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Optics & Photonics — Ultrasensitive Instrument Design — Analog Electronics

Resources from sci.electronics.design

The Usenet newsgroups sci.electronics.design and sci.optics are great places to talk about electrooptical stuff, but they're text-only. Sometimes a picture helps a lot, which is what this page is for.

Dr. Philip C. D. Hobbs, Principal

Last significant update April 17, 2015: Cascode e-pHEMT photodiode pictures

ElectroOptical Innovations LLC, 160 North State Road, Suite 203, Briarcliff Manor, NY 10510 (914) 236-3005

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Archive Sites
One of the nice things about sci.electronics.design is that it's widely redistributed by archive sites, some of which you can also use for posting, which is good since they made such a hash of the new Google Groups.

Some examples:
electronics-related
Electronics Point

It's really better to use a proper newsreader such as Thunderbird or Forte Agent. You can get a free Usenet account from Eternal September.


Transistor tester for laser noise cancellers. As discussed in medium-gory detail in this paper, these devices can let you do shot-noise limited measurements at baseband with lasers that are as much as 70 dB noisier than that.
They're limited by two main effects: beta nonlinearity (1/hFE-1/hfe) and log nonconformance (d ln(IC)/dVBE - kT/e). This tester measures both of these quantities directly.
It's a one-off, of course, so it's done with discrete logic and instrumentation
amp parts. It has certain points of interest, for instance the use of a unity gain instrumentation amplifier as a precision +1/+2 gain amplifier. One loose end: U1 is a LT1043 switched-capacitor building block—a glorified MUX that has very low charge injection and very good common-mode rejection.

Noise Peaks due to Linear Voltage Regulators
Errol Dietz is a remarkable fellow. He started out at National Semiconductor as a technician, and rose to become Chief Technology Officer.
Feedback amplifiers generally have an output impedance that rises linearly with frequency—in other words, it's inductive. As Dietz's paper from Electronic Design shows, this effective inductance can resonate with the output bypass capacitor to cause really nasty noise peaks in the 1-100 kHz region. If you have an inexplicable noise peak in that range, a small resistor (a few tenths of an ohm to an ohm or two) in series with the regulator output can be just the ticket.

High Dynamic Range FET RF Mixers
For RF folks, one of the perennial quests is for better frequency mixers: lower distortion, lower power, better spurious performance. FETs can help. Nowadays CMOS muxes are the devices of choice for HF mixers, but to get the best performance, you still have to know how they work. Ed Oxner was a long-time Siliconix apps guy, and his work is still right up there with the best. (From a Siliconix databook, 1985.)

How to handle tantalum capacitors
Solid tantalum capacitors have a lot of advantages: very low inductance in surface mount packages, ESR low but not so low that your LDO regulators start oscillating; and good capacitance per unit volume. Unfortunately they're also prone to burn up when mistreated, which makes many engineers wary of them. This article talks about how to treat them properly, and how a bit of TLC after soldering can restore their full performance.

Sine wave generation is a perennial problem. Direct-digital synthesis (DDS) uses a bunch of counters, lookup tables, and DACs, but that's a relatively heavyweight solution that doesn't fit all problems. BJT differential pairs naturally have a hyperbolic tangent (tanh) characteristic, which can be used to round off a triangle wave into a very passable sine. I'm not old enough to have invented this technique, but here are a couple of illustrations of how it works:
TANH Sine Wave Shaper (PDF)
TANH Sine Wave Shaper (Mathcad)

Linearizing a 2N7000/2N7002 Variable Resistor—not like a JFET!

Bidirectional open-collector level shifter
Cubic polynomial generator
Series Overvoltage Protection
On-Chip Temperature Stabilizer: BJT dissipation goes up at low temperature, with very high gain.
On Chip Temperature Stabilizer Step Response
Decap picture of a Terabeam CD3109 APD/TIA module, taken with a lens glued to a cell phone camera

Schematic of the current-amplifier calibrator
(Click for full-sized image.)

Cascoded pHEMT Current Probe: 30 electrons/sqrt(Hz) @ 100 MHz
This is a front end for a biochip research application, and I think it's pretty well the state of the current-amp art. Unfortunately I can't show it due to NDA requirements, but I can show the on-board calibrator design (LTspice schematic).
(There are a few oddities, e.g. series/parallel connected resistors, which are there to reduce the number of BOM items. Pick-and-place machines have a limited number of slots for different parts.)

At high frequency, the input impedance of the current amplifier is just that of its 0.9 pF capacitance. As in most high-speed current amplifiers, the noise is dominated by the amplifier's input noise (0.3 nV/sqrt(Hz)) differentiated by the reactance of the input capacitance, jωCin. Any added capacitance thus degrades the noise performance directly, so I couldn't just connect the calibrator there, or even hang a relay contact on it, which would be the usual methods. Still, I needed a nice rectangular +8 nA / -2 nA calibration signal with clean 1-ns edges and ~20% duty cycle. What to do?

The idea is to use a really triangular triangle wave generator, and let a tiny coupling capacitance (~0.05 pF) between two traces differentiate that into a rectangular current waveform at the amplifier input. The triangle wave generator is a positive and negative pair of BJT current sources, one of which is switched by a differential pair of pseudomorphic HEMTs. These work more or less like magic JFETs: ultra high transconductance, very low ON resistance, very low noise in the flatband, but horrible 1/f noise—this one (Avago ATF38143) has a 1/f corner frequency of 10 MHz, and the otherwise good Skyworks SKY65050's is 30 MHz.

(The amplifier that I can't show you uses the same pHEMTs in an integrating architecture followed by a differentiator, which de-emphasizes the low frequency noise but maintains excellent pulse fidelity.)

The resulting current charges and discharges a bunch of 39-pF capacitors, which are connected in parallel to reduce the series inductance. Another pHEMT makes a source follower, whose output drives the input of a Schmitt trigger made from two SiGe:C NPN transistors. This circuit switches in about 400 ps, so the corners of the triangle wave are sharper than 1 ns.

The output is the current through C5 at the far right of the schematic. The one really critical thing in this circuit is the inductance in series with the integrating capacitor—if that can be kept below 0.05 nH, all the ringing on the differentiated rectangle waveform goes away.

The Schmitt trigger and buffer part could have been replaced by a PECL comparator such as an ADCMP533, but the pHEMT parts were new to me, so I wanted to try them out in multiple roles. It turned out that the Avago ATF38143's drain impedance was too low for it to make a good follower, whereas the SKY65050 worked fine.


Cascode Enhancement pHEMT Photodiode Preamp:

Photo of cascode e-pHEMT front end
(Click for full-sized image.)

This is pretty small, because it has to be--those are microwave transistors, and will oscillate at the slightest provocation. The axial resistors and TO-92 parts are all for biasing---the actual amplifier is the part between the output coupling cap (the small orange thing in the middle) and the photodiode, which is the white square with the black middle at the right.
The 0.1-inch pitch holes round the outside and 25-mil pad pitch set the scale.
This amp works fine below 6 mA of bias current, but above there it wants to oscillate at 6 GHz. Interestingly this is just what SPICE predicts if the capacitance across the cascode device's base isolation bead is about 0,2 pF, which is a plausible number.
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Send me an email and let's discuss your application. Comments, corrections, suggestions, or questions are also welcome.


ElectroOptical Innovations LLC, 160 North State Road, Suite 203, Briarcliff Manor, NY 10510 (914) 236-3005