MOSFET overdrive…

a forced contract job for Ryan of Falcon Scott. he’s been having some issues finding an overdrive suitable to his hollow-body guitar. i just got a lot of BS170s and thought this might do the trick. the distortion is pretty squared off on the oscilloscope and crunchy. the left pot is post volume while the right biases the BS170. overall, it sounds pretty good and that’s saying a lot coming from someone who isn’t much into “distortion” (personal bias does make a difference. hah).

a big thanks to Jack Orman at for laying out the basis. i modified his schematic a bit to get the “best” results for my ear, but i couldn’t have done it without his work.


a good rule of thumb pulled from this forum thread (

  1. Need a switch to be fully-on fully-off and carry lots of current -MOSFET
  2. Need a switch that needs to have lowish capacitance – BIPOLAR
  3. Need a cheap, dirty 2 or 3 component current source – BIPOLAR
  4. Need a low voltage/noise amplifier – BIPOLAR
  5. Need an amplifier with VERY low input/bias current – MOSFET
  6. Need a low noise AND low input current amplifier – JFET
  7. Need a one component current source – JFET
  8. Need switch or amp that must cost almost nothing – BIPOLAR
  9. Need multiple transistor package that has matching – BIPOLOAR
  10. Need switch that may be over-voltaged – MOSFET
  11. need switch/amp that sits in nasty RF environment – MOSFET (BIPOLARS rectify & cause offsets)

MOSFETs can generally switch faster (they certainly require less complex and less power to drive their gates). But if I’m not mistaken, BJTs designed for the task can switch very very fast since they have no gate capacitance to charge and can also operate in quasi-saturation mode for even faster switching at the expense of conduction efficiency. MOSFETs have less losses when used as a switch at “lower” voltages (lower as in industry’s definition which is <~200V). MOSFETs act like a resistor when on while BJTs act more like diodes. The resistance can be modified by changing the “dimensions” of the MOSFET while the BJT’s “diode voltage drop” can’t be changed so easily unless the materials are changed. THis tends to make MOSFETs have less losses at the lower voltages but also means MOSFETs can be paralleled since current imbalances will cancel out. With parallel BJTs, the best BJT will hog the current from the other “not so good BJTs” and burn out and the cycle repeats with the remaining BJTs until they are all burned. This is similar to parallel diodes. You can correct for imbalances by manually tuning resistors in series with each BJT, but for power applications that’s needing massive resistors and wasting lots of power.

thanks guys…