[time-nuts] TNS-BUF update

[Gerhard Hoffmann]

Am 19.08.2018 um 10:22 schrieb Bruce Griffiths:
> Rick
>
> I devised the bias circuit for the TNS-BUF.
> It exploits the fact that for a RED LED at least the difference between the LED forward voltage and the transistor Vbe is ~ 1V and has a fairly low tempco and has low noise (at  least for RED LEDs).
> (Most of the LTSpice LED models do not correctly predict LED forward voltage drop tempco.)
>
> Most classical  schemes for biasing BJTs use a resistive voltage divider which inevitably couples power supply noise into the BJT collector current.
>
> John Miles changed the bias circuit of some classical series shunt amp buffers to one similar to this and the buffer flicker phase noise was significantly reduced.
>
> In principle an LED could be used to directly set the dc bias at the base of the amplifier transistors, however inductors may be required to shunt part of the emitter series resistance at dc to allow the desired dc collector current to be established. A pair of series connected LEDs buffered by an npn emitter follower would allow the bias voltage to be shared by all stages and allow the inductor to be replaced by a capacitor bypassing part of the emitter to ground resistance required to establish the desired collector current whilst achieving the required resistance from RF to ground for RF.
Not all red LEDs are created equal. For noise, by far the best I have found
is the HLMP6000 by HP / Avago / Whoever_owns_it_today.

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       >

0 dB is 1nV/rtHz, +20 dB is 10 nV/rtHz and so on.
LEDs are ineffective photo cells, probably because of the large band gap and
their built-in color filter. I have given up to apply the blob of black 
laquer,
I have never seen a difference.

What I find impressive is the noise performance of low voltage Z-Diodes.
We are always told that Zeners are noisy. No. Avalanche breakdown is noisy.
Take a look at these NXP BZX84C2V7 and C3V3. Admire the low 1/f corner
and note how things turn bad when we approach 5V.

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The precious 7V LM399 reference is a complete joke in comparison.
Its oven is no help here.

> Classical bias schemes are usually much noisier especially at low frequencies. Even regulators like the LT3042 are quite noisy at frequencies below the the pole of the reference circuit low pass filter.
That does no justice to the LT3042. It features 2nV/rtHz to _very_ low 
frequencies.
It is very easy to bypass a constant current source while it is very 
costly to filter
a low impedance LED. The LT3042 even has a startup circuit so that it 
does not
take forever to get to the right voltage. Most LEDs are much worse, and 
especially
at low frequencies.

The LT3042 is a piece of art. It leaves the rest of the regulators that 
we know
in the dust, by 40 dB or better.

Most of the noise < 50 Hz or so goes on my 89441A and the too-small input
coupling capacitor of my preamp. (20 ADA4898 op amps in par, 220 pV/rtHz)
The preamp has been fixed with a costly :-( wet slug tantalum in the 
meantime.

regards,
Gerhard