Re: Twin T circuit wanted

I'm not a big SPICE user in general, though I've done more SPICEing in the last year than in the 20 previous.

For some things, such as really weird photodiode preamps where nonlinear capacitances and so on are important, it can help a lot--provided the models are semi-decent. For most other things, I prefer algebra. You learn a lot more about how the circuit works by crunching the math than by poking the simulation until it seems to work.

Cheers

Phil Hobbs

Spice is handy for tweaking control loops, especially higher-order loops that matter, or when anything nonlinear is going on. Or when you don't have a hard definition of what "right" is. Simple opamp type things are better to do with guesswork, or a scribbled Bode plot. You lose points if you use a calculator.

Spice is good for filters, too.

I did a constant-voltage/constant-current crossover power supply sort of thing recently, analog ORing two loop error signals, with a wide range of possible customer loads, and it was great for tweaking. The actual implementation will be firmware.

The other good use for Spice is grinding out the numbers on voltage dividers, pure DC analysis, just to save a lot of calculator stuff.

We never simulate whole products, or even whole circuits, just little pieces, or control loop abstractions.

John

th

=A0If

oes

OK I wasn't talking about saturation, which is a whole other regieme. (sp)

y at

ce

to

Cool, Though I'm not sure I'd call that peculiar. You're lining up all those spins in a piece of iron, that stores energy.

I never use Z5U for bypass. X7R is what I like... though some X5R creeps in via purchasing.

George H.

What's the 6th part, I wonder?

I'm sure he would, but why should I, over the years you've insulted me at least as much as him, and perhaps more aggressively?

Anyway, he did explain it, SFAICT.

Note the BJT is over-biased - plenty of base current, that if left unchecked would charge the base-to-ground capacitor and over-current the transistor. So the oscillator runs and examining cycle-by-cycle, the collector swings higher and higher until it goes negative with respect to the base voltage, close to saturating the transistor, and turning on the base-collector diode a bit, robbing current from the base capacitor. This process servos the BJT current to just the right level to sustain an oscillation collector-voltage level where just the right amount of current is robbed each cycle to control the base voltage. Thereby insuring that the collector goes close to the emitter on each cycle, establishing a tightly-controlled amplitude, which as John pointed out, is temperature independent to first order since Vce(sat) is relatively temperature independent.

John said Vcc peak, but actually it must be closer to Vcc - Vce(sat).

I'll buy that the collector forwards biases, and you enter a limit cycle. Thus I'd call it ALC. I don't see any _gain_ variation that "AGC" would imply. ...Jim Thompson

Hi Jon, so good to see a post from you. Everything OK?

I'm going to have to go an reread Fenyman on this... (I thought I got it the last time, but that's how it is with Feynman.) I think there's a magnetic field everywhere in the magnetic circuit. In an electro- magnet it starts with the current in the coils and is transfered to the gap by the magnetic material...iron. But the field is everywhere in the circuit. It may very well be that the gap determines the field strength. But the magnetic energy is stored over the entire volume of the field.

George H.

So that makes the A_L

Picky, picky. To my mind, the base current robbed by the collector starves the base, lowering the CE stage's gain, until the exact equilibrium is achieved. ALC, AGC, pick your name as you like. Either way it gets the job done rather nicely, and is a bit different from what we've seen elsewhere, such as in old radio circuits. I see that it has been analyzed as a possible RF oscillator technique. But it seems to me that, working as we imagine, Vce(sat) and all, this trick would be limited to far far below fT.

----------------------------------------- ^^^^^^^^^

----------------------------------------- maximized

Yep, one winding has a well-establish inductance, not at all like a common transformer, but the second winding is well coupled to it, and with low leakage inductance, just like a good transformer.

We really need a new name for these beasts, inductors with transformer windings. BTW, toroid cores need not apply. Very poor AL control.

Nah. Still horribly busy around here -- probably going to be killing myself through the summer... too much to do (or, at least, I hope I can force myself to keep at it without giving up at some point.)

I _imagine_ aligned atoms as 'short circuits' that bridge vacuum. Vacuum is where the energy gets stored. (I think a tiny amount of energy is 'used' to align an iron atom, for example. But it is tiny and doesn't represent recoverable energy... I think it gets converted to heat in the end.) I'm very much open to being wrong, of course.

With all this in mind, it's easy to imagine the concept of permeability arising. For air core, it's vacuum all the way down, so to speak. All of the loop length is, in effect, the magnetic path length where energy gets packed away. But when you insert iron atoms chained end to end they act as short circuits, each of them bridging across a small bit of vacuum to make the effective length of vacuum shorter (when aligned

-- the molecules of O2 and N2 don't align and don't bridge so in effect they simply don't count and don't short the vacuum.) So if you imagine a core material inserted there, what counts is the interstitial vacuum that remains when computing the magnetic path length. But what remains?? Well, that depends. You can take a ruler and measure that core piece, of course. But that ruler won't help you figure out what the effective remaining vacuum length is and that is what you need to know. So in order to come up with a number that represents the percentage of the measurable length, you need a factor of some kind. This is the permeability figure. A permeability of 1000 would mean that if you had a toroid with a mean centerline path length measurable with a ruler of say 100mm, that the actual vacuum path length would be 100 microns and the other 99.9mm would be shorted out by the iron that is present there. A permeability of 100 would mean that

Jon

The peak detection can put a tiny flat on the negative swing of the sine wave. A small resistor in series with the collector fixes that and doesn't seem to do a lot of harm otherwise.

But he didn't insult your wife, too.

Oh, picky picky. It's actually, probably, a bit more complex, since the emitter is a little negative at the instant of collision, and the transistor is almost saturating, so some of the stolen base current is going into the collector and some is going into the emitter. It would be interesting to simulate, just to see where the currents really go. Adding the collector resistor changes things, too.

But the AGC thing does work, and the TC is close to zero.

John

When I post circuits, JT and JF pull out dictionaries and start arguing about what words mean. Circuits don't spend much time reading dictionaries.

John

The permalloy power cores are fairly good about Al. I think I did the ship fog bell with them. Can't remember for sure, after 3.5 decades.

John

SPICE 1 was introduced at a conference in 1973. It wasn't very good, and SPICE 2, 1975, was better. DRAMS were introduced in 1970.

John

Just to clarify, the RF versions I posted are similar to, but not the same as John's. They're standard UHF designs, Class A, without John's precision AGC. I don't think they can use John's AGC method directly-- if saturated, the transistors would be too slow--but maybe a Baker-ish clamp thing would do the job.

Oh, and John's oscillator really swings ~ 2* (Vcc + Vbe), not 2* (Vcc

- Vbe). Reason being, the AGC operates as the average base voltage gets sucked down to near 0v, killing the gain.

The collector has to swing downward from Vcc to ~Vbc _below_ GND to forward bias the b-c junction on the negative peaks, and swings up a symmetrical amount on the positive peaks.

-- Cheers, James Arthur

Z5U, and to a lesser extent X5R, are so bad you have to use almost 1uF when you wanted 0.1uF. Since X7Rs are the same size and price (almost), I get X7Rs too. :)

Tim

MPPs are often specified to high tolerances. Still gotta keep 'em away from DC though.

Tim

I did one of those the other day. Works perfectly. 5V, 100A variable output. Jam a screwdriver on the output... all you'll get is a warm screwdriver. ;-)

Tim