The Bipolar Transistor Help
Bipolar transistors have two p-n junctions connected together. This can be done in either of two ways: a p -type layer between two n -type layers or an n -type layer between two p -type layers.
Npn And Pnp
A simplified drawing of an npn transistor and the symbol that is used to represent it in schematic diagrams are shown in Fig. 16-6. The p -type, or center, layer is the base . The thinner of the n -type semiconductors is the emitter , and the thicker is the collector . Sometimes these are labeled B, E , and C in schematic diagrams, but the transistor symbol indicates which is which (the arrow is at the emitter). A pnp transistor (parts c and d ) has two p -type layers, one on either side of a thin n -type layer. In the npn symbol, the arrow points outward. In the pnp symbol, the arrow points inward.
Fig. 16-6 . Pictorial diagram of npn transistor (a), schematic symbol for npn transistor (b), pictorial diagram of pnp transistor (c), and schematic symbol for pnp transistor (d).
Generally, pnp and npn transistors can perform identical tasks. The only difference is the polarities of the voltages and the directions of the currents. In most applications, an npn device can be replaced with a pnp device, or vice versa, and the power-supply polarity reversed, and the circuit will still work if the new device has the appropriate specifications.
There are various kinds of bipolar transistors. Some are used for rf amplifiers and oscillators; others are intended for audiofrequencies (af). Some can handle high power for rf wireless transmission or af hi-fi amplification, and others are made for weak-signal rf reception, microphone preamplifiers, and transducer amplifiers. Some are manufactured for switching, and others are intended for signal processing.
The normal method of biasing an npn transistor is to have the emitter more negative than the collector. In most cases, the emitter is at or near zero potential while the collector is connected to a source of positive dc voltage. This is shown by the connection of the battery in Fig. 16-7. Typical voltages range from 3 V to approximately 50 V.
Fig. 16-7 . Typical biasing of an npn transistor.
The base is labeled “control” because the flow of current through the transistor depends on the base bias voltage, denoted E B or V B , relative to the emitter-collector bias voltage, denoted E C or V C .
When the base is not connected to anything, or when it is at the same potential as the emitter, a bipolar transistor is at zero bias. Under this condition, which is called cutoff , no appreciable current can flow through a p-n junction unless the forward bias is at least equal to the forward breakover voltage. For silicon, the critical voltage is 0.6 V; for germanium, it is 0.3 V.
With zero bias, the emitter-base ( E-B ) current I B is zero, and the E-B junction does not conduct. This prevents current from flowing in the collector unless a signal is injected at the base to change the situation. This signal must have a positive polarity for at least part of its cycle, and its peaks must be sufficient to overcome the forward breakover of the E-B junction for at least a portion of the cycle.
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