Physics 160Lecture 13R. JohnsonJFET Amplifiers• Typically, the place you want to use a JFET amplifier is where you need very high input impedance, for example because your signal source has a very high impedance (cannot deliver muchsignal source has a very high impedance (cannot deliver much current).– Probably best used in differential amps or source-followers, not common-source amps.• Another example is where the base current of a bipolar transistor will cause a significant error.g– The LF411 Op-amp that you will soon use in several circuits uses JFETs at its inputs. This is very nice, because the current flowing into the inputs is negligible in all cases.• Otherwise, bipolar transistors will usually give much better performance in terms of gain, predictability, etc.May 13, 2014 Physics 160 2JFET Amplifiers• You need to keep in mind:– The gate-source junction must be reverse biased at all times (or at worst, zero volts).worst, zero volts).– The gate does need some bias current, although it can be very small.–The drain-to-source voltage cannot be too small, especially if the gpydrain current is substantial. For a bipolar transistor the collector-emitter voltage can be a fraction of a volt, but for a JFET count on a few volts.U lik th f bi l t i t th i i l f l f–Unlike the case of a bipolar transistor, there is no simple formula for the transconductance. You must consult the data sheet, and it will be small compared with the bipolar transistor transconductance. •Or equivalently the effective dynamic resistance of the sourceOr equivalently, the effective dynamic resistance of the source will be a few hundred ohms, not the bipolar transistor value of 25 divided by the current in mA.May 13, 2014 Physics 160 3JFET Source FollowerVery high impedance JFET has rather high output impedance, so a BJT follower issource is no problem (except at high frequency)BJT follower is useful to boost the output drive.R210Meg1.382pAV215VdcVS1.382pAJ1J2N54854.011mA0 VDriving sourceC210Meg6.401mAVS1Vac0VdcQ1Q2N390413.89uA2.390mA-2.404mAVJ2N5485+0.5 V01VC212uFVV315VdcR36.2k2.404mARL1MegR13.9k3.997mAVoltage division between Zoutof the JFET (1/g)RS0.1 V6.401mA1Meg0Athe JFET (1/gm) and RS.results in gain less than unity May 13, 2014 Physics 160 4Setting the Bias CurrentEssentially the same procedure as used for the JFET current source. kR 9.345.15AV4mAMay 13, 2014 Physics 160 5JFET Follower Gain 2501mg94.049.3149.31mSmSgRgRGMay 13, 2014 Physics 160 6Reminder from Lecture 12:J31 053pA6.869mAV1V1J31 381 A4.254mA15.00VIDSSJ2N5485-1.053pAV115Vdc6.869mAV115Vdc4.254mAR1125J2N5485-1.381pA531.8mV 125mA4V5.0RMay 13, 2014 Physics 160 7Improving Follower PerformanceJ1J2N54854.274mAVS1Vac0Vd1.380pA2 380mAV215Vdc6 654mAR210Meg1.380pA0 V0VdcQ1Q2N390413.83uA2.380mA-2.394mAV6.654mAC2+0.5 V0.1 VR36.2k2.394mARL1MegV315VdcJ2J2N5485-1.464pA4.260mA12uF0.1 V145V15 V0AR11256.654mAReplace the source 14.5 VMay 13, 2014 Physics 160 8resistor by a JFET (or BJT) current source.Gain with current source = 0.99!The lack of high frequencyThe lack of high frequency performance is related to the high driving source impedance. We will see how to improve this with a “b t t ”“bootstrap”.May 13, 2014 Physics 160 9Output ImpedanceJFET Zin>>10 MegZoutis independent of source resistance!!J1J2N54854.274mAV215VdcR210Meg1.380pAVS1.380pAJFET Zout=1/gm=250 ohmsV6.654mAQ1Q2N390413.83uA2.380mA-2.394mAC21Vac0VdcBJT Zin=22.5170=3800 ohmsBJT Zout=250/170 + 25/2.4=12 510382/25:BJTrVRL12100uFV315VdcR36.2k2.394mAJ2J2N5485-1.464pA4.260mA5.1038.2/25 :BJTer12 ohm load (t )R11250A6.654mA2380/13 83 172(extreme case)May 13, 2014 Physics 160 10=2380/13.83=172Voltage Division of Impedances95.020038003800JFET Output48.0127.111295.0 BJT OutputMay 13, 2014 Physics 160 11Bootstrapping the Drain of the FollowerMake the drain follow the gate to minimize the voltage changes between Gate and Drain. This almost eliminates the lowpassHigh pass filterCGSalmost eliminates the low-pass filter formed by the source impedance and the G-S capacitance.GSLow pass filter0 V+7.8 VThis trick can +0.5 V4 mAgreatly increase the response at high frequency.0.1 V15 V14.5 VMay 13, 2014 Physics 160 12Don’t make RD too much larger or else the VDSwill become too small.No Bootstrap on DrainThe low-pass filter10 Meg-ohm Source The lowpass filter formed by the source impedance and gate-drain itiImpedance1 k-ohm loadparasitic capacitance kills the gain above about 10 kHzNote that the gain is less than unity above because of charge division between the output impedance of the BJT emitterfollower and the 1 kohmMay 13, 2014 Physics 160 13between the output impedance of the BJT emitter-follower and the 1 k-ohm load resistor.With Bootstrapped Drain10 Meg-ohm Source Impedance1 k-ohm loadBandwidth is now nearly 1 MHzNote that the gain is less than unity above because of charge division between the output impedance of the BJT emitterfollower and the 1 kohmMay 13, 2014 Physics 160 14between the output impedance of the BJT emitter-follower and the 1 k-ohm load resistor.Bootstrapped Drain with Heavy LoadRoll-off due to AC coupling to the load10 Meg-ohm Source The frequency response has moved down from 1 MHz to 100 kHzVoltage division between Zoutand the 100 ohm loadImpedance100 ohm loadthe 100 ohm load has reduced the gain by about 10%.With the load greatly increased (100 ohm load resistor), then the gain is so much less than unity that the bootstrap no longer works very well We lose a decade ofMay 13, 2014 Physics 160 15less than unity that the bootstrap no longer works very well. We lose a decade of frequency response.AC Coupled InputThe gate doesn’t need much current, but about 1pA still has to flow, so a bias resistor is essential!resistor is essential!The capacitor can be very small, since the bias resistor (input impedance) is large.Zinis now completely dominated by the bias resistor (well above the 3dB May 13, 2014 Physics 160 16(point).Bias resistor lowered Zinto 100 MNote the 10% loss of gain gdue to voltage division between the 10 Msource impedance and the 100 Minput pimpedance.May 13, 2014 Physics 160 17AC Coupling with Bias BootstrapWe can bootstrap the bias network to raise it to even higher impedance at signal frequencies just assignal frequencies, just as for a BJT follower.Only a small, inexpensive R10at signal frequencies lookscapacitor is