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AN105

更新时间： 2024-10-29 06:37:11

品牌	Logo	应用领域
威世 - VISHAY		电阻器
页数	文件大小	规格书
7页	95K
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FETS AS VOLTAGE-CONTROLLED RESISTORS

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AN105 数据手册

AN105

FETs As VoltageĆControlled Resistors

Introduction: The Nature of VCRs

A voltage-controlled resistor (VCR) may be defined as a Figure 1 details typical operating characteristics of an n-

three-terminal variable resistor where the resistance val- channel JFET. Most amplification or switching operations

ue between two of the terminals is controlled by a voltage of FETs occur in the constant-current (saturated) region,

potential applied to the third.

shown as Region II. A close inspection of Region I (the un-

saturated or pre-pinchoff area) reveals that the effective

slope indicative of conductance across the channel from

drain-to-source is different for each value of gate-source

bias voltage. The slope is relatively constant over a range of

applied drain voltages, so long as the gate voltage is also

constant and the drain voltage is low.

For a junction field-effect transistor (JFET) under certain

operating conditions, the resistance of the drain-source

channel is a function of the gate-source voltage alone and

the JFET will behave as an almost pure ohmic resistor.

Maximum drain-source current, I , and minimum re-

DSS

sistance r , will exist when the gate-source voltage

DS(on)

is equal to zero volts (V = 0). If the gate voltage is in-

creased (negatively for n-channel JFETs and positively

for p-channel), the resistance will also increase. When the

drain current is reduced to a point where the FET is no

longer conductive, the maximum resistance is reached.

The voltage at this point is referred to as the pinchoff or

Resistance Properties of FETs

The unique resistance-controlling properties of FETs can

be deduced from Figure 2, which is an expanded-scale

plot of the encircled area in the lower left-hand corner of

Figure 1. The output characteristics all pass through the

origin, near which they become almost straight lines so

cutoff voltage and is symbolized by V = V

. Thus

GS(off)

the device functions as a voltage- controlled resistor.

that the incremental value of channel resistance, r , is

essentially the same as that of dc resistance, r , and is a

Locus Curve

= V - V

GS GS(off)

function of V

GS.

Region 1

Ohmic Region

Region 2

= 0

DSS

Figure 2 shows an extension of the operating characteris-

tics into the third quadrant for a typical n-channel JFET.

While such devices are normally operated with a positive

Current Saturation Region

drain-source voltage, small negative values of V are

possible. This is because the gate-channel PN junction

must be slightly forward-biased before any significant

t 0

amount of gate current flows. The slope of the V bias

line is equal to I /V = 1/r . This value is controlled

→ V

GS(off)

by the amount of voltage applied to the gate. Minimum

, usually expressed as r , occurs at V = 0 and

DS(on)

is dictated by the geometry of the FET. A device with a

channel of small cross-sectional area will exhibit a high

- DrainĆSource Voltage (V)

and a low I . Thus a FET with high I

should

DSS

DS(on)

DSS

Figure 1. Typical NĆChannel JFET

Operating Characteristics

be chosen where design requirements indicate the need

for a low r

DS(on)

Updates to this app note may be obtained via facsimile by calling Siliconix FaxBack, 1-408-970-5600. Please request FaxBack document #70598.

Siliconix

10-Mar-97

12 3 4 5 6 7

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