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.
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!
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.
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.
formatting link
ff-anyhow His apprentice did a later piece on it which does include (unbroken) images :
formatting link
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?
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
formatting link
Interesting article. Someone needs to do a simple varactor front end in LTspice that shows how it works.
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. :^)
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?
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).
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. ;)
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.
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.
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.