square wave oscillator using transistors

dl324

Joined Mar 30, 2015
16,988
I breadboarded the circuit using the 6 MOSFETs in a CD4007. It's oscillating at around 20 30 Hz with a 1M resistor and 0.1uF cap. I doubled the cap and the frequency dropped to about 10 7 Hz.
I did some calculations for the Schmitt inverter in post #12. Something wasn't right about the frequency not scaling by a factor of 1/2 when capacitance was doubled.

I measured Vt+ and Vt- and did the calculations to determine the expected frequency. For 1M ohm and 0.1uF, that would be:

upload_2019-7-21_9-27-4.png

The axial ceramic cap I used still had some tape residue on the leads. Once I cleaned the leads, I started getting the expected results.
 

Thread Starter

Dut Chen

Joined May 19, 2017
27
I've done some more testing and i think the noise on the falling edge is due to the diodes reverse recovery time. I'm using 1n4007 diodes which have a typical recovery time of 30us and that does match what I'm seeing on the scope.
Unfortunately i don't have any other types of diodes, is there anything i can do to resolve the issue without having to buy new ones?
 

iimagine

Joined Dec 20, 2010
511
if you have a spare transistor, you can use it as a diode, just for testing.
The circuit still function without it, so you can remove it and check
 
Last edited:

Thread Starter

Dut Chen

Joined May 19, 2017
27
Using a BC547 transistor instead of a diode reduces the noise to 5us (which i assume is the recovery time of such a transistor).
Removing the diode entirely causes the capacitor to discharge through the output, which is why the diode is there in the first place.
 

iimagine

Joined Dec 20, 2010
511
Removing the diode entirely causes the capacitor to discharge through the output, which is why the diode is there in the first place.
You were suspecting that its causing it and you didnt have another diode, so its one quick solution to check and see if that was the case...
 

iimagine

Joined Dec 20, 2010
511
Getting rid of the 1k power consumption:
DeleteMe.PNG
Replace R6 with a pot to adjust discharging duty cycle
Note: Will not oscillate without a load (R5)
 
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