Current Sources and Voltage References

Using the JFET as a current source

Using JFETs and CRDs to Create Current Sources

151

where

r

s

is the JFET’s internal source resistance;

g

fs

i

s the transconductance or gain;

Z

OUT

is the output impedance in ohms; and

R

s

is the value of the external source

resistor. As an example, let’s obtain an approximate value of output impedance for

several popular N-channel JFETs, all with the same source resistance of 2.2 KΩ (Fig-

ure 6.12). Their typical g

fs

specs are as follows:

2N 5434 = 17 mS

2N 4867A = 1 mS

2N 3819 = 5 mS

2N 4416A = 6.5 mS

Using equations 6.8 and 6.9, the results are:

2N 5434 r

s

= 58.82 Ω; Z

OUT

= 57.5 Ω

2N 4867A r

s

= 1 KΩ; Z

OUT

= 688 Ω

2N 3819 ...