Multivibrator
by
L. Padilla
This document describes the recipe of an extremely simple, inexpensive
and easy-to-make autoexcited multivibrator or square wave oscillator. This
design is not of my own, you can find it in any electronics book. To make
this design you will only need 2 general purpose NPN silicon transistors, 2
capacitors and 4 resistors. The circuit is as follows:
+----------------------+
| |
| Rc1 | C Q1 E |
+-----| |---+---\/\/\/---+---\___/>-----+ __|__ Cb2
__|__ | | B| __|__ _____
___ Vcc | | ___ |
_ | | _ |
| Rb1 | |
+---\/\/\/---------+---------------===--+---o Output
| | |
| Rb2 | |
+---\/\/\/---------+----------------+ |
| | |
| Rc2 B_|_ |
+---\/\/\/---+---/ \>-----+ __|__ Cb1
| C Q2 E __|__ _____
| ___ |
| _ |
| |
+--------------------------+
The most usual case is to produce a symmetric square wave, in that case
Rb1 = Rb2 = Rb, Rc1 = Rc2 = Rc and Cb1 = Cb2 = Cb. The period of the
oscillation is approximately T(seconds) = 1.6 * Rb(Ohms) * Cb(Farads). The
frequency is then f(Hz) = 1/T(s). To obtain steep rising edges it must be
provided that Rc <= 10 * Rb.
The exact value of the different components depends on the application
for which the square wave function is required. For example, if you are
going to work with TTL circuits, choose Vcc = 5 Volts. Note that this will
provide a signal varying between 0 and 5 Volts, i.e., it is not pure AC
current, it has a DC component. By adding a capacitor in series with the
output you will obtain a pure AC signal which varies between -2.5 and 2.5
Volts.
For best performance the value of Rc should be as low as possible. This
has two advantages: better rising edges (steeper) and less influence of the
external load connected to the output. The lower limit is placed by the
maximum current you can allow, because either you want to save current
(preferred if powered with batteries) or you don't want to burn the
transistors. A value of Rc between 100 and 1000 Ohms should be well suited
for most applications.
The maximum current through the transistors is approximately Imax =
Vcc/Rc, which, with the values I mentioned, will be around 10 mA. The power
dissipation is approximately P = Vcc * Imax, in our case around 50 mW.
The value of Cb and Rb depends on the exact frequency you need, however
it is convenient a value of Cb not too low to avoid the influence of
dispersed capacitances. I would suggest Cb > 100 pF. If you fix Cb then
you have to choose Rb to match the frequency you need according to the
formula above. You could use a variable resistor and tune it to select
different frequencies. However it has the inconvenient that you have to
tune two resistors.
With the values I mentioned and using normal components I have
generated frequencies up to 1 MHz successfully, thought the transistors
have to be suited for those high frequencies. As you can see this circuit
gives you a wide range of frequencies with great stability. For higher
frequencies or higher stability you should use more complex circuits
including quartz crystals.
E-mail: padilla at
domain "gae ucm es" (my PGP/GPG public key)
First version: 17-Jun-1997, last update: 12-Dec-2002
This link: http://www.gae.ucm.es/~padilla/extrawork/multivibrator.html
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