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
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