# RC circuit, Multivibrator.

Cuthbert Nyack
The RC circuit is used for timing applications where the time for the capacitor to charge from one voltage to the next is used as the basic timing interval.
Applications include all kinds of multivibrators and triggering circuits. The timing interval can be controlled by varying the value of one of the components, usually R. The astable multivibrator circuit using an opamp shown above is one example of this application. The circuit is used to produce a square wave whose period is twice the time for the capacitor to charge from -bVo to +bVo and is controllable by the RC time constant and the feedback ratio
b = R1/(R1 + R2). The period Tp is given by
Tp = 2 T ln(1 + b)/(1 - b)).
The applet below shows the voltages at the 2 inputs and output of the opamp as the time constant T = RC of the circuit is changed. The numerical value of the voltages at any time can be seen by changing B/L.
For the parameters (2.4, 10.0, 0.9, na, na, na, na, (208:738), 2.0, 1.0) The period is 14.13s while from the expression it is 14.13s.

When applet is enabled its appearance is illustrated by the gif image below. 