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From the Q&A

With TJ Byers


Transistor Basics

Question:

I know this may be very basic for most of your readers, but I'm trying to do some simple tests with transistors before assembling a more complex design and I cannot get the PNP transistors (2N3906) to work. Here are the two test circuits — (a) and (b).

Closing the switch in circuit (a) turns on the LED; when I do the same in circuit (b) it doesn't work. What am I doing wrong?

Gregory M. Kiyoi
via Internet


Answer:

Well you've done a lot of things right. For example, you reversed both the LED diode and the polarity of the power supply. That's smart. Your mistake is that you don't have a current path through the base of the transistor.

Basically, a transistor is two back-to-back diodes with the base as common. For a transistor to work, you need to have current flowing from the base to the emitter. This current causes current to flow from the collector to the emitter; the amount of current flow is determined by the gain (hfe) of the transistor. If the transistor has an hfe of 100, 1 mA through the base-emitter will produce a flow of 100 mA through the collector-emitter.

In circuit (b), there is no current path through the base-emitter. Since you're mixing NPN and PNP transistors in a more complex circuit, the correct way to handle the situation is to invert the 2N3906 transistor (c) so that you can run off a single-polarity power source. In this configuration, current will flow from the emitter (+6V) through the base via the 1K resistor when the switch is closed. This causes the LED to glow. Good luck with your project!


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