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AN211A

Fifeld Effect Transistors in Theory and Practice

Motorola => Freescale 

AN211A

Freescale Semiconductor, Inc.

6000

5000

VDS = 15 V

TA = 25°C

f = 1.0 kHz

3000

2000

2N4220, A

1000

700

2N4222, A

2N4221, A

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.

500

300

0.1 0.2

0.5

1.0

2.0

ID, DRAIN CURRENT (mA)

5.0 10

Figure 14. Forward Transfer Admittance versus Drain

Current for Typical JFETs

5.0

gfs

yfs

4.0

VDS = 15 Vdc

VGS = 0

3.0

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.

2.0

- bfs

1.0

0

10

20 30 50

100 200 300 500 1000

f, FREQUENCY (MHz)

Figure 15. Forward Transfer Admittance versus

Frequency

AC

VTVM

VDS

D

GI

S

Vout

RS

AC

VTVM

VDD

BYPASS

YOS =

Vout

VDS RS

RS OF SUCH VALUE AS TO CAUSE

NEGLIGIBLE DC DROP.

RS SENSES AC DRAIN CURRENT

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.

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.

8

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