Page 100 - Complete Wireless Design

Source: Complete Wireless Design

Chapter
3

Amplifier Design

An amplifier is an active device that has the ability to amplify voltage, cur-
rent, or both, at zero frequency (a DC amplifier), low frequencies (an audio
amplifier), or high frequencies (an RF amplifier). Since power is P VI, then
power amplification is only a normal outcome of this capability, since raising
the current and/or the voltage will create power amplification.
AC amplifiers, whether for high or low frequencies, operate by allowing a
small fluctuating external input signal to control a much greater DC output
bias current. This small input signal changes the amplitude of the larger bias
current, with the varying bias current then sent through a high-value output
impedance or resistance component, which creates an AC output voltage due
to V IR. Depending on the amplifier’s designated purpose, these output com-
ponents may be composed of either a resistor, inductor, or tuned circuit.
There are assorted circuit configurations to allow an amplifier to achieve dif-
ferent frequency responses, input and output impedances, gains, and phase
shifts. Various bias circuits can be adopted to produce amplification at differ-
ent efficiency and thermal stability levels, while special coupling methods can
be applied to match impedances and filter out undesired frequencies with oth-
er stages or loads.

Amplifier circuit configurations. Amplifiers come in three different basic fla-
vors, each with its own distinct application and capability. They are referred
to as common-base, common-collector, and common-emitter amplifiers, depend-
ing on whether the base, collector, or emitter is common to both the input and
output of the amplifier circuit.
With an input signal inserted at the emitter and the output taken from the
collector circuit, we have our first configuration, the common-base amplifier
(Fig. 3.1). The common-base configuration can be found operating as a voltage
amplifier for low input impedance circuits. It also possesses a high output
2
impedance and a power amplification due to P V /R, but current gain will
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