We’re keeping the potentiometer, but also applying an AC source, a sine wave, to the base. Now, imagine for a moment our simple DC circuit used as an AC amplifier. At this moment the transistor is said to be saturated, or fully turned on, passing a current that’s limited by R1. There will be a region during which the relationship between base current and collector current is close to linear, but eventually a point will be reached at which the collector current stops increasing no matter how much you increase the base current. As our potentiometer moves upward the base voltage will increase, and there will be a corresponding increase in base current with an attendant increase in collector current. When the base voltage is just over 0.6 V, a little current flows in the collector, but not much as the transistor is barely turned on. You will see it quoted on data sheets, and for example a transistor with a gain of 100 would pass 100 mA at the collector for a base current of 1 mA. We’re back to the little man in the Horrowitz & Hill cartoon, and the relationship between base current and collector current is called the transistor’s gain. A small current flows into the base, and since this is a transistor we’re talking about that results in a larger current flowing through the collector. A silicon diode starts to conduct when the voltage across it reaches about 0.6 V, and when the voltage from our potentiometer across our base-collector junction reaches 0.6 V, it also starts to conduct. In an NPN transistor, the connection between base and collector is a PN junction, so as you might expect it shares its properties with the PN junction in a diode. In this circuit if you were to start with the potentiometer at the grounded end then the transistor would be turned off, and no current would flow. Because the emitter is grounded, even if sometimes via a resistor, this transistor configuration is referred to as a Common Emitter amplifier. Imagine for a moment a simple transistor circuit involving a single NPN transistor with its emitter grounded, its collector tied to the positive supply by a resistor, and a potentiometer between ground and supply allowing any voltage to be supplied to the base. The Simplest Biasing Example The potentiometer can be used to explore transistor biasing Fortunately it is possible to work with transistors without such an in-depth understanding of their operation, but before selecting the components surrounding a device it is still necessary to go a little way beyond transistor man. The “better” description of these aspects of the device fills the heads of first-year electronic engineering students until they never want to hear about an h-paramater or the Ebers-Moll model of transistor function again in their entire lives. Of course the base-emitter junction is a diode and it is not a simple potentiometer that sits between collector and emitter. It’s a simple but effective way of explaining the basic operation of a transistor, but it stops short of some of the nuances of how a transistor works.
If you wind back the base current, he drops it back. If you apply a little more base current, he pushes up the collector a bit. A transistor is shown as a room in which “transistor man” stands watching a dial showing the base current, while adjusting a potentiometer that limits the collector current. If you open up the perennial favourite electronics textbook The Art Of Electronics and turn to the section on transistors, you will see a little cartoon.
0 kommentar(er)
