Low voltage negative resistance oscillator design, lessons learned

Same here... windows and chrome. And thanks for the schematic.

Hey if you are into analog electronics and haven't read it yet the Jim Williams collection, "Analog circuit design, art science and personalities" Is a fun read. I think it has the P2 story by B. Pease that piglet mentioned up thread.

George H.

Reply to
George Herold
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I had another look at the teardown photos and I think the resistor from the 2nd stage bipolar npn emitter to ground that didn't get it's colors written on the first sketch is "red, vio, gold, silver" which would be

2.7ohm and makes sense since you want to feedback only a tiny fraction.

BTW thanks for drawing the resistors on the new drawing the US way - I have always preferred zigzags myself!

piglet

Reply to
piglet

A brilliant piece of sleuthing. I have to concede.

You are helped by the fact there is no other device like the AD310/311 in the AD catalog. I would have expected a ceramic baseplate for better rigidity and resistance to humidity, but I guess what looks like an early version of FR-4 worked well enough to avoid the expense.

Your analysis has to be one of the highest level I have seen on this newsgroup. Congratulations. I'm impressed.

Thanks

Reply to
Steve Wilson

Thanks. Already conceded.

Reply to
Steve Wilson

Yes, FR4 (or G10 or whatever) is fine when the hi-Z stuff is all on teflon standoffs and doesn't touch the FR4 :)

Thanks very much Steve, credit should go the carefully taken teardown photos and schematic sketch.

piglet

Reply to
piglet

e

Wow, thanks! That's nice to hear.

liams collection, "Analog circuit design, art science and personalities" Is a fun read. I think it has the P2 story by B. Pease that piglet mentioned up thr ead.

Analog is absolutely my hobby and career path. I've read/scanned the book, I liked it quite a lot.

Bob Pease has an article on a single-transistor varactor-based opamp.

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ff-anyhow His apprentice did a later piece on it which does include (unbroken) images :
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er-stuff-anyway I never did get around to designing one of those. I never fully understood how it works either. The varactor tank couples some signal into the base if there's a dc input, but how could that ever have a large enough influence to make a reasonable amount of gain?

Reply to
j.ponte

Yes. I mentioned up thread the link is

If you have not tried LTspice yet, it is invaluable. It has a steep learning curve but there are plenty of examples all over the web and YouTube. Run into a problem? Just google it. The trick is how to phrase the search. Check out the Wiki at

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Interesting article. Someone needs to do a simple varactor front end in LTspice that shows how it works.

Reply to
Steve Wilson

Ahh why spice? (D = delta, the change in) and - is goes to) so DV-DC-DI, as the bridge unbalances, DI depends on the modulation freq.

Well that's my thinking, and then synchronous detection with the fet. Is the fet demodulator a V= +1, 0 thing, rather than V= +1, -1?

We use this capacitance difference sensor, which has a similar front end, but doesn't use magnetics, and a bridge, but a diode ring de-modulator (there's a ton of leakage in the diode ring, and you learn about CMRR. :^)

George H.

George H.

Reply to
George Herold

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Yes, I used it as an undergrad. I still use it at home, although mostly as a way to store simulateable schematics and check my intuition. Good stuff. I collect LTspice functions that aren't in the help file and other tricks. So far my favorite are the AKO command and using VCCS/ICVS as time-variable resistors.

-s

fi

If you're referring to how the bridge produces an error signal, it's quite simple. An applied voltage shifts both varactors along their C-V curves and that unbalances the bridge, just like e.g. a strain-gauge based wheatstone bridge except with an AC stimulus. The cute thing is that the AC stimulus is generated by the two windings that make up half the bridge. Then the conversion gain is just the dC/dV slope of the varactors.

I think the simplest way to look at it is as an AM-modulator where the inpu t is the "data signal" and the stimulus the carrier.

What I don't entirely understand about that single-transistor unit is how i t makes any gain. It seems to phase-sensitively feed the AC signal out of t he bridge back into the base of the transistor by using the quasi-DC input error. But why are those 330 Ohm resistors there? I guess the change in collector voltage comes from some (presumably relativ ely subtle) change in oscillation amplitude due to the signal injection at the base, causing a change in Ic which modulates the IR drop of the collect or resistor. It seems extremely finicky to me, not like something that has a nice design approach.

Yes I think so. Chopping the + or - part of the waveform gives the phase se nsitivity, you just lose 6dB of gain.

r

That sounds interesting. The diode ring acts like a set of chopper switches ? Can you say a bit more about it?

Reply to
j.ponte

Now I'm wondering if perhaps it uses the fact that it's running fast w.r.t. the input signal changes to have the amplitude decay or increase fast, relatively speaking, by pushing the loop gain slightly above or below the Barkhausion gain criterion. I guess in that case the gain is mostly limited by the ratio between the oscillation frequency and the desired speed of operation (how slow the output filter is).

Reply to
j.ponte

ersonalities_0750696400.pdf>

tuff-anyhow His apprentice did a later piece on it which does include

: er-stuff-anyway

I haven't looked at it yet, but it sounds like a riff off the usual transformer-based differential capacitive position gauge. You can get down to picometer displacement sensitivities if you don't mind waiting a bit. ;)

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
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Reply to
Phil Hobbs

snipped-for-privacy@student.utwente.nl wrote:

I went ahead and made a bridge using bipolar transistor per your schematic.

It appears the input DC signal turns on either transistor as it sweeps from

- to + 200mV.

The input current is the current when the junction is forward biased. It is in nA, not in the pA level. That is hardly electrometer (1e-14) level.

Here are the LTspice files. Please let me know if you find something wrong.

Version 4 SHEET 1 1204 680 WIRE 432 -16 352 -16 WIRE 512 -16 432 -16 WIRE 608 -16 512 -16 WIRE 432 16 432 -16 WIRE 608 16 608 -16 WIRE 352 64 352 -16 WIRE 368 64 352 64 WIRE 80 160 32 160 WIRE 208 160 80 160 WIRE 352 160 288 160 WIRE 400 160 352 160 WIRE 432 160 432 112 WIRE 432 160 400 160 WIRE 608 160 608 96 WIRE 656 160 608 160 WIRE 688 160 656 160 WIRE 720 160 688 160 WIRE 832 160 800 160 WIRE 688 176 688 160 WIRE 832 176 832 160 WIRE 32 192 32 160 WIRE 432 208 432 160 WIRE 608 224 608 160 WIRE 352 256 352 160 WIRE 368 256 352 256 WIRE 688 256 688 240 WIRE 32 288 32 272 WIRE 432 320 432 304 WIRE 512 320 432 320 WIRE 608 320 608 304 WIRE 608 320 512 320 FLAG 832 176 0 FLAG 400 160 Q1E FLAG 512 -16 Q1C FLAG 512 320 Q2E FLAG 32 288 0 FLAG 80 160 Vin FLAG 688 256 0 FLAG 656 160 R2C1 SYMBOL res 304 144 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R1 SYMATTR Value 100k SYMBOL voltage 608 0 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V1 SYMATTR Value PULSE(-50m 50m 0 1u 1u 4u 10u) SYMBOL voltage 608 208 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V2 SYMATTR Value PULSE(-50m 50m 0 1u 1u 4u 10u) SYMBOL voltage 32 176 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V3 SYMATTR Value PULSE(-0.2 0.2 0 10m 0 0 0 1) SYMBOL res 816 144 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R2 SYMATTR Value 100k SYMBOL cap 672 176 R0 SYMATTR InstName C1 SYMATTR Value 10n SYMBOL npn 368 16 R0 SYMATTR InstName Q1 SYMATTR Value 2N3904 SYMBOL npn 368 208 R0 SYMATTR InstName Q2 SYMATTR Value 2N3904 TEXT 408 -104 Left 2 !.tran 0 10m 0 TEXT 408 -136 Left 2 ;'AD310 Bipolar Bridge

[Transient Analysis] { Npanes: 2 { traces: 1 {524290,0,"V(vin)"} X: ('m',0,0,0.001,0.01) Y[0]: ('m',0,-0.24,0.04,0.24) Y[1]: ('u',1,1e+308,2e-007,-1e+308) Volts: ('m',0,0,0,-0.24,0.04,0.24) Log: 0 0 0 GridStyle: 1 }, { traces: 1 {34603012,0,"I(R1)"} X: ('m',0,0,0.001,0.01) Y[0]: ('n',0,-7e-008,1e-008,7e-008) Y[1]: ('u',0,1e+308,6e-006,-1e+308) Amps: ('n',0,0,0,-7e-008,1e-008,7e-008) Log: 0 0 0 GridStyle: 1 } }
Reply to
Steve Wilson

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Interesting, the beta seems to take an absolute nosedive below about 8nA. P retty much the entire input current is the difference between the base curr ents. Is that large base current physical? I always thought low-end beta ro ll-off was mostly junction leakage.

I assume you would be able to get very close to the saturation current of t he BJTs: the drive amplitude is only 4 thermal voltages at room temperature , so Ic would be about 6.4x higher than Is.

THat does sound like it would be the exact same setup, yes.

Reply to
j.ponte

When used with overrall feedback of course the input voltage will be essentially zero.

piglet

Reply to
piglet

Yes that's a crucial point. What's bothering me is that about 650pA still flows in the simulation when the bridge is in balance. That's odd, either Is is ~100pA or there is leakage somewhere.

Reply to
j.ponte

You need to sit your computer on teflon standoffs :)

piglet

Reply to
piglet

Of course, how could I have forgotten :^)

Reply to
j.ponte

The feedback in the op's schematic does not affect the input current to the bridge.

Reply to
Steve Wilson

How? The feedback is to the source of the input jfet. It has no effect on the input current into the bridge

Reply to
Steve Wilson

The model for the 2N3904 shows IS = 1e-14:

.model 2N3904 NPN(IS=1E-14 VAF=100 Bf=300 IKF=0.4 XTB=1.5 BR=4 CJC=4E-12 CJE=

8E-12 RB=20 RC=0.1 RE=0.1 TR=250E-9 TF=350E-12 ITF=1 VTF=2 XTF=3 Vceo=40 Icrating=200m mfg=NXP)
Reply to
Steve Wilson

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