CHAPTER 16 ■ TRANSISTOR SWITCHESWhen looking at a datasheet, pay attention to the rounded and the flat sides of the transistor’s package.
Otherwise, you might connect the leads in reverse by mistake (however, no damage will result if improperly
inserted into a multimeter).
- For the 2907A, the meter should display a number between 75 hFE and 300 hFE.
This is the unique amount of gain for this individual transistor.
Understanding the Importance of Transistor Gain
Current applied to the transistor’s base is multiplied by the hFE to determine the amount of current the
transistor will produce. A transistor with 30 hFE gain and 2 mA current on its base lead allows 60 mA of
current to flow between the emitter and collector.
2 mA × 30 hFE = 60 mA
The circuit attached to the collector isn’t required to use all of the current, but it is restricted to that
maximum (in this case 60 mA).
By varying the amount of current provided to the transistor’s base, you can control the amount of
current provided to the attached circuit. This technique is used in some battery chargers to limit the
maximum current that the battery can draw, thus preventing the battery from overheating or exploding.
A transistor is a current-controlled device. Just like an LED, the output of a transistor depends largely on
the current provided to it.
Gain Varies from Transistor to Transistor
The hFE varies from transistor model to transistor model. It even varies from transistor to transistor within
the same batch from the factory. In a lot of ten 2907A transistors, I got results from 151 hFE to Q 173 hFE. The
datasheet guaranteed a minimum of 75 hFE, so the actual numbers were very good.
The variations in gain cause headaches to stereo manufacturers as they result in speaker outputs with
different sound volumes. However, since the LM393 comparator is going to be giving at least 6 mA to the base
of the transistor, even a minimum gain of 75 is going to result in 450 mA available to the attached circuit.
Switching Off and On Rather Than Amplifying
When a PNP transistor is used as a switch, the base lead is provided either no current to turn the transistor
completely off or plenty of negative power at the base lead to turn the transistor completely on. Putting a
transistor into a fully powered state is called saturation. This is a legitimate and common technique.
When a transistor is used as an amplifier, for example for an audio speaker, the transistor is provided
user-adjustable levels of current to the base lead through a potentiometer. This results in audio volume
control.