I'm having problems with these gaps in between the sawtooth waveforms produced by a free-running bootstrap ramp generator triggered by a schmitt trigger. Why are they occurring and how could I fix this?

At first I thought it was because of the pnp transistor, but now I'm not so sure.

I tried to redesign my circuit multiple times, but I still get the same problem. What could I be doing wrong? Or where should I look?

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This is the circuit producing that: enter image description here

  • \$\begingroup\$ I recommend you read this post: electronics.stackexchange.com/questions/28251/… and "normalize" your schematic to more conventional form. While it may not be immediately apparent, schematics are a visual language, and standard-ish form helps greatly in making your work "readable" by experienced electrical engineers. It also may help you reason more clearly about your circuit yourself :) \$\endgroup\$
    – scanny
    Commented Oct 28, 2015 at 4:33
  • \$\begingroup\$ You have (onbiously enough) and undesired delay between the end of the reset action and start of ramp up. Things like the diode D1 driving R4/R2 MAY be involved. As R1-R2 are not driven low the must "float low". I haven't followed that through in detail but things like that need accountingg for. (Quick mental check suggests it is the wrong polarity to matter but maybe not). \$\endgroup\$
    – Russell McMahon
    Commented Oct 28, 2015 at 4:54

1 Answer 1


Look at the BJT that discharges the cap .I generaly used small mosfets like BS170 and BST72 when I did circuits like yours.There are now many more small mostets to choose from compared to 1988.If you still want to use the BJT then you will have to do something about its turnoff.

  • \$\begingroup\$ Thanks! I consulted with my prof and she said the same thing. We aren't allowed to use mosfets though, so I'm currently trying to figure out what causes that problem with its switching. \$\endgroup\$
    – Val Croft
    Commented Oct 30, 2015 at 9:18

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