A Planet Analog article, “2N3904: Why use a 60-year-old transistor?” by Invoice Schweber, impressed some curiosity on this previous transistor and the way it’s generally used, and if any unusual makes use of may exist. Right here’s one we performed round with.
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The Linear Expertise Software Be aware 47-D: “Excessive Pace Amplifier Strategies” by Jim Williams presents an attention-grabbing aspect street to common transistor use, the place a typical quick pulse transistor is utilized in avalanche collector-to-emitter breakdown Vbceo to create sub-nanosecond pulses. The 2N3904 will work on this configuration, however requires a excessive voltage (>100 V) like the heart beat transistor to achieve the Vbeco breakdown, and produces a slower pulse, being a slower GP transistor.
Some time again, I had measured the reverse breakdown of the 2N3904 base-emitter junction and famous the small space of adverse resistance the place the junction present reduces as utilized reverse voltage will increase (Determine 1).
Determine 1 Measurement of the reverse breakdown of the 2N3904’s base-emitter junction, exhibiting a small space of adverse resistance.
This base-emitter breakdown is way decrease than the collector-emitter breakdown and may function a decrease voltage model of the avalanche pulse technology methodology described in App Be aware 47-D.
A easy circuit was created with the 2N3904 emitter linked by a 100-kΩ resistor to a variable provide set to ~14 VDC. A shunt capacitance of 10 nF from the emitter to floor and a 50-Ω resistor from the collector to floor. Simply two resistors, a capacitor, and the 2N3904 are all that’s required to create a easy leisure oscillator (really, the 50-Ω resistor isn’t required).
Determine 2 exhibits the end result with the DSO AC-coupled blue hint, the relief voltage on the transistor emitter, and the DC-coupled magenta hint, the voltage throughout the 50-Ω resistor from the collector to floor (keep in mind the NPN is the other way up or inverted!).
Determine 2 Waveforms of the easy leisure oscillator circuit with the AC-coupled blue hint and DC-coupled magenta hint.
The heartbeat throughout the 50-Ω resistor in Determine 3 exhibits the avalanche present in additional element, the place this present is ~ 2 V peak throughout the 50-Ω resistor, or ~40 mA peak. This isn’t quick, nevertheless, the 2N3904 is a general-purpose (GP) transistor that isn’t supposed for velocity.
Determine 3 Avalanche present proven in additional element on the DSO, exhibiting a ~40 mA peak.
Using sooner transistors such because the 2N2369 ought to produce narrower pulses with sooner rise instances. Whether or not these produce sooner rise instances and narrower pulse widths than within the collector-emitter avalanche breakdown methodology from App Be aware 47-D stays an experiment ready for these . Instinct signifies the “regular” avalanche collector-emitter mode might be sooner, although!
Anyway, I hope people discover this easy and weird use of those previous standby 2N3904 transistors attention-grabbing, I actually did!!
Michael A Wyatt is a life member with IEEE and has continued to get pleasure from electronics ever since his childhood. Mike has an extended profession spanning Honeywell, Northrop Grumman, Insyte/ITT/Exelis/Harris, ViaSat, and retiring (semi) with Wyatt Labs. Throughout his profession, he collected 32 US Patents and, previously, printed a couple of EDN Articles, together with Finest Thought of the Yr in 1989.
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