PP logo

 

Schmitt trigger astable multivibrator.

Contents


Introduction.

This is some basic calculations for Schmitt trigger astable multivibrators.
In addition there is a list of threshold voltages for some CMOS gates.


Schmitt trigger astable multivibrator.

Astable multivibrators with logic gates and waveforms.
Fig.1: Astable multivibrators with logic gates and waveforms.

The waveform shown above has exactly 50% duty-cycle ( it is a calculated example with ideal components ).
If you need 50% duty-cycle, you should use a divide-by-two circuit after the multivibrator.

Oscillation period:
tp=RC*A

where:
A=ln(((Vh-Vol)/(Vl-Vol))*((Voh-Vl)/(Voh-Vh)))

For a CMOS gate where Voh≈VDD and Vol≈0:
A=ln(((Vh)/(Vl))*((Voh-Vl)/(Voh-Vh)))

Other calculations:
f=1/(RC*A)
RC=1/(A*f)
RC=tp/A

There is no need to go to extremes and select a very high value for R in order to save power.
This circuit bias the gate's input stage in the linear region and supply current is much higher than many will expect from a CMOS gate.
The plot below shows the supply current vs. input voltage for one CD40106 gate at 9 V supply.
More modern and faster gates will draw much more current than this.
A 74LVC14 gate draws, as far as I remember, around 5 mA when it is used in this way.

CD40106 supply current vs. input voltage for one gate at 9V supply voltage. Unused gates' inputs on GND.
Fig.2: CD40106 supply current vs. input voltage for one gate at 9 V supply voltage. Unused gates' inputs on GND.

Astable multivibrators with OP-AMP / comparator.
Fig.3: Astable multivibrators with OP-AMP / comparator.

Fig.3a is an OP-AMP or comparator based multivibrator for use with dual supplies.
Vl=Vol*R12/(R11+R12)
Vh=Voh*R12/(R11+R12)

Fig.3b is an OP-AMP or comparator based multivibrator for use with single supplies.
Vl=(R21*R23*VDD+R22*R23*Vol)/(R21*R22+R21*R23+R22*R23)
Vh=(R21*R23*VDD+R22*R23*Voh)/(R21*R22+R21*R23+R22*R23)

Fig.3c is an open-collector comparator based multivibrator for use with single supplies.
If R34 << R and R31, calculation is like fig.3b. If not, calculations becomes somewhat complex as the time-constant differs in the 2 states.
Use a simulator.


Downloads.

Calculations and a list of Vl and Vh for different gates.


Copyright and disclaimer.

Copyright Notice.
This web-page, including but not limited to all text, drawings and photos, is the intellectual property of Poul Petersen, and is Copyright ©.
Reproduction or re-publication by any means whatsoever is strictly prohibited under International Copyright laws.
The author grants the reader the right to use this information for personal use only.
Any commercial use is prohibited without express written authorization from Poul Petersen.

The information is provided on an "as-is" basis and is believed to be correct, however any use of the information is your own responsibility.

This web-site may contain links to web-sites outside Poul Petersen domain ( www.poulpetersen.dk ).
Poul Petersen has no control over and assumes no responsibility for the content of any web-site outside Poul Petersen own domain.

Poul Petersen does not use cookies to "enhance your experience" on this website.


Poul Petersen, C/Faya 14, 35120 Arguineguín, Las Palmas, Spain.
E-mail: pp@poulpetersen.dk

Poul Petersen notes index

Poul Petersen diy index


Copyright © Poul Petersen 2020. Last update: 20200211. Valid HTML!