# Bistable Multivibrator Design and Simulation

Multivibrator circuits are critical timers, oscillators, and pulse sensors.

Think back to your electronics classes in high school or college; you probably built a multivibrator circuit from an op-amp or discrete transistors. These circuits are often viewed as simple educational tools, but they have some useful applications as moderate frequency and fast electronic pulse detectors.

If you want to build a fast pulse detection circuit, a 555 timer configured as a bistable multivibrator won’t do the trick. Instead, you might consider building a bistable multivibrator from discrete components. Once you’ve built your circuit, there are some simple simulations you can perform to verify its functionality.

## What is a Bistable Multivibrator?

There are three types of multivibrator circuits: monostable, astable, and bistable. The bistable multivibrator operates like a pulse detector. When an electronic pulse is input to the circuit, the output will switch between two possible states (a high and low output voltage). This switching behavior does not trigger a free-running oscillation, in contrast to an astable multivibrator. 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.

The transistors used in these circuits will limit the switching time that will be seen on the output pulse. 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. 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. However, if you need highly accurate triggering with sub-μs precision, a bistable multivibrator is a simple, low-cost option.

### Symmetric and Asymmetric Triggering

The construction of a bistable multivibrator circuit depends on how the circuit is triggered. With symmetric triggering, a single input pulse is used to provide regenerative feedback in the circuit. This type of circuit is shown below.

Synchronous triggering with a bistable multivibrator.

The difference between these two circuits lies in the use of two input pulses (asymmetric) vs. one input pulse (symmetric). With asymmetric triggering, two input pulses are required for triggering, which can be configured to provide two outputs in opposite states. This type of circuit is effectively a Schmitt trigger.

Asynchronous triggering with a bistable multivibrator.

## Faster Trigger Speeds and Oscillators

Perhaps the 555 timer is the most popular IC for building 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. 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.

The 555 timer can be run as a bistable multivibrator

### Multivibrator Circuits from Discrete Components

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. Newer 555’s can reach low MHz frequencies, but the high parasitic capacitances in these chips sets a lower limit on the oscillation period. Getting beyond 100 kHz free-running oscillator and below 1 ns rise time takes much faster components. Similarly, for a bistable multivibrator, working with discrete components provides an application for fast electronic pulse detection with binary output.

By using discrete CMOS transistors, you can get the astable free-running oscillation to ~50 MHz with ~800 ps rise time. Just for comparison, the older timer circuits used for these applications have transition times ranging from 10 to 100 ns. 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.

### Op-Amp Multivibrator Circuits

The other option for building a bistable multivibrator with an op-amp, some resistors, and a capacitor. 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. 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. Once triggered, the output will switch between the positive and negative saturation voltage values. This type of circuit is basically an integrator, but with a defined threshold.

Op-amp circuit as a bistable multivibrator with symmetric triggering.

Getting beyond the MHz regime and into the GHz regime with either approach requires integration. Placing everything into a single chip provides a number benefits for switching integrity and signal integrity. The use of multivibrator circuit architectures as oscillator and triggered switch elements with GHz bandwidth has been an active area of research for the past few decades. These high bandwidth circuits are normally built on GaAs, although GaN may provide even higher switching rates/oscillation frequencies.

## Suppressing Relaxation Oscillations on the Output

One point that is not often discussed with triggered oscillator circuits, whether they incorporate discrete transistors or amplifiers, is relaxation oscillations (i.e., underdamped oscillations that appear superimposed on the stable output voltage level). These oscillations arise in a multivibrator when the RC time constant becomes too small. In addition, at very high frequencies with discrete components, unintended feedback can occur in a PCB due to parasitic capacitance between the output/input ports of the circuit and the nearest reference plane. This unintended feedback can drive one of these circuits to exhibit relaxation oscillations during switching.

When simulating one of these circuits, you’ll need to use a pulsed voltage source on the input.  You’ll then want to measure the output and perform a transient analysis. This will help you identify the switching time. You can also sweep the input pulse peak level to determine the triggering threshold. Make sure to use verified component models in your simulations.

If you plan to build a bistable multivibrator circuit from discrete components or an integrated circuit, you’ll need to use the best PCB design and analysis software. The design and simulation tools in PSpice Simulator for Allegro and the full suite of analysis tools from Cadence are ideal for building and simulating your bistable multivibrator circuit on its own or as part of a larger system.