I have to make the frequency variable of an square waveform generator built with the help of an astable. I modified a circuit a found on the internet and everything seems good with only one exception: I cannot bring the astable to a low frequency without destroying the signal.

Whenever I raise the resistance of R13 (thinking about it as a potentiometer) to make the frequency lower, the signal waveform is getting worse and I can't figure out why.

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I really don't have enough experience (I'm student) to make it work between 20-60 kHz without ruining the signal waveform. I thought if I add a variable resistor before the capacitors I can limit the current so the charging time of it, but I don't know why the signal is destroyed at a certain frequency. Do I have any chance to make it work like this or should I just think about another method? Or what should i modifiy to make it work?

  • \$\begingroup\$ CMOS give much wider ranges due to very low bias current. i.stack.imgur.com/m4HEm.gif \$\endgroup\$ Aug 18, 2018 at 15:39
  • \$\begingroup\$ Adjust here tinyurl.com/yagza4uj Rmin ~100, Rmax=10M \$\endgroup\$ Aug 18, 2018 at 15:50
  • \$\begingroup\$ I forgot to mention i am not allowed to use mos transistors..only bipolar. And also no integrated circuits. \$\endgroup\$
    – Bogdan
    Aug 18, 2018 at 16:02
  • \$\begingroup\$ Well Q3,4,5 are reversed \$\endgroup\$ Aug 18, 2018 at 16:05
  • \$\begingroup\$ I fixed them now, but doesn't seem to change anything..Q1 and Q2 still work properly. \$\endgroup\$
    – Bogdan
    Aug 18, 2018 at 16:26


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