Amusingly, but not surprisingly, the "nudge the swing" class-C version has the best spectral purity :-) ...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
The only thing bipartisan in this country is hypocrisy
It is not. Simulate it and look at the whole cycle. Pay attention to the directions of the various currents.
The pic above is zoomed up on the small interval, about 50 us, when the collector swings below the base. That's when the AGC happens, as charge is pulled out of the base capacitor and transistor bias, and transconductance, are servoed down. In fact, the drive into the tank is sustained over the whole rest of the cycle. The emitter current is a nice smooth curve and it doesn't cut off at the opposite peak of the cycle.
One reason the amplitude is so predictable and stable is that it's NOT running "class C."
Try it. Note especially the waveform at the base.
No. The gain mechanism is active through almost the whole cycle. But the naming of "class A" versus "class B" is a ham-radio sort of thing. It doesn't explain what the circuit is actually doing. The circuit doesn't care what letter you call it. I don't either.
Young bucks proceed at your own peril. ...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
The only thing bipartisan in this country is hypocrisy
It's a cute circuit, but letting the BJT saturate causes horrible close-in phase noise. Cutoff is a much better behaved self-limiting mechanism than saturation, if you can't use separate AGC.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net
The original requirement was for very high amplitude stability, to excite the LVDT-like inclinometer thing. The signal was synchronously detected, so phase noise, or a little distortion, didn't matter. This was done about 35 years ago, when it was harder to do this sort of thing. One transistor did it, with amplitude stable to a very small fraction of a per cent. Even today, with a bucket of opamps, it would be non-trivial to do an externally AGCd oscillator with this sort of longterm, low-TC amplitude stability.
How could you use cutoff to get this sort of amplitude stability? Actually, this sort of does use cutoff, in that increasing collector swing backs off the base voltage and reduces Gm. James' addition of a Baker schottky is even better, because that limiting happens a bit above saturation type voltages. I didn't have schottkies when I did this one.
For the requirements, that was a good choice--I was making a more general point about self-limiting oscillators.
Saturation is certainly more stable with temperature than cutoff, and has that switching action that you're using. I don't know if a Schottky would be an improvement from that point of view--you still have the 2 mV/K slope.
Widlar did a cute temperature-compensated breakpoint amp using saturation, by driving several feedback networks from the summing junction via emitter followers with collector resistors. When the BJT saturated, the beta dropped and the extra resistors suddenly became part of the feedback network. (National AN4, Figure 8,
formatting link
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net
The base voltage needs to be +1 jd for the thing to oscillate at equilibrium. The clamp device, c-b junction or schottky, has about the same tc, but in the opposite direction. In the c-b case (with a low secondary swing, not some awful class C thrashing, and a big base cap, so it's not a blocking oscillator) the collector swing that pulls current out of the base capacitor touches zero, near where the transistor saturates. In fact, the tc of the c-b junction is canceling the tc of the e-b junction. Not perfectly, because they are effectively used at different currents. Because of reverse beta, some of the tank energy can be clamped through the emitter, which is a bit negative at that instant. But even that current flow happens when the c-b junction is forward biased.
Schottky: suppose we need +0.6 on the base for stable oscillation. The collector will dip down to, say, 0.3, the schottky will conduct and pull charge out of the base cap, and it will stabilize there. The tc's almost cancel. If you assume the amplitude limiting is just brute clamping of the swing, the tc's still cancel. No saturation, since the c-b junction doesn't get forward biased. This assumes you've chosen a suitable base resistor, not jamming too much current into the base, and the transistor has reasonable beta.
Actually, the base AGC thing must be happening. With small feedback voltage into the emitter, a sim shows that the transistor is on throughout the cycle, "class A." So Ic ~= beta * Ib, on average. Average Ic would be large if the current through the base resistor were all actually going into the base. But it's not: the base cap is being discharged at the negative swing of the collector, stealing base current and reducing transconductance, and that is exquisitely sensitive to p-p amplitude. Some tank energy does of course get lost to the emitter... both mechanisms are at work. Fortunately, both stabilize the amplitude.
In my sim, if you short the tank, the supply current goes up about
30x. (Hmmm, Rb could be bigger!) Output swing is 10.11 volts p-p with a 5-volt supply.
Several people have modeled this circuit with small base caps and low transformer ratios and lots of base bias current. That works differently.
Interesting, thanks. I agree that the Baker clamp pretty well fixes the saturation problem.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net
Phil, Don't you have means to simulate Larkin's oscillator?
If you bothered to do that you'd find some of Larkin's statements above are _absolutely_ incorrect, and others are so hand-waving as to be hilarious.
But, hey! Ignorant young bucks with no knowledge of basics are good for business :-) ...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
The only thing bipartisan in this country is hypocrisy
All the schools teach anymore is CAD. No one seems to know even the very basic behavior of transistors, or resistors, and even less about energy storage devices such as capacitors and inductors. Sigh :-( ...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
The only thing bipartisan in this country is hypocrisy
"Jim Thompson" wrote in message news: snipped-for-privacy@4ax.com...
Well, as I've suggested before, I think there's reasonably good money to be made in giving courses on this stuff. Granted, giving live courses involves travel and finding people willing to pay the average going rate of ~$500/day for professional instruction, but selling DVD at, e.g. $99-$399 works pretty well too. Even books can be OK, although it seems to me that there's rather more grunt work involve with books for each dollar you can charge than with DVDs, and from a student's perspective DVDs allow for a somewhat more hands-on feel anyway.
Check out, e.g.,
formatting link
... $110 for 50 minutes of "Introduction to the Smith Chart?" I doubt the author, Glenn Parker, spent more than a week working on it!
I'm hip deep in measuring very small nonlinearities in InGaAs photodiodes just now, though, and since (unlike some folks) I have no axe to grind, there's no reason for me to do that. I just like talking about electronics.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net
I have no axe to grind, other than intellectual honesty.
ANYONE who posts crap is subject to my wrath, particularly when they insist on defending crap :-) ...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
The only thing bipartisan in this country is hypocrisy
Actually I'm also hip deep in writing a program to estimate active core area of a chip from the netlist... basically mathematically "drawing" each cell, then calculating its area, then summing up all cells. ...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
The only thing bipartisan in this country is hypocrisy
Low distortion sine wave + DC driving IR LED PD goes to TIA with x64 AC gain and x1 DC gain Auxiliary light source (fibre coupled laser in this case) provides CW light Bridge circuit (like an old HP THD meter) to null out sine wave and improve dynamic range
Set up IR LED to give some decent AC photocurrent like 5 uA, null carefully, move laser in and out while changing PD reverse bias voltage.
Look at residual on digital scope with FFT.
This allows me to see nonlinearities down to about -70 dB. It could go further, but the silly laser mode hops like mad, and it takes forever to integrate it away.
Interim conclusion: InGaAs is a lot less linear than Si. The quantum efficiency improves by something like 5% for bias levels between 0 and 5 V.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net
So you are sort of summing light sources. I guess.
If you measure the magnitude of the AC component of the PD signal, at various points of added laser light, you get the slope gain at various points, and the distortion of the LED doesn't matter. Is that the idea?
LEDs are pretty limear at higher currents, but I don't know about 70 dB. Actually, it wouldn't be all that hard to find out.
Does that imply amplitude nonlinearity at fixed bias?
What about temperature? Maybe the bias voltage needs a TC correction.
Exactly. Just like a two-tone tester, where you sum the tones right at the output to avoid IMD internal to the tester.
Yes. That measures the slope rather than the actual curve, which is what I'd really prefer in this instance.
Yes, and it's worst at zero bias--maybe 2x or so, in the range I care about. Interestingly the AC output signal shows a straight-line growth with bias--effectively the quantum efficiency goes up about 5% from 0 to
5V bias.
I hope not--this gizmo is going to have to work in a fairly harsh environment eventually. Today I'm going to do an eyeball fit to the distortion polynomial, to see if I can predict how much the output gets compressed at higher powers. It's on the order of 2% at 100 uW in a
300-um diameter detector at zero bias, which is really stinky compared with silicon.
And InGaAs is the best of the infrared detector materials. Infrared is just a lot harder than visible, for a whole bunch of reasons.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net
Turn them on and off in all possible states, measure PD current with a very good meter, and analyze the results. See how well things sum that ought to sum.
You could also trim the individual LED currents. It would be reasonable to do a series of experiments that would precisely adjust the LED light outputs to be equal, or to be precise 2:1 (optical binary DAC) steps. Then you can play with combinations. LED self-heating would be the biggest error, so something clever would need to be done about that.
That's the sort of thing that would be fun to do as a product, if you thought that anybody would buy it.
Knowing nothing about semiconductors doesn't keep me from speculating. Maybe the high defect density causes recombinations at low drift velocities.
At any rate, I should keep my PD power supplies stiff.
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