[sdiy] MOSFET cap discharge

[Scott Gravenhorst]

Many thanks, Ian, being a MOSFET-ophile, I may try this one.  Very cool.

Ian Fritz <ijfritz at earthlink.net> wrote:
>Hi listers --
>
>We often hear that MOSFETs shouldn't be used for the cap discharge switch 
>in a saw VCO, because they have too much leakage current.  Indeed, the spec 
>sheet for the BS170 (for example) lists a max drain cutoff current of 500 
>nA.  Pretty bad.
>
>But if you look in Horowitz and Hill, they say that the channel resistance 
>in the off state is usually over 10000 M, i.e., 10^10 Ohm.  So with  1V 
>across the channel, leakage should be under 100 pA.  How bad is it in 
>practice?  I have heard that some people have had success with MOSFET 
>discharge switches.
>
>So I wanted to try this, and I also wanted to try to make a VCO with a pair 
>of dual opamps (for simplicity) and with a 1V saw amplitude (to keep a 
>reasonable switching speed).  The circuit I ended up with is posted here:
>http://home.earthlink.net/~ijfritz/xfer.htm
>
>The front end uses a precision device (OPA2227) to reduce offset errors and 
>drift.  The ramp/reset circuit uses an OPA2134.  This has been used before 
>in a VCO integrator, and with bias current in the pA range and a 20 V/us 
>slew rate it works quite well.  As a comparitor it's not so hot, but using 
>the MOSFET switch and keeping the ramp voltage low keeps the reset time 
>down to about 600 ns (see photo of  the reset part of waveform). Note that 
>the voltage divider feeding the FET gate keeps the gate near 0 V in the off 
>state, so the comparitor only needs to slew a couple of volts to turn the 
>switch on.
>
>With the current source disconnected, the oscillator free-runs with a 
>period of about 5 min (600 s or 3 mHz).  This corresponds to a total 
>leakage current of only 40 pA.  Moreover, when the MOSFET is removed the 
>ramp rises at about the same rate!  So the MOSFET leakage is under 
>something like 20 pA.  40 pA of leakage is a bit large for the 2134, but 
>not out of specs.  There is probably also some leakage from my grungy old 
>whiteboard.
>
>Capacitive coupling is also an issue with MOSFET switches and you can see 
>some ringing on the waveform.  Most of that can be filtered out in 
>combination with whatever amplifier is used to raise the signal up to the 5 
>or 10 V level.
>
>This VCO tracks quite well.  With correct tracking set at the octaves 
>starting at 200 Hz and 5 kHz, tracking is within 0.2% up to ~16 kHz.  Above 
>that it goes a bit sharp.  (If the comparitor is replaced with an LM311 the 
>high end goes flat, so it must be opamp phase shifts that make it go sharp.)
>
>Anyway, don't be afraid to try MOSFETs!
>
>   Ian
>

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-- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
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