AC Millivoltmeter Adapter

  
Diode detectors are good at changing AC into DC, even at high frequencies. The main problem with diode detectors have inherent nonlinearities, particularly for small signals. To minimize the effects of the nonlinearities, a preamplifier boosts the amplitude of the input signal by 16X, thereby reducing the the diode detector’s nonlinearity effects by the same factor. The detector is followed by a buffer and adjustable gain stage. This last stage compensates for DC offset that results from bias on the diode detector and reduces the full scale amplitude so that the output signal DC level is equal to the input peak-to-peak value.
AC Millivoltmeter Adapter - schematic

The input net work consists of AC coupling made of a 0.047 uf capacitor and a 10 Meg resistor and transient protection. The -3 db corner of the high pass filter is 0.34 Hz. Some degree of transient protection is obtained by two back-to-back emitter-base junctions, which work a lot like back-to-back Zeners. The main difference between these and Zeners is that the reverse biased junction capacitance is lower and the knee fairly sharp, meaning that it should have no measurable effect on a 200 kHz, 200 millivolt input signal. The input of the preamp is the gate of the 2N5485 JFET, and since the drain drives the forward-biased base-emitter junction of the 2N3906, the signal on the JFET’s drain is very small, thus there is little miller capacitance, keeping the input capacitance low. Negative feedback thorough the 20k resistor from the collector of the 2N3906 and the source of the 2N5485 JFET stabilizes the DC operating point of both transistors and stabilizes the gain of the stage overall. I measured the gain of the one I built at 16X. The 10K resistor was selected for the individual 2N5485 to set the DC level on the output of the stage. On the one I built, the DC level at the output is 4.7 VDC. This resistor (10k in this schematic) may need to be changed for individual JFETs because of variation in the pinch-off voltage. The AC signal from the preamp is connected to a half-wave voltage doubler employing Schottky...



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