Unusual 2N3904 transistor circuit
A Planet Analog article, “2N3904: Why use a 60-year-old transistor?” by Bill Schweber, inspired some interest in this old transistor and how it’s commonly used, and if any uncommon uses might exist. Here’s one we played around with.
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The Linear Technology Application Note 47-D: “High Speed Amplifier Techniques” by Jim Williams offers an interesting side road to usual transistor use, where a typical fast pulse transistor is utilized in avalanche collector-to-emitter breakdown Vbceo to create sub-nanosecond pulses. The 2N3904 will work in this configuration, but requires a high voltage (>100 V) like the pulse transistor to reach the Vbeco breakdown, and produces a slower pulse, being a slower GP transistor.
A while back, I had measured the reverse breakdown of the 2N3904 base-emitter junction and noted the small area of negative resistance where the junction current reduces as applied reverse voltage increases (Figure 1).
Figure 1 Measurement of the reverse breakdown of the 2N3904’s base-emitter junction, showing a small area of negative resistance.
This base-emitter breakdown is much lower than the collector-emitter breakdown and might serve as a lower voltage version of the avalanche pulse generation method described in App Note 47-D.
A simple circuit was created with the 2N3904 emitter connected by a 100-kΩ resistor to a variable supply set to ~14 VDC. A shunt capacitance of 10 nF from the emitter to ground and a 50-Ω resistor from the collector to ground. Just two resistors, a capacitor, and the 2N3904 are all that’s required to create a simple relaxation oscillator (actually, the 50-Ω resistor isn’t required).
Figure 2 shows the result with the DSO AC-coupled blue trace, the relaxation voltage at the transistor emitter, and the DC-coupled magenta trace, the voltage across the 50-Ω resistor from the collector to ground (remember the NPN is upside down or inverted!).
Figure 2 Waveforms of the simple relaxation oscillator circuit with the AC-coupled blue trace and DC-coupled magenta trace.
The pulse across the 50-Ω resistor in Figure 3 shows the avalanche current in more detail, where this current is ~ 2 V peak across the 50-Ω resistor, or ~40 mA peak. This isn’t fast, however, the 2N3904 is a general-purpose (GP) transistor that is not intended for speed.
Figure 3 Avalanche current shown in more detail on the DSO, showing a ~40 mA peak.
Utilizing faster transistors such as the 2N2369 should produce narrower pulses with faster rise times. Whether these produce faster rise times and narrower pulse widths than in the collector-emitter avalanche breakdown method from App Note 47-D remains an experiment waiting for those interested. Intuition indicates the “normal” avalanche collector-emitter mode will be faster, though!
Anyway, I hope folks find this simple and unusual use of these old standby 2N3904 transistors interesting, I certainly did!!
Michael A Wyatt is a life member with IEEE and has continued to enjoy electronics ever since his childhood. Mike has a long career spanning Honeywell, Northrop Grumman, Insyte/ITT/Exelis/Harris, ViaSat, and retiring (semi) with Wyatt Labs. During his career, he accumulated 32 US Patents and, in the past, published a few EDN Articles, including Best Idea of the Year in 1989.
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