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Rarely Asked Questions – Issue 105, May 2014
Discretion is the better part…
Q: How do I choose a discrete transistor?
A: Discreetly. (Sorry, I couldn't resist that.)
Actually, not too carefully. I have seen engineers agonizing for days about the best choice for a discrete transistor for a slow logic inverter, and I have seen them near breakdown because they could not obtain exactly the same transistor that was used as an emitter follower in the application note.
In fact, provided that some basic issues are addressed, many very different transistors will work equally well in a large number of circuits.
In many applications, it really doesn’t matter whether you choose a bipolar junction transistor (BJT) or a MOSFET. They’ll need slightly different circuitry, of course: the transistor will probably need base resistors, while the MOSFET will not; and the MOSFET will have a larger input capacitance, which must be considered when assessing stability. Both work well as solid-state triodes in amplifiers, oscillators, and logic, however. The BJT draws base current but has lower capacitance; the MOSFET has infinitesimal gate current, but quite large gate capacitance. The physical properties and choice of device do matter in some cases, of course, such as where the thermal characteristics of a BJT’s base-emitter junction are used for temperature measurement.
Take care is to ensure that whatever device you choose has the correct polarity, (Is it NPN/N-channel or PNP/P-channel?) and make sure that your circuit does not allow whatever transistor you choose to exceed its absolute maximum ratings, both steady state and transient.
For the rest, you must understand what characteristics are important and which ones can vary greatly without having much effect on your circuit’s performance. Choosing Transistors considers these issues in some detail, and concludes that the best procedure for choosing a transistor should conclude, “Any device having characteristics better than xxxx, yyyy, and zzzz is likely to work in this circuit; SPICE analysis has shown that the 2Naaaa, 2Nbbbb, and 2Ncccc all work in simulations; and prototypes built with the 2Naaaa definitely work well. However, many other transistors having similar characteristics should be equally acceptable.”
Whatever device you choose, do make sure it is discreet as well as discrete. You don’t want it to babble to all your colleagues about how little care you seem to take when selecting transistors, even though it’s actually good engineering not to over-specify. In the words of Henry Ford, “An engineer can do for a nickel what any damn fool can do for a dollar.”
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