The Circuit Designer's Companion - diagramas.diagram...

Active components 137order of 15−20p for plastic packages.Current regulatorsA frequent niche application for JFETs is as a one-component current regulator. If thedevice is connected with gate shorted to source (Figure 4.29) then once V DS exceedsthe pinch-off voltage the drain current will limit to I DSS and remain constant over a widerange of applied voltage. The current can be adjusted downwards from I DSS byincluding a resistor in series with the source, which effectively gives a constant gatebias.I DSSI DSSI D < I DSSconstant current regionorR = V G/I DFigure 4.29 The JFET as current regulatorThe circuit cannot be used as a precision current source/sink because its outputimpedance is on the low side for such applications, and the control current istemperature dependent, though like the zener there is a zero-tempco crossover point.Also, the wide variation in pinch-off voltage and I DSS between devices results in asimilarly wide variation of current. Even so, it can be useful where the absolute valueof current is unimportant, such as in amplifier biasing. It is possible to obtain “currentregulator diodes” which are gate-source-shorted JFETs specially characterised for thepurpose and available in reasonably tightly selected current bands from 0.2 to 5mA,though these are relatively expensive, being around 50p for plastic packaged types.4.4.3 High impedance circuitsThe JFET is very useful for the design of high input impedance amplifiers. Because thegate under normal operating conditions is a reverse-biased junction, the low-frequencyinput impedance of a JFET front end is limited only by gate leakage and by theresistance of any bias resistor that may be necessary. Gate leakage currents of a fewpicoamps at room temperature are readily achievable, although making use of them isanother matter − leakage currents due to stray paths across pcbs and connectors areusually the limiting factor. The JFET does not always live up to its promise of highimpedance, though.Firstly, because the current is due to reverse-biased junction leakage, it increasesexponentially with temperature in the same way as was seen for the ordinary silicondiode junction. Thus at the maximum commercial temperature limit of 70˚C theleakage current is more than twenty times that at room temperature, though it is stillbetter than that of most other devices. At 125˚C, the maximum military temperaturelimit, it is a thousand times worse, and a well-designed bipolar input will have a betterperformance. This is one reason why JFET input circuits are rare in military designs.The gate current breakpointSecondly the common mode voltage range is restricted. Obviously if the input voltageexceeds the supply rails then incorrect operation may occur − though you might want