Common·Drain Amplifier Figure 8-17 shows a JFET connected as a common-drain amplifier. Note that the drain terminal is connected directly to the supply voltage VOD, so the drain is at ac ground. Since the input and output signals are taken with respect to ground, the drain is common to both, which accounts for the name of the configuration. Since the ac output signal is between the source terminal and ground, it is the same signal measured from source to drain as measured across Rs. Note that the JFET is biased using the combined self-bias/voltage-divider method, though either of the other two bias methods we studied for the common-source configuration could also be used. As can be seen from the figure Figure 8-18 shows the ac equivalent circuit of the common-drain amplifier. Although this circuit appears at first glance to be the same as the common-source configuration (Figure 8-11(b». note that Viii is not v••. Rather, v •• is related to VMS by equations 8-22 and 8-23. Neglecting the signal-source resistance rs for the moment, we derive the voltage )!ain v(./v”, as follow It is clear from equation 8-25 that the voltage gain of the common-drain configuration is always less than 1. Note that there is no phase inversion between input and output. When g”,(r,/ II R, II RI) » 1 (often the case), equation 8-25 shows that In other words, the load voltage is approximately the same as the input voltage, in magnitude and phase, and we say tbat lite output follows the input. For this reason, the common-drain amplifier is called a source follower. The input resistance of the source rollower can be seen Irom Figure 8-17 to be the same as it is for a common-source amplifier: Similariy, rill is the same as it is for a common-source amplifier when the other bias arrangements are used. When the voltage division between signal-source resistance and rin is taken into account, we find the overall voltage gain to be The output resistance of the source follower (r,,(stage), looking from RI. toward
the source terminal) is R,~in parallel with the resistance “looking into” the JFET at the source terminal. The resistance looking into the JFET at the source terminal can be found using equation 8-25. Since we are looking to the left of Rs and Rt., the parallel combination of those resistances is not relevant in the equation. Substituting r, for r”IIRsIIRt.. equation 8-25 becomes Like the BJI emitter follower. the source follower is used primarily as a buffer
amplifier because of its large input resistance an-a small output resistance.