How high is the input impedance of 'a' jfet transistor?

[Hans L]

What kind of value can I presume a jfet transistor has for its input impedance? Tens of MOhms? GOhms? I've searched datasheets, but apparently the input impedance (which naturally is presumed 'very high' for jfet) isn't stated. Can it be determined from other specs in the datasheet? For instance via the Zero-Gate Voltage Drain Current? I'm currently testing (read: learning) with the 2n5459 (http://www.fairchildsemi.com/ds/2N/2N5459.pdf) and the BF245A (http://www.fairchildsemi.com/ds/BF%2FBF245A.pdf).

Any ideas how to easily measure it with reasonable accuracy given that it is 'very large'?

Cheers,
Hans.

[richie00boy]

Look at the gate-source and gate-drain capacitances and work out the impedance at the frequencies of interest.

[janneman]

Originally posted by Hans L
What kind of value can I presume a jfet transistor has for its input impedance? Tens of MOhms? GOhms? I've searched datasheets, but apparently the input impedance (which naturally is presumed 'very high' for jfet) isn't stated. Can it be determined from other specs in the datasheet? For instance via the Zero-Gate Voltage Drain Current? I'm currently testing (read: learning) with the 2n5459 (http://www.fairchildsemi.com/ds/2N/2N5459.pdf) and the BF245A (http://www.fairchildsemi.com/ds/BF%2FBF245A.pdf).

Any ideas how to easily measure it with reasonable accuracy given that it is 'very large'?

Cheers,
Hans.

Basically it is so high that the leakage current across the leads over the package dominates any input current at DC. At AC, the input impedance is dominated by the input capacitance, which should be stated in the data sheet. Trying to measure a static input R is probably fruitless.

Jan Didden