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.
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.
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
nonconformance (d ln(IC)/dVBE -
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
Peaks due to Linear Voltage Regulators Errol Dietz
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
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.
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
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.
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
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:
(Click for full-sized
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.
hobbs @ electrooptical.net
Send me an email and let's discuss your application. Comments, corrections, suggestions, or questions are also welcome.
Innovations LLC, 160 North State Road, Suite 203,
Briarcliff Manor, NY 10510 (914) 236-3005