Low voltage negative resistance oscillator design, lessons learned

What did you tear down and where did you get it?

According to whit3rd, the AD 310 sat in a teflon bushed socket. Your circuit would definitely not fit in a teflon socket.

(Still trying to make sense of this thing:)

Thanks

The other connection, b-e shorted, makes superb diodes with higher reverse voltage. I guess a bit of reverse beta makes the "forward" conduction a little better.

A BFT25 is a better diode than practically any official diode.

The teardown AD310 in the wonderful photos had been soldered into the wider world judging by the tinned tips of the tall pins. However page 98 of the AD catalog at

does describe a mating socket AC1017 that users could elect to use instead of direct soldering. I guess the two input sockets of that AC1017 would have teflon bushings. Teflon wouldnt have been necessary on all the other i/o pins so it is possible those were insulated less extravagantly. In the teardown photos one can clearly see the "hot" hi-Z input circuit internally is on teflon standoffs so it makes sense to keep up the high grade insulation.

piglet

It also greatly delays the onset of high-level injection conditions by sucking the carriers out of the base ASAP.

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

Never tried that--good idea. The Philips model has BF=16.9 for the BFT25A vs. BF=85, fwiw.

Yup, an old fave for that. One of our products uses a small InGaAs photodiode with a TIA whose feedback is 200M parallelled with two BFT25A C-B junctions in series, with a feedback T-network that preserves the full 5-V output range. It's for a more qualitative sort of application, so we don't care much about the diode tempco. That combination gives high sensitivity and constant gain in low light, and in brighter light it preserves the the highest sensitivity consistent with not railing the amp.

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

BR=16.9 for the

FTFM.

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

The data for the AD310 and 311 starts on page 99. It shows socketed versions of the ic's. Both ic's and the mating socket dimensions are shown on page 100.

The circuit that snipped-for-privacy@student.utwente.nl showed does not correspond to the information on either of these pages. The teardown was of some other circuit. I am trying to find out what it was.

Thanks for the AD catalog link!

My bad, I gave you the page numbers as shown on the pages not the pdf file numbering.

In what way does the teardown circuit not correspond to the datasheet?

piglet

The pcb shown in the teardown circuit is too big to fit on the AC1017 socket.

I haven't had time to study the patent, but I suspect the AD310-311 schematic will be competely different from the circuit shown in the hand- drawn schematic.

The teardown circuit is completely different from the AD310-311. I am trying to find out what it is from and where it came from.

According to the catalog page dimensions the AC1017 socket pins are pitched 0.2" (c.5mm) apart and the two rows are 2.2" (c.56mm) apart.

The teardown photos do not include a ruler but the dimensions of TO-92 transistors are well known, body height and diameter are both approx

0.2" (c.5mm). Using TO-92 as the yardstick on the photos I estimate the pins pitch at 0.2" and rows spacing (11 x TO-92) at 2.2". It looks to me very like the teardown device would nicely fit the AC1017 socket. The pin functions on the teardown device agree with the 310 datasheet.

Do you have any other source for the AD310 circuit to say why the teardown device is not a AD310? The patent is many years older than the teardown device and the exemplar circuits in the patent might bear no relation to any real world production. Fig 5 in the patent does look rather like the teardown device bridge configuration. Please can you share the reasons for doubting the teardown device is a 310?

piglet

There's a nice 'how it works' schematic of 310 in the catalog you linked to. That mostly lines up with the hand drawn schematic above (by the student.) Though the hand drawn one seems to be missing the synchronous detection piece... done with diodes... I'm still trying to piece out how the whole circuit works... Looking at it as a capacitance sensor makes some sense to me. But I don't quite get the input bridge type circuit.

Fun stuff. (Steve gets grumpy sometimes.)

George H.

Hi George,

The synchronous detection in the teardown is a synchronous chopper switch, pretty much the same as in the Philbrick P2 and chopper amps going way back. Effectively works the same way as the commutating diodes detector depicted in the principle of op sketch but requires more filtering I think. I guess a one transistor switch is easier than messing around with yet another transformer and matching diodes and so on.

I never tried to equate these varactor dc op-amps with RF parametric amps. Instead I envisage them as a breed of chopper amp but with (probably) lower conversion gain but the great advantage of zero dc load current. So to backtrack, in a 1930-60s style chopper amp 1mV of dc input gets chopped and makes 1mV of AC for easy amplification with old technology. But the input gets loaded somehow. In a varactor op-amp with zero volts input the two sides of the bridge are balanced and no AC output exists. As the input unbalances the operating point shifts and the two varactors capacitances unbalance and a small AC output exists. Depending on the diode's V-C slopes the sensitivity could be lower - e.g. 1mV of dc input may make less than 1mV of AC but hey gain is cheap at AC. But because the diodes are never conducting the DC input bias current is near zero. A very creative solution way back.

Bob Pease's story of the P2 is fun reading, especially how a quirk of the board layout stymied competitors' attempts to copy.

Yes, fun stuff!

piglet

Hmm, perhaps. Although I would expect it to be quite a bit speedier than the opamp-based integrator thingy, I doubt its associated poles show up above 0dB.

I'm not sure what makes you think it is not an AD310. I got it on aliexpress of all places, the case has some wear and clearly came off something they salvaged. It was about 15$. The bridge design also matches the one shown in the AD documents.

I could put the case up if you really want to see but there's nothing to it, just a metal shell.

The synchronous detection is done by the second JFET, to the right in the schematic, acting as a sort of chopper switch. The result of that operation is integrated.

Re 2nd Jfet: Ahh good! There were a lot of nodes labeled C or G and it wasn't clear to me what went where.

GH

I finally decided to properly draw the schematic. It should be up at the end of the teardown imgur post. Otherwise here's a direct link:

It shows up black for me (firefox) unless clicked for some reason.

If anyone wants the postscript file or sees errors, please let me know.

Oh the oscillator is pretty badly wrong. I shouldn't do these when I'm tired. Maybe I'll fix it later.

Floating gate on the first jfet?

Thanks. This new drawing will help, I think you also missed the 22Meg gate bias resistor on the first fet, the resistor (I can't see the value) on the 2nd bjt emitter to ground, and the C & R on the 2nd fet gate.

The uA709/LM709 was my first op-amp and usually went around with a compensation 1.5k and couple-nF series that are on the board but not in your sketches. You probably don't need to copy them unless for completeness.

piglet