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Electronics Circuits Reference Archive Multivibrators, relaxation oscillators and their allies. No – not at all: What exactly is a multivibrator? I suppose one definition would be ‘a circuit which has several states’. This will do for now, it’s quite loose so astablw plenty to the imagination!

Conventional multivibrators have only two stages and come in three flavours: And bistable, where the circuit can be flipped from state one to state two. The fact that it can be ‘flopped’ back again leads coura another term ‘flip-flop’ – but you can call an astable an astable flip-flop!

Meaning of “multivibrateur” in the French dictionary

Then you can get tristables – and one circuit I shall introduce is a ‘donkey simulator’ where 3 transistors are combined to make 3 separate astable mulivibrators! And what is a relaxation oscillator?

A circuit which changes ‘slowly’ from one level to another at which it resets to the first state and starts changing again.

The two descriptions may seem totally different – but are in fact the same thing – or at couurs there is a distinctly grey area where they overlap, as I will show. There are also a lot of other oscillator circuits which aren’t quite conventional multivibs or relaxation oscillators.

The ‘common’ Astable multivibrator. A circuit which I find pretty boring but, if I don’t introduce it first, how am I to elaborate on all its more interesting relatives?

Operation Assume that the Tr1 is turned hard on if you’ve indexed this site by ‘hard on’ sorry – it’s not for you! Assume C1 is not charged: At a certain point there will be enough voltage on the right end of C1 to cause Tr2 to start conducting.

As Tr2 starts to turn on, Tr2’s collector will start to fall and will pull down the right hand end of C2.

Since C2 is a capacitor, the voltage across it cannot suddenly change so the left hand end of C2 will also start to fall. This will rob current from Tr1 which will start to turn off and its collector voltage will start to rise. As it does so, C1 will feed this rising voltage into Tr2’s base, helping it to turn on – so the circuit will ‘collapse’ into a state where Tr2 is fully on and Tr1 is fully off. As Tr2 quickly switches from off to on, its collector falls from Vcc to 0v and by capacitor action the base of Tr1 will be reversed biased from about 0.

Now R3 starts to charge C2’s left end positively from near -Vcc below the 0v line towards 0v and past 0v towards Vcc.

MULTIVIBRATEUR – Definition and synonyms of multivibrateur in the French dictionary

Remember that Tr2 is turned on, so the right end of C2 is clamped to 0v via Tr2. So the circuit switches between two states, Tr1 on Tr2 off and Tr2 on Tr1 off. The time in each state is determined by R1C1 and R3C2 and is 0. So the circuit gives a The values shown will give a frequency around 1kHz 0. I say ‘fairly’ reliable because there are a couple of points Imagine that Tr1 and Tr2 are both conducting and both capacitors are ‘discharged’: R1’s current flows into Tr2 and R3’s current flows into Tr1.


The capacitors are ‘discharged’ so no current is flowing in them: This condition happens and is probably the biggest single problem with the common multivibrator – and with several of its relatives. The ‘astable multivibrator’ to give this circuit its ‘proper’ name was invented in the days of valves and was common with early germanium transistors. These has a gain of maybe 20 gain is the ratio of collector current to base current: Then silicon transistors came out: This ‘failure to oscillate’ became common: R2 ofwhich would suit a transistor having a gain of at least I would not use such a simple multivib in a critical part of a commercial circuit today, because it can fail to start.

But it remains an interesting circuit and a good learning exercise! As I have said, the period of the multivib is 0. But this only applies for low voltages.

When one transistor turns on, the base of the other transistor gets pulled very hard negative well below the 0v line and the base is turned off. But transistor base junctions cannot stand a lot of reverse voltage: If you try and put a larger reverse voltage on the base – the transistor base emitter junction conducts like cuors zener diode.

This interferes with the formula – and you get a shorter time than you expect. The second problem with the simple circuit, above, is that its edges aren’t very sharp. Just after Tr1 turns off, full Vcc is present across C1 so there is a small voltage drop across R2 from this: The second circuit shows one way astble this.

Note that the values in one half are 10 times those in the other half: The trade-off is that Tr2’s collector waveform is made much worse by the larger C2 reacting with the increased R4.

The asymmetrical circuit also is more reliable: Astsble multivib in disguise. If you want to draw a multivibrator so it confuses an engineer, try this circuit.

If you compare it with the basic multivib – it is actually the same.

But it is drawn like a two stage a. It doesn’t look symmetrical, doesn’t appear to have two identical halves. The way you draw a circuit is important. A badly drawn circuit can hide the circuit’s function and confuse the reader!

An identical but complementary circuit results. This is one of the nice things you can do with transistors – but not with ICs.

Virtually all ICs have a negative earth and do not work in a complementary version. The reasons for this are mainly historical – the early silicon transistor fabrication made better NPN transistors than PNP. Although modern fabrication makes both, there are very small intrinsic performance differences due to differences in electron and hole flow and NPN is therefore the more common model.


4QD-TEC: Multivibrators and Relaxation Oscillator Circuits

But in most two transistor multivib circuits it is possible to replace one of the transistors asrable its complement i. The operation is changed, and sometimes some head scratching is required to see the result! Nevertheless it remains a fascinating and instructive exercise! The diagram above shows the standard multivib with one stage made into its complement.

The circuit may look incredibly similar – but the operation is dramatically changed! Instead of the transistors alternating now they both switch on together. The circuit, with an almost trivial change, has stops being a multivibrator and has become a relaxation oscillator! Which should explain why I do not consider these to be two to be distinct types of circuit! Names are language aatable Since both transistors conduct simultaneously, the on period is a short pulse followed by a long recovery symptomatic of a relaxation oscillator.

To increase the on time, resistors can be put in series with the capacitors as is done in the circuit below. Yes – this is a slightly different circuit to the courx Since the collector resistors are connected one each to 0v and to Vcc, why not use a common collector resistor? Indeed – why not also a common base resistor. Both varieties oscillate – so too does the circuit below. We have derived this, by a few transformations, from astabl common multivib. Yet this is a series connected multivib!

Another language thing, the changes in the real world are trivial! Sstable lead us into a whole new class of multivibrators – where the elements are connected in series rather than in parallel as the first example. But for a lot of simple circuits, consider the supply to be a two lead component a battery.

Circuit elements in series can be re-arranged, their order is not important. So let’s take our first two complementary multivibs and re-arrange them a bit A nice, well behaved oscillator couts. The diode isn’t really needed but it does improve the waveform available on the emitters so that a good sawtooth is available. The pulse output may astabe taken from asgable collector. Note also that both halves of this circuit are emitter followers and an emitter follower has a voltage gain of less than 1.

Have you been taught astale you need voltage gain to make an oscillator? It’s not true, current gain alone is enough! Here’s another very simple circuit which oscillates well, even if it gives a lousy waveform! Warning This page is going to be long: I have a lot of this type of circuit, there is a lot to say. I’m not finished yet but I guess you’d prefer half a page now than a whole page later, so here are a few more circuits in no particular order that I’m about to corus up There are many more I have not yet transferred to the computer!