Conclusions#

In this chapter, we have discussed in which way the amplification function can be implemented with the aid of electronic devices and power sources.

We have seen that a two-terminal, nonlinear, passive device, in conjunction with power sources can provide an available gain larger than unity, if such a device is biased in an operating point in which its small-signal resistance is negative. Biasing of such a device is performed by placing a current source in parallel with the device and a voltage source in series with the device, in such a way that the operating point of the device does not depend on the $v-i$ characteristic of a passive, DC termination. Either the bias voltage or the bias current can be selected freely, while the other one follows from the one selected and from the device’s $v-i$ characteristic and the temperature.

The implementation of amplification with three-terminal devices proceeds similarly. Three-terminal active devices such as MOS transistors, BJTs or vacuum tubes can be regarded as nonlinear two-ports of which the input port and the output port share one terminal. When properly biased, these devices can also provide an available power gain that exceeds unity. To this end, we need to select a proper operating point and bias each port with a current source and a voltage source in such a way that the biasing is independent of the passive DC termination resistance at both ports. Two of the four bias sources can be designed freely, while the other two follow from the device characteristics. As we will see later, important performance aspects, such as the noise performance, the dynamic behavior and the nonlinearity show a direct relation with the biasing quantities at the output port. For this reason, the bias current and the bias voltage of the output port are usually selected by design, while the bias voltage and current of the input port have to be adjusted such that the device’s operating point does not depend on the temperature and on the passive DC termination resistance at both ports.

Generalized biased active device#

In the following chapters, we will design amplifiers using biased active devices. Since the design theory itself is technology-independent, we will use one symbol for a generalized biased device. This symbol is shown in Fig. 72A. It refers to any type of biased active device, as indicated in Fig. 72B.