Amplifier Design with BJTs and FETs
3.1 Properties of Amplifiers
The goal of this chapter is to move from the details on the semiconductor devices themselves to our first practical application. One of the more interesting features of amplifiers is that they can easily become what will be covered in Chapter 5: oscillators. This is due to the high gain of the devices, unaccounted for parasitic elements, and other design flaws. The amplifiers we need will fall into three categories:
- low-noise amplifiers,
- high-gain amplifiers, and
- medium-to high-power amplifiers.
See Figure 3.1. The low-noise amplifier always operates in Class A, typically at 15%–20% of its maximum useful current. The high-gain amplifier can operate in Class A, as well as B (mostly push–pull). The higher dc current for the same device in a higher noise figure and more gain, and ultimately more output power. Class C operating mode is really reserve to either FM transmissions or constant-carrier modes like CW. Some of the modern digital modulation types are sensitive to phase distortion rather than amplitude changes, and because of the resulting output spectrum, designers have stayed away from Class C operation.