The common-source amplifier is a commonly used open-loop gain stage for JFET amplifiers, the basic circuit is shown in Figure 9-3.

Figure 9-3 Common-Source Amplifier

The equation for gain of the circuit in Figure 9-3 is shown in Equation 1.

Generally, higher gain results in improved noise performance. Gain increases as the bias current is increased as a result of increasing gm (see Figure 6-4). As a result, the input-referred noise decreases as bias current is increased (see Figure 6-14). Any JFET design must make a tradeoff between current consumption and noise performance. The JFE2140, however, delivers significantly lower noise performance than most operational amplifiers at the same current consumption. The bias current (I_DS) is set by the value of the source resistor, R_S, and the threshold voltage, V_T, of the JFE2140. For JFETs, this threshold voltage is equivalent to the gate-to-source cutoff voltage, V_GSC. A graph showing nominal I_DS vs R_S is shown in Figure 9-4.

Figure 9-4 Drain-to-Source Current vs R_S, V_DS = 5 V

The bias current varies according to the resistor and threshold voltage tolerances. Additionally, thermal noise associated with R_S couples directly into the gain of the circuit, degrading the overall noise performance. To improve the circuit in Figure 9-5, use a current-source biasing scheme. Current-source biasing removes the JFET threshold variation from the biasing scheme, and allows for lower-value filtering capacitance (C_S) for equivalent filtering due to the high output impedance of current sources.

Figure 9-5 Common-Source Amplifier With Current-Source Biasing