Please help me bias this FET bridge

Discussion in 'The Projects Forum' started by Darkstar, Sep 5, 2011.

  1. Darkstar

    Thread Starter Senior Member

    Sep 3, 2010
    Well, you've uncovered another mistake I made. Sometime after my moving the FETs outside I got the cables mixed up inside. So, when I was monitoring the gnd FET above (post 114) I was actually monitoring the antenna FET & vice versa. I re-ran the tests from post 114 using the schematic below. Here are a few circuit stats and graphs.

    Antenna FET:
    Output began about 15V and leveled off about 22V over 30 min later. A little sensitive to my movements.
    AC V is 2.23 V @ 60 Hz
    Measuring specifically for frequency gives 0.0 Hz
    Current through the antenna FET (4.7K resistor) = 22V/4680 Ω = 4.70 mV, 103.4 mW. See graph below.

    Ground FET:
    Output was steady about 17V.
    AC V is 47 mV @ 120 Hz – first time I've ever seen this freq.
    Measuring specifically for frequency gives 0.0 Hz
    Current through the antenna FET (4.7K resistor) = 17V/4680 Ω = 3.63 mV, 61.7 mW. See graph below.

    (We always have the option of going back to having both fets inside and using just one cable out to the antenna. Hint, Hint.)
  2. Hi-Z

    Active Member

    Jul 31, 2011
    Ah, good, now we're getting somewhere, I think. It looks like the antenna plus fet gate are slowly being charged up, and ending up at a potential of about 7V above ground. It seems to be picking up 60Hz, but we can probably live with this (with the right circuit). (Incidentally, how do you know the frequencies are 60/120 Hz if you measure 0.0 Hz?)

    The ground connection seems to be OK - it's probably sitting around 0V with a bit of 120Hz pickup (120Hz implies coupling to a power supply - probably the +/- 15V supply). Anyway, that's all perfectly liveable with, I'd say.

    I think you're absolutely right to have the fets outside - just think how slowly the antenna circuit would take to charge up if you had a significant cable capacitance to cope with!

    I think the next test ought to be a long run, probably connecting the two fet outputs to the in-amp via 2M2 resistors (with the 1uF filter capacitor etc. in place), and measuring the differential voltage by setting a gain of x1. (If that's not possible, just measure the differential voltage directly with the dvm - it should be able to reject the 60/120 Hz.)

    Hopefully, you'll be able to see some voltage variation over the course of a few hours (or a day). If the variation turns out to be very small relative to the 7V or so, then there are some things which can be done (like adding a zero pot).
  3. Darkstar

    Thread Starter Senior Member

    Sep 3, 2010
    There are 3 ways to measure frequency with my DVM:
    1. While monitoring DC volts, push a button to display the frequency of any AC being picked up. This gives me 0.0 Hz
    2. While monitoring AC volts, push the same button to display the frequency of the signal. This gives me 60-120 Hz.
    3. Turn dial selector to monitor frequency directly. This gives me the approximately 20 to 40 KHz recently measured.

    The meter can measure from 10 Hz to 4 MHz, but sensitivity varies depending on the range of the meter. See copy of a page from the instructions.

    Just a FYI here concerning antenna capacitance. The DVM will also measure capacitance in a somewhat small range, so I measured the antenna back when I built it and I got 13 pF.
    The capacitance of my 50' shielded cable, from shield to each center conductor, was about 2,000 pF.
    I thought it might be helpful to know these numbers when trying to design a filter.

    I'll begin a long run measuring between the 2 fet outputs. I'll do it first just with the DVM so we know what that looks like for starters.