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The Bipolar Transistor Help (page 2)

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By — McGraw-Hill Professional
Updated on Sep 11, 2011

Reverse Bias

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.

Forward Bias

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.

Saturation

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.

Fig. 16-8 . A family of characteristic curves for a hypothetical npn bipolar transistor

Pnp Biasing

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.

Fig. 16-9 . Typical biasing of a pnp transistor.

Practice problems of these concepts can be found at: Semiconductors Practice Test

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