Analog amplifiers: This chapter introduced important amplifier characteristics including voltage gain Av, current gain Ai, power gain AP, input resistance, and output resistance. Gains are expressed in terms of the phasor representations of sinusoidal signals or as transfer functions using Laplace transforms. The magnitudes of the gains are often expressed in terms of the logarithmic decibel or dB scale.
Biasing: It was demonstrated that bias must be provided to an amplifier to ensure that it operates in its linear region. The choice of bias point of the amplifier, its Q-point, can affect both the gain of the amplifier and the size of the input signal range for which linear amplification will occur. Improper choice of bias point can lead to nonlinear operation of an amplifier and distortion of the signal. One measure of linearity of a signal is its percent total harmonic distortion (THD).
Two-port representations: Linear amplifiers can be conveniently modeled using two-port representations. The g-, h-, y-, and z-parameters are of particular interest for describing amplifiers in this text. In most of the amplifiers we consider, the 1-2 parameter will be neglected. It was noted that the g-parameter description is highly useful for representing voltage amplifiers, whereas a current amplifier can be conveniently represented by an h-parameter description. These networks were recast in terms of input resistance Rin, output resistance Rout, open-circuit voltage gain A, and short-circuit current gain β. Ideal voltage amplifiers have Rin = ∞ and Rout = 0, whereas Rin = 0 and Rout = ∞ for ideal current amplifiers. SPICE transfer function analysis can be used to find numeric values of the two-port parameters at dc.
Frequency response: Linear amplifiers can be used to tailor the magnitude and/or phase of sinusoidal signals and are often characterized by their frequency response. Low-pass, high-pass, band-pass, band-reject (or notch), and all-pass characteristics were discussed. The characteristics of these amplifiers are conveniently displayed in graphical form as a Bode plot, which presents the magnitude (in dB) and phase (in degrees) of a transfer function versus a logarithmic frequency scale. Bode plots can be created easily using MATLAB.
In an amplifier, the midband gain Amid represents the maximum gain of the amplifier. At the upper- and lower-cutoff frequencies -- fH and fL, respectively -- the voltage gain is equal to Amid / square root of 2 and is 3 dB below its midband value (20 log|Amid|). The band-width of the amplifier extends from fL to fH and is defined as BW = fH - fL. Narrow band-pass amplifiers are characterized in terms of Q = fo/BW, in which fo is the center frequency of the band-pass circuit.
