Using a higher dissipation transistor package for the output stage is also the
strategy adopted in some commercial versions of these amplifiers.
Note that the amplifier with the 4:1 (turns ratio) output transformer and 300
ohm resistor in series with the primary of the input transformer is intended to
have its input connected in parallel with 5 others to form a 6 output
distribution amplifier.
If it is redesigned for a 50 ohm input with a 2: (turns ratio) output
transformer the PN floor is reduced significantly.
The biggest drawback for some applications is the 24-28V power supply.A 12V or
lower voltage supply would be nice.This can only be achieved (without unduly
raising the amplifier PN floor) by something like a folded cascade of NPN + PNP
+ NPN CB stages or by transformer coupling between stages. Using a CB input
stage has the advantage that the input signal voltage swing doesn't reduce the
available collector voltage swing as much as in a CE stage.
Bruce
From: Charles Steinmetz <csteinm...@yandex.com>
To: Discussion of precise time and frequency measurement <time-nuts@febo.com>
Sent: Wednesday, 30 March 2016 5:51 PM
Subject: Re: [time-nuts] high rev isolation amps
Bob wrote:
>There were (and maybe still are) SOT-89 versions of the 2N3804 and
>3906. They will handle more power than most of the other versions.
>That gives you better Vce on the string.
Bruce wrote:
>The PZT3904 and PZT3906 are still available.
>With most of these old circuits reducing the LF noise contribution
>to the emitter/collector current by the biasing circuit by utilising
>lower noise power supplies and/or using improved biasing methods can
>improve the clse in PN significantly.
The lower transistors on the totem pole operate in current mode and
typically have only 3-5v from C to E, so their dissipation is very
low even if the standing current is high. I used TO-92 and SOT-23
transistors there. The top transistor, which must develop the output
voltage, dissipates more than a TO-92 or SOT-23 should be asked to
handle, even with a good heatsink, so I typically used TO-5 (TO-39)
or SOT-89 transistors (with heatsinks) there.
It is definitely true that the published NIST designs did not
minimize circuit noise. I used *much* larger base bypass capacitors,
and improved capacitance multipliers. I also used separate
capacitance multipliers for the base divider string and the collector
supply. I played with ultra-low-noise DC regulators on each base and
for the collector supply, which gave modest improvements in both
noise and distortion -- but the added complexity was not justified by
the gains, IMO. I did find that doing without the interstage
resistors (from the collector of one transistor to the emitter of the
next higher transistor) reduced both noise (modestly) and distortion
(slightly). In the very rare cases when a circuit oscillated (I
experimented with *many* combinations of transistors), I used ferrite
beads in place of the resistors or on one or more of the base
leads. Choosing transistors with very low base spreading resistance
was another key to lowering noise.
Best regards,
Charles
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