In this case, the threshold is equal to the saturation input voltage dropped across R3 in the voltage divider (R2 and R3) forming the feedback loop to the non-inverting input. In this case, the switching trigger forces the op-amp into saturation as long as the peak voltage of the input pulse is above some threshold. The other option for building a bistable multivibrator with an op-amp, some resistors, and a capacitor. For a bistable multivibrator, using CMOS transistors would provide very fast pulse detection that could not be provided by a 555 timer or other slower circuits. Just for comparison, the older timer circuits used for these applications have transition times ranging from 10 to 100 ns. Similarly, for a bistable multivibrator, working with discrete components provides an application for fast electronic pulse detection with binary output.īy using discrete CMOS transistors, you can get the astable free-running oscillation to ~50 MHz with ~800 ps rise time. Getting beyond 100 kHz free-running oscillator and below 1 ns rise time takes much faster components. Newer 555’s can reach low MHz frequencies, but the high parasitic capacitances in these chips sets a lower limit on the oscillation period. The standard 555 timer has a limited free-running astable oscillation of about 100 kHz, thanks to the use of bipolar transistors as the active switching elements. Multivibrator Circuits from Discrete Components The 555 timer can be run as a bistable multivibrator The older TTL 555 timer has been with us for a long time, but newer logic families and transistor manufacturing processes have provided components for building faster multivibrator circuits. They can be set to run in free-running oscillation mode (astable multivibrator) or in triggered mode (bistable multivibrator) by adding a 555 timer IC to some simple passives. Perhaps the 555 timer is the most popular IC for building multivibrator circuits. This type of circuit is effectively a Schmitt trigger.Īsynchronous triggering with a bistable multivibrator. With asymmetric triggering, two input pulses are required for triggering, which can be configured to provide two outputs in opposite states. The difference between these two circuits lies in the use of two input pulses (asymmetric) vs. Synchronous triggering with a bistable multivibrator. With symmetric triggering, a single input pulse is used to provide regenerative feedback in the circuit.
The construction of a bistable multivibrator circuit depends on how the circuit is triggered. However, if you need highly accurate triggering with sub-μs precision, a bistable multivibrator is a simple, low-cost option. Bipolar transistors used in these circuits will normally have a slow rise time on the output compared to other transistor architecture and families, so they are not ideal for highly precise timing applications.
When switched the total circuit has some propagation delay (usually ~5 ns with bipolar transistors), which creates a delay between the input pulse trigger and the time at which the output switches. The transistors used in these circuits will limit the switching time that will be seen on the output pulse. There is some threshold for switching in a bistable multivibrator, which depends on the passives used in the regenerative feedback loop between the two transistors. This switching behavior does not trigger a free-running oscillation, in contrast to an astable multivibrator. When an electronic pulse is input to the circuit, the output will switch between two possible states (a high and low output voltage). The bistable multivibrator operates like a pulse detector. There are three types of multivibrator circuits: monostable, astable, and bistable. Once you’ve built your circuit, there are some simple simulations you can perform to verify its functionality. Instead, you might consider building a bistable multivibrator from discrete components. If you want to build a fast pulse detection circuit, a 555 timer configured as a bistable multivibrator won’t do the trick. These circuits are often viewed as simple educational tools, but they have some useful applications as moderate frequency and fast electronic pulse detectors. Think back to your electronics classes in high school or college you probably built a multivibrator circuit from an op-amp or discrete transistors. Multivibrator circuits are critical timers, oscillators, and pulse sensors.