Chapter 13 initiated our study of the basic amplifier circuits used in the design of more com-plex analog components and systems such as operational amplifiers, audio amplifiers, and Rf communications equipment. The chapter began with an introduction to the use of the transis-tor as an amplifier, and then explored the detailed operation of the BJT common-emitter (C-E)and FET common-source (C-S) amplifiers. Expressions were developed for the voltage gain and input and output resistances of these amplifiers. Several examples of the complete analysis of common-emitter and common-source amplifiers were included near the end of the chapter. The relationships between Q-point design and the small-signal characteristics of the amplifier were fully developed.

• The common-emitter amplifier can provide good voltage gain but often has only a low-to-moderate input resistance.
• In contrast, the FET stages can have very high input resistance, but typically provide relatively modest values of voltage gain.
• The output resistances of both C-E and C-S circuits tend to be determined by the resistors in the bias network and are similar for comparable operating points.
• The superposition principle is used to simplify the analysis and design of amplifiers. Circuits are split into two parts: a dc equivalent circuit used to find the Q-point of the transistor and an ac equivalent circuit used for analysis of the response of the circuit to signal sources.The design engineer often must respond to competing goals in the design of the dc and ac characteristics of the amplifier, and coupling capacitors, bypass capacitors, and inductors are used to change the ac and dc circuit topologies.
• The use of superposition requires linearity, and the ac analyses were all based on linear small-signal models for the transistors. The small-signal models for the diode, bipolar transistor (the hybrid-pi model), MOSFET, and JFET were all discussed in detail. The expressions relating the transconductance output resistance r_o, and input resistance r_π to the Q-point were all found by evaluating derivatives of the large-signal model equations developed in earlier chapters.
• The small-signal model for the diode is simply a resistor that has a value given by r_d = V_T/I_D.
• The results in Table 13.4 on page 916 for the three-terminal devices are extremely important. The structure of the models is similar. The transconductance of the BJT is directly proportional to current, whereas that of the FET increases only in proportion to the square root of current. The resistances r_π and r_o are inversely proportional to Q-point current. The resistor r_π is infinite for the case of the FET, so it does not actually appear in the small-signal model. It was discovered that each device pair, the npn and pnp BJTs, the NMOS and PMOS FETs, and the n- and p-channel JFETs, has the same small-signal model.
• The small-signal current gain of the BJT was defined as β_o = g_mr_π, and its value generally differs from that of the large-signal current gain β_F. Because β_F for the FET is infinite, the FET exhibits an infinite small-signal current gain at low frequency.
• The amplification factor, also known as the intrinsic voltage gain of the transistor, is defined as μ_f = g_mr_o and represents the maximum gain available from the transistor in the C-Eand C-S amplifiers. Expressions were evaluated for the amplification factors of the BJT and FETs. Parameter μ_f was found to be independent of Q-point for the BJT, but for the FET, the amplification factor decreases as operating current increases. For usual operating points, μ_f for the BJT will be several thousand whereas that for the FET ranges between tens and hundreds.
• The definition of a small signal was found to be device-dependent. The signal voltage v_d developed across the diode must be less than 5 mV in order to satisfy the requirements of a small signal. Similarly, the base-emitter signal voltage v_be of the BJT must be less than 5 mV for small-signal operation. However, FETs can amplify much larger signals without distortion. For the MOSFET and JFET, v_gs ≤ 0.2 (V_GS - V_TN) and v_gs ≤ 0.2 (V_GS - V_P) represent the small-signal limits, respectively, and can often range from 100 mV to more than 1 V.
• Common-emitter and common-source amplifiers were analyzed in detail. Table 13.5 on page 947 is another extremely important table. It summarizes the overall characteristics of these two amplifiers. The rule-of-thumb estimates in Table 13.5 were developed to provide quick predictions of the voltage gain of the C-E and C-S stages.
• The chapter closed with a discussion of the relationship between operating point design and the power dissipation and output signal swing of the amplifiers. The amplitude of the signal voltage at the output of the amplifier is limited by the smaller of the Q-point value of the collector-base or drain-gate voltage of the transistor, and by the Q-point value of the voltage across the collector or drain-bias resistors R_C or R_D.
• It is extremely important to be sure to understand the difference between ac analysis and transient analysis in SPICE. ac analysis assumes that the network is linear and uses small-signal models for the transistors and diodes. Since the circuit is linear, any convenient value can be used for the signal source amplitudes, hence the common choice of 1-V and 1-Asources. In contrast, transient simulations utilize the full large-signal non-linear models of the transistors. If we desire linear behavior in a transient simulation, all signals must satisfy the small-signal constraints.