Q. Common-Drain JFET Amplifier?

Figure (a) shows a CD JFET amplifier in which resistors R₁, R₂, and R_SS are selected by the bias design, and capacitors CG and CS are chosen to be large enough to act as short circuits at frequencies in the band of interest (known as the midband). Figure (b) gives its small-signal ac equivalent circuit. Analysis of this circuit yields

In many cases Av₁ = 1 and v_L = v1; that is to say the load voltage "follows" the input. Hence a CD amplifier is often known as a source follower, which becomes an excellent buffer to couple a high-resistance source to a low-resistance load with nearly no loss in signal voltage. The current gain, however, can be very large, leading to significant power gain.