COMMON-EMITER AMPLIFIER MODEL TO EXPLAIN WORKING PRINCIPLES

B. SOMANATHAN NAIR

            In this article, we present a physical model that will help to understand the behavior of a transistor amplifier in the common-emitter configuration. In the CE configuration, a small base current (in the microampere range) is controlling a much larger collector current (in the milliampere range). How can a small current induce control over a much larger current? To answer this, we use the following model.

            Figure 1 shows a model in which we have two interconnected paths. One path is narrow through which only one electron can move. Let this represent the base-emitter path. Similarly, let the second path, which represents the collector-emitter path, be much larger and can permit three electrons to move through it.

            Let electron E1 move around the base-emitter path and let electrons E2, E3, and E4 move around the collector-emitter path. The four electrons share the common path between base and emitter. It can be seen that it is in this region that E1 interacts with E2, E3, and E4. The question here is how do they interact?

The answer to this lies in the fact that there exists a tiny magnetic field surrounding every electron (i.e., electron is a tiny magnet). When base electron E1 meets collector electrons E2, E3, and E4 in the common (base-emitter) path, their magnetic fields interact with each other. When a signal voltage is applied to E1, the magnetic field surrounding it will vary (i.e., get modulated) according to the variations in the signal. These variations in the magnetic field around E1 will induce similar variations in the magnetic fields of E2, E3, and E4 while they traverse through the common path. This theory can be extended to the actual case where millions of electrons are involved.

Thus the variations in the tiny input base current due to the signal input voltage induce corresponding variations in the larger collector current by the interaction of magnetic fields surrounding individual electrons. This in turn produces amplification of the signal in the common-emitter. Figures 2, 3, and 4 illustrate the relevant operations further.

Figure 2 shows the situation when the electrons are injected from the emitter region under the action of supply voltages V­_BB and V_CC and input signal voltage V_s. It is in this region that the magnetic fields around the electrons start interacting with each other. The signal variations created in the base electron gets transferred to the collector electrons creating corresponding variations in the collector current. Since there are millions of electrons In the base current and millions and millions of electrons in the collector current, we find that signal variations in a very low-value base current is able to create corresponding variations in the much larger collector current, producing amplification, as stated above.

Figure 3 shows the situation when the base electron and collector electrons enter the base region. Here also interaction of respective magnetic fields occurs.

Figure 4 shows the condition when the modulated base electron comes out of the base and the modulated collector electrons come out of the collector. It can be seen that the collector current carries the larger variation (modulation) in it resulting in amplification of signal voltage as stated earlier.