COMMON-BASE CONFIGURATION STUDY OF OUTPUT CHARACTERISTICS USING HYDRAULIC MODEL

B. SOMANATHAN NAIR

Figure 1 shows the common-base (CB) configuration of a PNP-type bipolar junction transistor with its emitter-base junction forward biased (FB) and collector-base junction reverse biased (RB). It can be seen that, under these biasing conditions, emitter current I_E flows through the base to the collector. It can also be seen that the emitter current gets divided as collector current I_C and base current I_B. Since the base is very narrow, we find that I_B is much smaller than I_C and that I_C­ is almost equal to I_E.

The operation of CB configuration can be studied using Fig. 2, which shows a hydraulic model of the CB configuration. In this model, we have a driving pump in the first pipe section and a suction pump in the third pipe section. The driving pump is similar to the forward bias in the emitter-base region and suction pump is similar to the reverse bias in the collector-base region.   

The driving pump drives current from the first pipe section (equivalent to the emitter current I_E through the emitter) into the second and third pipe sections. A major portion of this current (i.e., I_E) flows to the third pipe section (i.e., collector region) as collector current I_C, which will be sucked away by the suction pump (i.e., acting similar to the reverse bias between collector and base). A small portion of the current (i.e., emitter current) flows through the narrow middle pipe section (i.e., base region) as the base current I_B. It can be seen that

I­_E = I­_C + I­_B

Now, if I_E varies, then I_C varies in the same fashion. Thus the changes in the input current are regenerated in the output current as such and hence the CB configuration can act as an amplifier.

It can be seen that current flows through the collector even if the suction pump is removed. This operation is similar to the CB configuration with V_CC removed (Fig. 3). We thus find that I_C­ flows even when V_CC= 0. This is because even though V_CC= 0 (by shorting the collector-base terminals), the path is completed for the emitter-supply voltage V_EE, which will drive the emitter current through the collector. Thus we find that I_C is almost equal to I_E­ even when V_CC = V_CB = 0 (here we use the notation V_CB to indicate the voltage between collector and base terminals since in plotting characteristics, we use terminal voltages rather than supply voltages). This is clearly indicated in the characteristic (Fig. 5).

Now, to make I_C = 0, we have to convert the suction pump (reverse bias) into a driving pump (forward bias). Then collector current will flow from collector into base so that I_C = I_E. Since the two currents oppose each other, the net collector current will become zero. This is shown in Fig. 4. Figure 5 shows one CB output characteristic.

In Fig. 5, we find that I­_C becomes zero at about V­_CB = 0.8 V. It may also be noted that the curve for +V­_CB resembles that of a forward-biased (C-B) diode. This condition is usually called saturation. Thus, when both the junctions are forward biased, we get the saturation condition of the transistor amplifier. It may be noted that since V_CC (or V_CB) = V_EE (or V_EB) for saturation condition, the net voltage V_CEsat (V_CB ‒ V_EB) = 0, ideally. However, since E-B junction is heavily doped and C-B junction is lightly doped, usually V_CEsat = 0.1 to 0.2 V.