At A, the dual-diode model of a simple NPN circuit. At B, the actual transistor circuit.
Imagine a bipolar transistor as consisting of two diodes in reverse series. You can’t normally connect two diodes this way and get a working transistor, but the analogy is good for modeling the behavior of bipolar transistors. A dual-diode NPN transistor model is shown in above figure A. The base is formed by the connection of the two anodes. The emitter is one of the cathodes, and the collector is the other cathode. Above Figure B shows the equivalent real-world NPN transistor circuit.
The NPN Case
The normal method of biasing an NPN transistor is to have the collector voltage positive with respect to the emitter. This is shown by the connection of the battery in above figure A and above figure B. Typical dc voltages for a transistor power supply range between 3 V and about 50 V. A typical voltage is 12 V.
In the model and also in the real-world transistor circuit, the base is labeled “control,” because the flow of current through the transistor depends critically on what happens at this electrode.
Zero Bias for NPN
Suppose the base of a transistor is at the same voltage as the emitter. This is known as zero bias. When the forward bias is zero, the emitter-base current, often called simply base current and de- noted IB, is zero, and the emitter-base (E-B) junction does not conduct. This prevents current from flowing between the emitter and collector, unless a signal is injected at the base to change the situation. Such a signal must, at least momentarily, attain a positive voltage equal to or greater than the forward breakover voltage of the E-B junction.
Reverse Bias for NPN
Now imagine that a second battery is connected between the base and the emitter in the circuit of above figure, with the polarity such that EB becomes negative with respect to the emitter. The addition of this new battery will cause the E-B junction to be reverse-biased. No current flows through the E-B junction in this situation (as long as the new battery voltage is not so great that avalanche breakdown occurs). A signal might be injected at the base to cause a flow of current, but such a signal must attain, at least momentarily, a positive voltage high enough to overcome both the reverse bias and the forward breakover voltage of the junction.
Forward Bias for NPN
Now suppose that EB is made positive with respect to the emitter, starting at small voltages and gradually increasing. This is forward bias. If the forward bias is less than the forward breakover voltage, no current will flow. But as the base voltage EB reaches the breakover point, the E-B junction will start to conduct.
The base-collector (B-C) junction of a bipolar transistor is normally reverse-biased. It will remain reverse-biased as long as EB is less than the supply voltage (in this case 12 V). In practical transistor circuits, it is common for EB to be set at a fraction of the supply voltage. Despite the reverse bias of the B-C junction, a significant emitter-collector current, called collector current and denoted IC, will flow once the E-B junction conducts.
In a real transistor circuit such as the one shown in below figure Fig. above figure B, the meter reading will jump when the forward breakover voltage of the E-B junction is reached. Then even a small rise in EB, attended by a rise in IB, will cause a large increase in IC, as shown in below figure. If EB continues to rise, a point will eventually be reached where the IC versus EB curve levels off. The transistor is then said to be saturated or in saturation. It is wide open, conducting as much as it can.
Relative collector current (IC) as a function of base voltage (EB) for a hypothetical NPN silicon transistor.
At A, the dual-diode model of a simple PNP circuit. At B, the actual transistor circuit.
For a PNP transistor, the situation is a mirror image of the case for an NPN device. The diodes are reversed, the arrow points inward rather than outward in the transistor symbol, and all the polarities are reversed. The dual-diode PNP model, along with the real-world transistor circuit, are shown in above figure. In the preceding discussion, replace every occurrence of the word “positive” with the word “negative.” Qualitatively, the same things happen: small changes in EB cause small changes in IB, which in turn produce large fluctuations in IC.