Posted on Dec 17, 2012

This FET probe has an input impedance of 10 M!J shunted by 8 pF. Eliminating the protective diodes reduces this impedance to about 4 pF. The frequency response of the probe extends from de to 20 MHz ( -1 dB), although higher frequency operation is possible through optimized construction and use of a UHF-type transistor. Zero de offset at the output is achieved by selecting a combination of a 2N5246 and source resistor that yields a gate-source bias equal to the V8E of the 2N3704 at approximately 0 V.

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At medium frequencies, the probe can be used unterminated for near-unity gain; for optimum impedance converter probe high-frequency response, the cable must be terminated into 50 !J. The voltage gain, when properly terminated, is precisely 0.5 X.

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