Inside of the PtT voltmeter showing inc eJcneu circuit oooru mou^iLM on the meter terminals. The ceran.tc switches are recommended for low leakage since the input impedance is 22,000,000 ohms.

megohms. All this of course means that the FET voltmeter gives a truer reading because it loads the circuit under test less.

The circuit works as follows; FE i Q2 is referenced to common or floating ground by R15. When the meter is zeroed by R12, the voltage drops across R10 and R14 are equal so no meter current flows. Effectively then, the gate of Ql is also at ground, but biased at the correct operating point with respect to its source. Any signal introduced onto the gate of Qi will change the operating point with respect to Q2 and the voltage drop will change across R10. Since a voltage difference now occurs across the meter a current proportional to the applied voltage will flow. Full scale deflection of the meter occurs with .5 volts input. The voltage divider string, R1 through R8 plus the 2 M. in the probe, is used so that full scale deflection occurs at several different voltages, R11 is the calibrate pot and serves to adjust the meter so that with .5 volts on the gate of QI the meter reads full scale. R13, although not needed on my unit, may be required with other FET's. It is used to set the linearity of the voltmeter so that .5 volts Occurs at full scale and .25 volts at exactly half scale. R13's function is to slide the operating point up or down the FET's characteristic curve to utilize the most linear portion. Try a 10 k pot at R13 if you have difficulty finding a linear portion of the curve* When a linear Scale is achieved measure the resistance required and prrrnanentl) wire (¡ii;.it value into the circuit. Interchanging Ql and Q2 may help if they arc nut matched too closely. The meter Ml can be of any sensitivity from 50 mA to 200 mA. But bear in mind that the final accuracy of your voltmeter depends on the use of precision resistors in the voltage divider and also in the basic accuracy of the meter used. Buy the best meter that you can afford. I would also recommend using ceramic switches for S.I and S2 to keep leakage paths to a minimum. All other resistors in the circuit may be half watt 5% carbon,

I won't go too deeply into construction as the photos are self explanatory- Use good construction techniques and if you use a printed board as I have—do by all means clean the board of excess resin after all the parts have been mounted- The resin comes oft quite easily with alcohol (not the chinking type), and besides looking professional, it eliminates a source of leakage on your board*

A matched pair of Siliconix U-112 field effect transistors is available from Siliconix, Inc., 1140 West Evelyn Avenue, Sunnyvale, California 94086 for only S6.00, This is a tremendous bargain for these excellent fETs that normal 1> sel! lor $16.50 in matched pairs.

After youVe tried this FET voltmeter, you'll never want to me a vacuum tube voltmeter again. - - - K3LCU

Fet Voltmeter
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