THE COMMON-SOURCE AMPLIFIER In this section, we consider the first of the three basic circuits; the common-source amplifier. We will analyze several basic common-source circuits, and will determine small-signal voltage gain and input and output impedances. A Basic Common-Source Configuration For the circuit shown in Figure 6.13, assume that the transistor is biased in the saturation region by resistors R1 and R2, and that the signal frequency is sufficiently large for the coupling capacitor to act essentially as a short circuit. The signal source is represented by a Thevenin equivalent circuit, in which the signal voltage source vi, is in series with an equivalent source resistance RSi. As we will see, RSi should be much less than the amplifier input resistance, Ri = R1 || R2 in order to minimize loading effects. Figure 6.14 shows the resulting small-signal equivalent circuit. The small signal variables, such as the input signal voltage Vi are given in phasor form. The output voltage is The input and output resistances of the amplifier can be determined from Figure 6.14. The input resistance to the amplifier is Ris = R1 || R2. Since the low-frequency input resistance looking into the gate of the MOSFET is essentially infinite, the input resistance is only a function of the bias resistors. The output resistance looking hack into the output terminals is found by setting the independent input source Vi equal to zero, which means that VGS = 0. The output resistance is therefore Ro = RD || ro.
THE COMMON-SOURCE AMPLIFIER In this section, we consider the first of the three basic circuits; the common-source amplifier. We will analyze several basic common-source circuits, and will determine small-signal voltage gain and input and output impedances. A Basic Common-Source Configuration For the circuit shown in Figure 6.13, assume that the transistor is biased in the saturation region by resistors R1 and R2, and that the signal frequency is sufficiently large for the coupling capacitor to act essentially as a short circuit. The signal source is represented by a Thevenin equivalent circuit, in which the signal voltage source vi, is in series with an equivalent source resistance RSi. As we will see, RSi should be much less than the amplifier input resistance, Ri = R1 || R2 in order to minimize loading effects. Figure 6.14 shows the resulting small-signal equivalent circuit. The small signal variables, such as the input signal voltage Vi are given in phasor form. The output voltage is The input and output resistances of the amplifier can be determined from Figure 6.14. The input resistance to the amplifier is Ris = R1 || R2. Since the low-frequency input resistance looking into the gate of the MOSFET is essentially infinite, the input resistance is only a function of the bias resistors. The output resistance looking hack into the output terminals is found by setting the independent input source Vi equal to zero, which means that VGS = 0. The output resistance is therefore Ro = RD || ro.