1.9 GROUND IN A CIRCUIT
When we consider a CE amplifier, the emitter is a common terminal between the input and output, commonly referred to as the ground terminal. Consider the CE amplifier shown in Fig. 1.50(a). Let an n–p–n device be used in the amplifier. VCC is connected to one end of RC and the negative terminal is connected to the ground. IC flows downwards and VC is positive with respect to the ground.
If on the other hand, a p–n–p device is used in the amplifier circuit, the biasing is exactly opposite, as shown in Fig. 1.50(b).
−VCC is connected to one end of RC and the positive end of the battery is connected to the ground. IC flows upwards and VC is negative with respect to the ground. This means that the directions of the currents and polarities of voltages are exactly opposite in an n–p–n device when compared to a p–n–p device. Hence, these two types of devices are called complementary devices. Now consider a practical case of a two-stage amplifier using n–p–n devices shown in Fig. 1.51(a)
If for some reason, Q2 needs to be replaced and the only replacement available is a p–n–p device with similar characteristics, resultant circuit, when the devices are biased, is as shown in Fig. 1.51(b).
FIGURE 1.49(a) Multi-node network
FIGURE 1.49(b) Redrawn circuit of Fig. 1.49(a) using dual of Millers theorem
FIGURE 1.50(b) Supply voltage for biasing of a p–n–p device is negative
FIGURE 1.51(a) Two stage amplifier using n–p–n devices
FIGURE 1.51(b) Resultant circuit with the crossovers
FIGURE 1.51(c) A p–n–p device connected in its upside down fashion
FIGURE 1.51(d) Circuit in which a p–n–p device is connected in an upside down fashion
With the crossovers it is not possible to run a + VCC and – VCC bus bars and make the circuit look simple. Now consider an alternate arrangement for Fig. 1.50(b). Instead of calling point X as the ground, call point Y as the ground and connect the p–n–p device in its upside down fashion as shown in Fig. 1.51(c).
Once the p–n–p device Q2 is connected in it’s upside down fashion, as in Fig. 1.51(d), it is possible to run a common bus bar. Thus, the ground in a circuit is nothing but an arbitrarily chosen reference terminal with respect to which we measure voltages.