By Robert J. Widlar
The free-running multivibrator shown in Figure 1 is an excellent example of an application where one does not normally consider using an operational amplifier. However, this circuit operates at low frequencies with relatively small capacitors because it can use a longer portion of the capacitor time constant since the threshold point of the operational amplifier is well determined. In addition, it has a completely-symmetrical output waveform along with a buffered output, although the symmetry can be varied by returning R2 to some voltage other than ground.
Figure 1. Free-Running Multivibrator
*Chosen for oscillation at 100 Hz.
Another advantage of the circuit is that it will always self start and cannot hang up since there is more dc negative feedback than positive feedback. This can be a problem with many “textbook” multivibrators.
Since the operational amplifier is used open loop, the usual frequency compensation components are not required since they will only slow it down. But even without the 30 pF capacitor, the LM101 does have speed limitations which restrict the use of this circuit to frequencies below about 2 kHz.
The large input voltage range of the LM101 (both differential and single ended) permits large voltage swings on the input so that several time constants of the timing capacitor, C1, can be used. With most other amplifiers, R2 must be reduced to keep from exceeding these ratings, which requires that C1 be increased. Nonetheless, even when large values are needed for C1, smaller polarized capacitors may be used by returning them to the positive supply voltage instead of ground.
1580 08 December 2007