CHAPTER 5. TRANSISTOR CIRCUITS95RciciBREvERB1RB2CBVccvcFigure 5.13: Basic common base ampliﬁer.The input impedance looking into the emitter (Rin = hie/hfe) is quite small. The outputimpedance of this circuit is never greater than RC.Because of its high-frequency response and small input impedance, this circuit is oftenused to receive high-frequency signals transmitted via a coaxial cable. For this purposethe input impedance of the ampliﬁer is adjusted to match the distributed impedance of thecoaxial cable – usually 50 to 75 Ω.5.5The Junction Field Eﬀect Transistor (JFET)Bipolar junction transistors have low input impedance, small high-frequency gain and arenon-linear when|VCE| < 2 V. The input impedance is naturally restricted by the forward-biased base-emitter junction. There are always problems due to the main charge carrierspassing through the region where the majority carriers are of opposite polarity.The junction ﬁeld eﬀect transistor (JFET) overcomes some of the problems of the bipolarjunction transistor. JFETs come in two types: N-channel and P-channel, and are shown inﬁgure 5.14.The designation refers to the polarity of the majority charge carriers in the bar of semi-conductor that connects the drain terminal D to the source terminal S. Since the channel isformed from a single-polarity (unipolar) material, its resistance is a function only of the ge-ometry of the conducting volume and the conductivity of the material. The JFET operateswith all PN junctions reverse-biased so as to obtain a high input impedance into the gate.5.5.1Principles of OperationFigure 5.15 shows an N-channel JFET with DC bias voltage applied. Just as for a simplediode, the depletion region grows as the reverse bias across the PN junction is increased,thereby constricting the cross section of the conducting N-channel material and increasingthe resistance of the channel. The major current ID in the channel is caused by the appliedvoltage between the drain and source, VDS, and is controlled by the applied voltage betweenthe gate and source, VGS.