Conclusions

In this chapter, we have briefly studied the operation and modeling of modern active devices. We have seen that those devices can be considered as nonlinear two-ports, of which the input port and the output port share one terminal. When biased properly, they can provide an available power gain that exceeds unity.

Although the fabrication, the operation and the modeling of active devices differs for each device, they have a lot in common:

1. The physical operation mechanism of all these active devices is that the current in the output port is controlled by the voltage across the input port, while building up this voltage requires a nonzero charge. This charge controls the current in the output port.

   A current will flow in the output port only if both the input port voltage and the output port voltage differ from zero.

2. The static intrinsic noise sources of all devices are the noise currents associated with the static (DC) port currents. We have studied in which way they can be converted into equivalent-input noise sources.

3. When properly biased, the active devices, together with their bias sources, can provide an available power gain that exceeds unity. In the following chapters we will design amplifiers using biased active devices. Since the design theory itself is technology-independent, we will often use generalized biased three-terminal devices from Fig. 72. Fig. 124 shows a simple small-signal of such a device that can be used during early stages of the design. It models the small-signal behavior of an intrinsic device. For BJTs we have \(g_{i}=g_{m}/\beta_{AC}\). For field effect devices \(g_{i}\) equals zero.

Fig. 124 Generalized small-signal model of the active devices.

Fig. 125 {SLICAP} html page with circuit data.

Fig. 126 {SLICAP} html page with circuit data.