.

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. :-)

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.

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.

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.

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.

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. :-)


--
Thanks,
- Win
.

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