I thought the idea was to go to higher currents. Reducing the 33-ohms might help. Do you have a lot of high-freq noise to deal with? High-freq noise is easily dealt with, using R L and C parts and ferrite beads.
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Thanks,
- Win
The ZTX450 are nice, output noise ~1.5 nV/rtHz. For what ever reason the ZTX851's filtered less noise than the 2n4401's.
George H.
Maybe higher C-B capacitance. I often use an RC in the collector as well, like 1 ohm and 10 uF. With a low-V_CEsat transistor, that doesn't cost any dropout voltage.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Does ZTX851 have SMD version (SOT-23 prefered)?
Driving home I was thinking I should describe the circuit. From the voltage ref. I've got two series 500 ohm R's and two 100 uF Al. electros to gnd. Between the base and the last C/R junction I've got ten ohms and a ferrite bead, Lower r_bb' can only help down to the ten ohm level. (bead and R_b added to stop oscillations.)
George H.
I use that approach too, though I put in a single 0603 in the base, which starts out as a jumper and gets hacked if necessary. (I've only seen one or maybe two cap multipliers oscillate.) The electros probably have tens to hundreds of milliohms ESR, so a 1-uF MLCC can help.
A bead to prevent oscillation plus another big cap on the emitter will get rid of the residual noise pretty well.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Yes, it is not even ripple problem. We have very thin spikes from Traco Pow er's isolated dc-dc converter. And those spikes are really wideband and dif ficult to suppress. They leaks through Murata BLMs and capacitors somehow.. . I'm considering to replace Traco with isolated push-pull converter built around LT1683 (which is slew rate controlled). But it costs a lot of PCB sp ace and some time to understand adum3190 compensation...
I simulated cap mult with single FMMT620 transistor (without PNP) at 500mA load in LTSpice. Behaves very good. Especially in Phil's "5-Pole 2-Stage C apacitance Multiplier" setup. May be FMMT620's model is too optimistic. Hav e to test on bench.
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