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From the Q&A

With TJ Byers


Transistor Selection

Question:

What is the determining factor in transistor choice? How interchangeable are the different types? I have a circuit that calls for a 2N3707, which I have never even heard of.

B. Brown
via Internet


Answer:

From an engineering point of view, there are four critical parameters when it comes to choosing transistors:

  1. Voltage
  2. Current
  3. Gain
  4. Frequency

Supposedly, each transistor is custom fit (as in a specific 2N number) to fill a niche for performance and cost. That is, you don’t want to pay for performance you don’t need, but you do not want to be caught short with a marginal part, either. Hence, all the various numbers.

However, like most electronic devices, a transistor with better specs can always replace one with lesser expectations.

As for the 2N3707 — which is now obsolete — it’s an NPN general-purpose audio transistor rated at 30 volts, 200 mA, and a gain of 100 to 400.

To me, that sure sounds like a 2N2222, which is rated at 60 volts, 800 mA, with a gain of 300 at 150 mA. Both are of the same sex — NPN as opposed to PNP — both are silicon (not germanium), and both have an upper frequency limit of 250 MHz. So, why does your design specify the 2N3707? Probably because the designer got a good deal (price break) on this transistor at a time when he needed those parameters.


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