Multivibrators and their Types – Everything You Need To Know

As circuits got more and more complicated over the years, it became impossible to have advanced applications without multiple states. Just think about it. What if a certain device of yours had only one state and no settings? This was the sort of problem faced earlier, when circuits were growing in complexity, and required components that could facilitate more efficient operation with multiple outputs. In modern terms, what if your microwave oven could only heat for say 30 seconds, nothing more, nothing less? They’re not directly related of course, but it’s just something we’re used to, electronic devices with multiple functions and outputs, multiple states, so to speak. So a multivibrator can be defined as an electronic circuit that is used to implement two state systems like oscillators, timers and flip-flops. Here’s all you need to know about them and their types!

History of Multivibrators

Back in 1919, Henri Abraham and Eugene Bloch described the first multivibrator circuit, also called a plate-coupled multivibrator. Made from vacuum tubes and introduced in 1920, it was christened the ‘Abraham-Bloch multivibrator oscillator’. It was simple yet progressive, and used harmonics to calibrate a wavemeter in the center. The whole setup was rather large because of the use of vacuum tubes, and gave way to more popular circuits. In fact, the Eccles-Jordan trigger is often said to be the original multivibrator circuit, when in reality, it was derived from the Abraham-Bloch multivibrator oscillator. It is, however, considered to be the first electronic flip-flop, and is named after William Eccles and F.W. Jordan.

The Eccles-Jordan trigger also made use of vacuum tubes, and was used in early computers, even in transistorized form. Basically, the fundamental flip-flop circuit introduced here was in use for a long time after, hence the popularity. Flip-flop types as we know them, were first described in 1954 by Montgomery Phister, in a course on computer design at the University of California, Los Angeles (UCLA). The Eccles-Jordan trigger was the first bistable multivibrator, a type that we will describe below. Over the years, the terminologies for multivibrators and flip-flops became more defined, and the terms as we know it were formed. Earlier, even monostable multivbrators were sometimes called flip-flops, which we now use only to refer to bistable multivibrators.

Types of Multivibrators

The name ‘multivibrator‘, originally came from the original oscillator version of the circuit, because of the harmonics seen in the output waveform. It became the generalized term for the circuit, and over the years more specific types of the circuit arose, which are:

Astable Multivibrator

A multivibrator circuit in which there is no single stable state is known as an astable multivibrator, as the name suggests. Basically the circuit keeps switching from one state to another, and while that theoretically sounds undesirable, there are actually many practical applications for it, namely, relaxation oscillators. It consists of two amplifying stages connected by coupling networks, with a positive feedback loop. The actual amplifying elements used range from vacuum tubes to op-amps and many more, including field effect transistors and bipolar junction transistors.

The circuit keeps switching from one state to another because of the positive feedback, and has only one amplifying element running at a time. The charge cycle is much faster compared to the discharge cycle, and is facilitated by the coupling capacitors, since voltage change in a capacitor cannot be instantaneous.  Just like other circuits, it requires some time to begin conducting, which depends on the amplifying elements used.

Applications

• Used in applications where low clock frequency clock pulse train is required
• Relaxation oscillators, which are parts of vehicle indicator lights, disco strobe lights, early oscilloscopes and television receivers
• Timing signals

Monostable Multivibrator

A monostable multivibrator, as the name suggests, is a circuit with a single stable state. Conceptually and practically, it can be termed as a half astable multivibrator. While an astable multivibrator uses two resistive-capacitive networks, a monostable multivibrator uses one resistive-capacitive and one simple resistive network, hence it being called a half astable multivibrator. It outputs a perfect square waveform because of the absence of a loaded capacitor, and is used in precise applications.

The circuit, when triggered by an input pulse, switches to the unstable state for some time, then returns to the stable state. Depending on the application, the circuit can be tweaked to stay in the unstable state for as long as required, even sometimes requiring multiple input pulses to keep it in the unstable state. This is also called a retriggerable monostable multivibrator. Consequently, if the trigger pulses don’t affect the time spent in the unstable state, it is called a non-retriggerable monostable multivibrator.

Applications:

• Used as pulse generators
• Used to produce time delay in circuits
• Used to regenerate pulses in old telecommunication systems
• Used to reduce pulse distortion in computer systems
• Used as gated circuits

Bistable Multivibrator

A bistable multivibrator is a circuit that has two stable states. Instead of using two resistive-capacitive networks like an astable multivibrator, a bistable multivibrator uses two resistive networks only. Therefore it uses only direct or resistive coupling, and the latch type has no charge or discharge time because of the lack of any capacitors. Switching between the states can be done by two terminals called set and reset. Bistable multivibrators are among the most important components of digital computer systems, because they’re used to store data. It is also the basic storage element in sequential logic, and can be clocked or unclocked. Just like other types of multivibrators, bistable multivibrators also use multiple control elements, such as vacuum tubes and bipolar junction transistors. In complex circuits, multiple bistable multivibrators are cascaded as well.

As flip-flops, it can be divided into a few common types:

1. Set-Reset Latch: An electronic device that requires no control input, and depends entirely on the state of the S and R signals. It is a simple circuit made from both NOR and NAND gates. As in, there are SR NOR latches, as well as SR NAND latches.
2. Gated Latches: Similar to set-reset latches, these electronic devices often have an extra pair of NAND/NOR gates. So it requires an extra input before the set-rest inputs can be activated. This extra input is also termed as the ‘Enable’ input, since it enables the usage of the set and reset inputs.
3. D Flip-Flop: Also known as the ‘Data’ or ‘Delay’ flip-flop, this widely used electronic device finds application in memory, delay lines or a zero cell hold. Practically, it’s the basis for shift registers, a highly important part of many electronic devices. It basically holds the current value of D input.
4. T Flip-Flop: T Flip-Flops are also known as toggle flip flops, and change their output on each clock edge, which is usually half the frequency of the signal to the T input. They are used extensively in frequency dividers, binary counters and binary addition devices.
5. JK Flip-Flop: Known as the universal flip-flop, JK flip-flops are similar to SR NAND latches, except that there is no forbidden input, even when both set and reset are 1. It is basically a gated SR flip-flop with an additional input clock circuit for extra stability. There are four possible outputs in this case, namely logic 1, logic 0, toggle and no change.

Bistable multivibrators, particularly flip-flops, are subject to something called metastability, which happens when both inputs change at the same time. It can cause unforeseen problems like extra time to settle into a state, unintended oscillations and the like. In digital computer systems, this can lead to data corruption and crashes because of the inconsistent state change. This is fixed by setting proper time constraints and cascading multiple flip-flops.

Applications:

• Used in counting circuits
• Used in memory storage units
• Used as frequency dividers
• Used in pulse generation circuits

Conclusion

Thus, multivibrators are very important electronic devices, used in a wide range of applications. As seen above, they’re more of the basic building blocks of electronics and computer systems, and began from simpler vacuum tubes. While earlier they found use in television sets, frequency dividers and the like for synchronizing purposes, evolved versions of them are now used in complex computer systems and form the very basic forms of memory. On the other hand, they’re also used in simple turn signal circuits seen in vehicles around the world, which is testament to their versatility. Our basic notion of multiple operation, types and states stems from these devices, without which we’d probably be stuck with much simpler devices. It’s quite interesting to see how multivibrators give way to devices like oscillators and flip-flops, which in turn turn into something more complex, so on and so forth. Basic components like these are what make much more complex systems possible, and whether you’re a budding engineer or inventor, you’ll still have to familiarize yourself with these components to get anywhere. After all, we all have to start somewhere, and for understanding electronics, this is one of the places you can begin! Do let us know if you’d like to know more about these topics and feel free to ask us anything you didn’t understand via the comments below!