The Bipolar Transistor Help (page 2)
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.
Suppose that another battery is connected to the base of the npn transistor at the point marked “control” so that the base is negative with respect to the emitter. The addition of this new battery will cause the E-B junction to be in a condition of reverse bias . Let’s assume that this new battery is not of such a high voltage that avalanche breakdown takes place at the E-B junction.
A signal might be injected to overcome the reverse-bias battery and the forward-breakover voltage of the E-B junction, but such a signal must have positive voltage peaks high enough to cause conduction of the E-B junction for part of the cycle. Otherwise, the transistor will remain cut off for the entire cycle.
Suppose that the bias at the base of an npn transistor is positive relative to the emitter, starting at small levels and gradually increasing. This is forward bias . If this bias is less than forward breakover, no current flows. However, when the voltage reaches forward breakover, the E-B junction conducts current.
Despite reverse bias at the base-collector ( B-C ) junction, the emitter-collector ( E-C ) current, more often called collector current and denoted I C , flows when the E-B junction conducts. A small rise in the positive-polarity signal at the base, attended by a small rise in the base current I B , causes a large increase in I C . This is the principle by which a bipolar transistor can amplify signals.
If I B continues to rise, a point is reached eventually where I C increases less rapidly. Ultimately, the I C versus I B function, or characteristic curve , of the transistor levels off. The graph in Fig. 16-8 shows a family of characteristic curves for a hypothetical bipolar transistor. The actual current values depend on the particular type of device; values are larger for power transistors and smaller for weak-signal transistors. Where the curves level off, the transistor is in a state of saturation . Under these conditions, the transistor loses its ability to efficiently amplify signals. However, the transistor can still work for switching purposes.
For a pnp transistor, the bias situation is a mirror image of the case for an npn device, as shown in Fig. 16-9. The power-supply polarity is reversed. To overcome forward breakover at the E-B junction, an applied signal must have sufficient negative polarity.
Either the pnp or the npn device can serve as a “current valve.” Small changes in the base current I B induce large fluctuations in the collector current I C when the device is operated in that region of the characteristic curve where the slope is steep. While the internal atomic activity is different in the pnp device as compared with the npn device, the performance of the external circuitry is, in most situations, identical for practical purposes.
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