I don't know about this one, but I've seen TO-220 MOSFETs oscillate by doing nothing more than soldering three ceramic capacitors across the leads, applying a high DS voltage, and biasing the gate a tiny bit. It hisses and gets nice and warm. Maybe this MOSFET would be fast enough to have similar problems with it's stubby little legs.
Or maybe the resistor symbolizes something that burns out when the gate drive is too high.
-- I will not see posts from astraweb, theremailer, dizum, or google because they host Usenet flooders.
Especially if the source is not grounded. Bipolar emitter followers love to oscillate, too. I tend to blame the oscillation on wire bond inductances, but that's conjecture on my part. A series gate or base resistor is the usual fix.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
They oscillate because the impedance viewed into the base is negative under certain levels of emitter load capacitance. ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: skypeanalog | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
With the two lags (from C_GS and C_DG), resistance in the source turns into negative resistance at the gate. You need a big enough series resistor to make the net resistance seen by the gate positive.
I'm currently building a 200-400 MHz low noise TIA using one of those nice Avago ATF55143 e-pHEMTs cascoded by a BFP640 40 GHz SiGe.
The transconductance is about 3k, not bad for a single stage at that frequency. (Well, two stages technically--there's also a BFT92A emitter follower driving the coax.)
Interestingly, its stability is limited by the capacitance of the 10-ohm bead in the cascode's base. It works great at bias currents below 6 mA, but much above there, it wants to oscillate at 6 GHz. Interestingly this is just what LTspice predicts if the capacitance across the bead is
0.2 pF, which is a plausible value.Still not bad for a hand-wired protoboard.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
We love those Avago e-phemts. We can drive them on/off from ECL. The bigger ones make nice laser drivers.
It might work better as a real surface-mount circuit on a multilayer board. It will certainly work different!
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
You got that right. I didn't have time to send it out--the electronics for this gizmo have to be working by Monday.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Thankfully--I've still got two reels in the workshop for hobby-stuff.
Nice fusible ESD-detectors, too.
Cheers, James
I don't think I've ever zapped one. I certainly aren't careful handling them. Must be the humidity.
They do switch fast. Most "modern" SOT23s have a low Rds-on and a lot of capacitance, so 2N7002 looks pretty good there.
FDC2612 is 200 volts and very fast.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Would the FDC2512 be faster? It's rated at 150V and has similar capacitance, but half the Rds(on). And it can switch with a considerably-lower Vgs, conducts 2A at Vgs = 3.7V compared to 5.4V. Both show a spec'd required Qg of 8nC at Vgs = 10V, but the '2512 can be fully switched at 6V, taking 5nC.
Or is lower Rds(on) not helpful?
--
Thanks,
- Win
Too much theory for me. We needed to make 100 volts into 50 ohms, through a transmission-line transformer, and we tried a bunch of fets. A few FDC2612s in parallel worked great, with ca 1 ns edges.
2N7002s work fine at 60 volts, if you pick the right manufacturer. The data sheets may be the same, but the parts sure aren't.-- John Larkin Highland Technology, Inc picosecond timing laser drivers and controllers jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
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