Astable multivibrator using CMOS gates - mathematical analysis


Joined Feb 17, 2009
I take some real world measurement. I build this gen using CD4069 supply from 5V PSU and R1 = 100kΩ; R2 = 10kΩ ; C1 = 100nF.
And the results are
The Output voltage and V4 voltage (at R1,R2,C joint)


As we can see the Duty-cycle is not equal to 50% because the gate threshold voltage is not equal VDD/2 instead we have a threshold voltage around 3V also the "positive" and "negative" threshold voltage ar not equal (3.08V and 2.9V).
This is why the output frequency is out from ideal case Fo = 1/( 2*R2C*ln(3) ) = 1/( 2*10kΩ100nF*ln(3) ) = 455Hz but this do not include component tolernce.

And the V1 voltage, at the first gate input when we can see the protection diodes in action.


If I replace R1 resistor with a short circuit (R1 = 0Ω) the situation changes.


With R1 = 0Ω the output frequency goes up (435Hz) and some strange things happen with diode clamper.

Thread Starter


Joined Aug 7, 2012
ps. I use simulation software Multisim for analog electronics but I have problems using it for digital electronics. If someone here uses Multisim, please post here simulation of this circuit (or original one).