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AN211A Datasheet(PDF) 8 Page - Motorola, Inc |
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AN211A Datasheet(HTML) 8 Page - Motorola, Inc |
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8 / 12 page  AN211A 8 MOTOROLA SEMICONDUCTOR APPLICATION INFORMATION 6000 5000 3000 2000 1000 700 500 300 0.1 0.2 0.5 1.0 2.0 5.0 10 2N4222, A 2N4221, A 2N4220, A ID, DRAIN CURRENT (mA) VDS = 15 V TA = 25°C f = 1.0 kHz Figure 14. Forward Transfer Admittance versus Drain Current for Typical JFETs gfs - bfs yfs VDS = 15 Vdc VGS = 0 1000 500 300 1.0 2.0 5.0 200 100 50 30 20 10 0 4.0 3.0 f, FREQUENCY (MHz) Figure 15. Forward Transfer Admittance versus Frequency VDS AC VTVM S D RS GI YOS = Vout VDS RS SENSES AC DRAIN CURRENT RS OF SUCH VALUE AS TO CAUSE NEGLIGIBLE DC DROP. Vout AC VTVM VDD BYPASS RS Figure 16. yos Measurement Circuit for Depletion FETs Voltages and frequencies for measuring yos should be exactly the same as those for measuring yfs. Like yfs, it is a complex number and should be specified as a magnitude at 1 kHz and in complex form at high frequencies. µ Closely related to yos and yfs is the amplification fac- tor, µ: µ = ∆VDS/∆VGS ID = K The amplification factor does not appear on the field-effect transistor registration format but can be calculated as yfs/yos. For most small-signal applications, µ has little circuit significance. It does, however, serve as a general indication of the quality of the field-effect manufacturing process. Ciss The common-source-circuit input capacitance, Ciss, takes the place of yis in low-frequency field-effect transistors. This is because yis is entirely capacitive at low frequencies. Ciss is conveniently measured in the circuit of Figure 17 for the tetrode JFET. As with yfs, two measurements are necessary for tetrode-connected devices. At very high frequencies, the real component of yis becomes important so that rf field-effect transistors should have yis specified as a complex number at the same conditions as other high-frequency parameters. For tetrode-connected rf FETs, reading of both Gate 2 to source and Gate 1 tied to Gate 2 are necessary. In switching applications Ciss is of major importance since a large voltage swing at the gate must appear across Ciss. Thus, Ciss must be charged by the input voltage before turn-on effectively begins. Crss Reverse transfer admittance (yrs) does not appear on FET data sheets. Instead Crss, the reverse transfer capacitance, is specified at low frequency. Since yrs for a field-effect transistor remains almost completely capacitive and relatively constant over the entire usable FET frequency spectrum, the low-frequency capacitance is an adequate specification. Crss is measured by the circuit of Figure 18. For tetrode FETs, values should be specified for Gate 1 and for both gates tied together. Again, for switching applications Crss is a critical characteristic. Similar to the Cob of a junction transistor, Crss must be charged and discharged during the switching interval. For a chopper application, Crss is the feedthrough capacitance for the chopper drive. Cd(sub) For the MOSFET, the drain-substrate junction capacitance becomes an important characteristic affecting the switching behavior. Cd(sub) appears in parallel with the load in a switching circuit and must be charged and discharged between the two logic levels during the switching interval. Noise Figure (NF) Like all other active components, field-effect transistors generate a certain amount of noise. The noise figure for field-effect transistors is normally specified on the data sheet as “spot noise”, referring to the noise at a particular frequency. The noise figure will vary with frequency and also with the resistance at the input of the device. Typical graphs of such variations are illustrated in Figure 19 for the 2N5458. From graphs of this kind the designer can anticipate the noise level inherent in his design. Freescale Semiconductor, Inc. For More Information On This Product, Go to: www.freescale.com |
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