AN105 데이터 시트보기 (PDF) - Vishay Semiconductors

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AN105

FETs As Voltage-Controlled Resistors

Vishay Semiconductors 

AN105

Experimental Results

Figures 8 and 9 show low voltage output characteristic

curves for a typical Siliconix n-channel voltage-controlled

resistor, VCR7N. Bias conditions are shown both with and

without feedback. Figure 8 shows a two-volt peak-to-peak

signal on the VGS = 0 V bias curve, with the VCR operating

in the first and third quadrants. The VCR is operated without

feedback.

The forward-biased gate-drain PN junction may be seen

at approximately –0.6 V, and bending of the bias curve is

apparent in the third quadrant. The photo also demon-

strates the comparison between a fixed resistor (the linear

line superimposed on the bias curve) and the distortion

apparent in the VCR without feedback compensation; the

VCR signal is unusable with the indicated amount of dis-

tortion at 2 V peak-to-peak.

In Figure 9, the same VCR7N FET is shown operating

with the addition of the feedback resistors. Distortion has

been reduced to less than 0.5%, and the characteristics of

the VCR are now closely comparable to those of a fixed

resistor.

In Figures 8 and 9, the same VCR FET characteristics are

shown, with VGS adjusted for higher rDS. No feedback

network is employed in Figure 8, and measured distortion

is greater than 8%. In Figure 9, the feedback resistors

have been added and distortion has been reduced to less

than 0.5%.

Some degree of non-linearity will be experienced in both

the first and third quadrants as VGS approaches the FET

cut-off voltage. For this reason, it is important that the

feedback resistors be of equal value so that the non-linear-

ities likewise will be equal in both quadrants.

200

VGS = VCONTROL

100

VGS = 0 V

200

VGS = VCONTROL = 0 V

100

0

–100

VGS = –2.5 V

0

–100

VGS = –3 V

VCONTROL = –6 V

–200

–1.0

–0.4

0

0.4

1.0

VDS – Drain-Source Voltage (V)

Figure 8. VCR7N Without Feedback

–200

–1.0

–0.4

0

0.4

1.0

VDS – Drain-Source Voltage (V)

Figure 9. VCR7N With Feedback.

Temperature

(_C)

+125

+25

–55

Table 1: Distortion vs. Temperature

Without Feedback

rDS = rDS(on)

>13%

>10%

3.9%

rDS = 10 rDS(on)

>6%

>5%

3.2%

With Feedback

rDS = rDS(on)

<0.5%

<0.5%

<0.5%

rDS = 10 rDS(on)

<0.5%

<0.5%

<0.5%

4

Siliconix

10-Mar-97

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