The LT3042 is still inherently very noisy at ultra low frequencies approaching and below the pole frequency (can be well below 1Hz) of the reference filter (There's a limit to the maximum capacitance available/feasible). Forward biased diodes including LEDs are quieter in this region.
Bruce > On 19 August 2018 at 23:51 Gerhard Hoffmann <[email protected]> wrote: > > > > > 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. > > < > https://www.flickr.com/photos/137684711@N07/24354944411/in/album-72157662535945536/ > > > > > 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. > > < > https://www.flickr.com/photos/137684711@N07/24411798996/in/album-72157662535945536/ > > > > > 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 > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
