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Multivibrators,
relaxation oscillators and their allies.

Multivibrators: an old, well known and boring subject. No - not at all: there
are lots of different ways of looking a them and they are not at all what they
may seem.

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 leaves
plenty to the imagination! Conventional multivibrators have only two stages and
come in three flavours: astable (where neither state is stable and the circuit
oscillates between the two states); monostable (where one state is stable, the
other transient). And bistable, where the circuit can be flipped from state one
to state two. The fact that it can be 'flopped' back again leads to another term
'flip-flop' - but you can call an astable an astable flip-flop! 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 least 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..

1: 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!) and Tr2 is not conducting, so
the right end of C2 is at Vcc: current is flowing into the base of Tr1 via R3
causing it to conduct, so the left end of C1 is at 0v.

Assume C1 is not charged: its left end is clamped to 0v by Tr1 but its right
end is being fed from Vcc via R1, so it will start to charge up through R1, its
right end moving positive towards Vcc. 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
0.6v to (Vcc-0.6v) below earth as the left end of C2 follows the right end
downwards.

Now R3 starts to charge C2's left end positively from near -Vcc (below the 0v
line) towards 0v and past 0v towards Vcc. Remember that Tr2 is turned on, so the
right end of C2 is clamped to 0v (via Tr2).

As the left hand end of C2 gets to around +0.6v, Tr1 starts to conduct and
the sequence repeats.

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 0.69CR (but
read on). So the circuit gives a 50:50 mark space and is a 'fairly' reliable and
predictable oscillator. The values shown will give a frequency around 1kHz (0.5
mSec on, 0.5mS off).

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: the circuit is in a stable state
and is not oscillating. 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: feed 1 microamp into the base of a transistor and, with a gain of 20,
you will get 20 microamps collector current). With these typically R2 and R4
might be 1K, R1 and R3 10K. Then silicon transistors came out: these had higher
gains. This 'failure to oscillate' became common: silicons had more gain and the
circuit oscillates more reliably if the transistors are not turned too hard on:
I've chosen R1:R2 of 100, which would suit a transistor having a gain of at
least 100.

Nevertheless: 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 0.69CR. 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: modern
ones seem to be around 10v. If you try and put a larger reverse voltage on the
base - the transistor base emitter junction conducts like a zener diode. This
interferes with the formula - and you get a shorter time than you expect.

2: Asymmetrical multivibrator.

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 charging of C2 tends to round the
edges and make them less than ideally sharp.

The second circuit shows one way round this. Note that the values in one half
are 10 times those in the other half: the times of both halves remain the same
but the 'rounding' effect on Tr1 is reduced by the smaller C1. 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: since the two halves are not
equal the chance of them both switching on together is reduced.

3: A 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.c. coupled amplifier, with overall a.c.
feedback. 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!

4: Complementary Multivibs.

An obvious fact: for all transistor circuits there exists a complement,
replace all PNP transistors by NPN and vice versa, reverse all diodes and other
2 terminal polarised components. 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.

But in most two transistor multivib circuits it is possible to replace one of
the transistors by its complement (i.e. turn half the circuit 'upside-down' and
the circuit still works. 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 things: the real world is a continuum and
doesn't neatly fit the pigeonholes of language!

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 above: one of the many
circuits I've collected over the years. Source unknown.

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 a common multivib. Yet
this is a series connected multivib! Another language thing, the changes in the
real world are trivial! It lead us into a whole new class of multivibrators -
where the elements are connected in series rather than in parallel as the first
example. And of course, you can do the same PNP - NPN transformations as for the
conventional circuit.

5: Series Multivibs.

The above S.M.V. had was common emitter (the emitters were connected to the
supply lines). 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 circuit. The diode isn't really needed but it
does improve the waveform available on the emitters so that a good sawtooth is
available. The values shown give a pulse of about 30µSec every 37mSec. The pulse
output may be taken from either 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 that
you need voltage gain to make an oscillator? It's not true, current gain alone
is enough!

6: Emitter Coupled Multivibs.

Here's another very simple circuit which oscillates well, even if it gives a
lousy waveform!

More Multivibs.

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 write up... There are many more I have not
yet transferred to the computer!

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Page's Author: Richard Torrens

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