Multivibrators constitute an important class of pulse and switching circuits.
Multivibrators perform a variety of useful functions like:
i. Generation of square waveforms,
ii. Counting,
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iii. Frequency division,
iv. Generation of time-delays, and
v. Storage of binary bit of information.
Types of Multivibrator:
Multivibrators are of the following three types:
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1. Astable Multivibrator (also called Free Running Multivibrator):
It generates a square wave of known period. It does not have any permanent stable states. They have two quasi-stable states (i.e., temporary states).
The circuit changes state continuously from one quasi-stable state to another without any external stimulus or trigger after a predetermined length of time. This predetermined length of time is decided by circuit time constants and parameters. Thus, an astable multivibrator generates continuous square waveform without any external circuit. The output waveform of an astable multivibration.
2. Monostable Multivibrator (Also Known as One-Shot Multivibrator or Univibrator):
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It has one stable state and another quasi-stable state or unstable state. When an external trigger or stimulus is applied, the circuit state is changed abruptly to unstable, i.e., quasi-stable state for a predetermined length of time and returns to stable state automatically. The time duration of unstable state is by the circuit time constants and parameters and is independent of triggering pulse duration.
It is used to generate precise time delays. The waveforms of a monostable multivibrator.
3. Bistable Multivibrator (Also Known as Flip-Flop, Binary and Scaler of Two Circuits):
The word ‘bistable’ means two stable states. Bistable circuit can stay in a particular state indefinitely. For this reason it can be used to store the binary bit of information.
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To change the state of the binary, it is required to apply the external trigger. At the occurrence of each triggering pulse, the circuit state changes abruptly from one stable state to another. An important application of bistable circuit is in counting circuits.
BJT Saturating Astable Multivibrator:
Fig. 3.17 indicates the circuit diagram of an astable multivibrator using BJTs as active devices while Fig. 3.18 indicates the waveforms obtained. RC1 and RC2 are the collector resistances for T1 and T2 respectively. The capacitors C1 and C2 are the coupling capacitors. The capacitor C1 connects the output of the transistor T1 to the input base terminal of the transistor T2. Resistors RB1 and RB2 provide ON-state base current to the transistors T1 and T2 during saturation region.
For a symmetrical astable multivibrator, we should have-
To explain the operation of a saturating astable multivibrator, we proceed as follows:
i. At t = 0, when the power supply voltage VCC gets applied abruptly due to slight mismatch, the current IC1 flowing in the transistor T1 is little more than the collector current IC2 of the transistor T2. Hence, the rate of fall of VC1 is more than that of VC2.
ii. For the transients the capacitors act as short circuits and voltage across them cannot change instantaneously. Thus the changes in collector voltages of T1 from the initial voltage VCC to VC1 (VC1 < VCC) will make the base of the transistor T2 negative which will reduce the conduction and increase the collector voltage VC1 towards the supply voltage VCC.
iii. This increase in VC2 will be transferred through C2 to the base of T1 making its voltage more positive and thus, increasing the conduction in T1. It will further reduce the collector voltage VC1 which in turn will make the base of T2 more negative, thus, reducing its conduction more.
iv. In this way due to positive feedback action with the loop-gain greater than unity, whole of the above sequence of operation occurs instantaneously and the transistor T1 is in saturation region and T2 is in the OFF region, which is a limiting condition.
Thus, when an astable multivibrator is switched, we have the following two conditions:
i. Transistor T1 is in saturation region.
ii. Transistor T2 is in OFF region.
b. Circuit Conditions for the Transistor T1 in Saturation Region:
For the circuit indicated in Fig. 3.17 we have-
In the saturation region, the following relationship should apply:
The base voltage of transistor T1 should be about 0.7V, for the silicon transistor. With collector- to-emitter saturation voltage VCE sat ≈ 0.2V, this base voltage will be forward bias for both the junctions.
If we make the simplifying assumption that the transistor T1 is an ideal one then equations (3.33) to (3.36) will reduce to the following forms:
Circuit behaviour in Quasi-Stable State:
For the time interval 0 < t < t1 the capacitor voltage will rise from (VB2)0ƒƒ/t = 0 towards VBB. The charging path for capacitor C1 is indicated in Fig. 3.19 assuming that (VCE) sat of the transistor T1 is negligible.
At any instant of time, the voltage on C1 or (VB2)0ff/t can be written as-
Fig. 3.20 shows the charging of capacitor as function of time, due to the presence of emitter-base junction of the transistor T2 which enters into conduction, when the instantaneous base voltage equals (VB2)on. This occurs at the time instant t = t1.
We have-
a. Behaviour at the Time Instant t = t1:
At the time instant t = t1, transistor T2 goes into conduction. The collector voltage of T2 begins to fall which is communicated to the base of transistor T1 through capacitor C2. As a matter of fact the conduction of T1 reduces, with the increase of collector voltage of T1. This increase is communicated to the base of T2 through capacitor C1 and thus, increases its conduction. This regenerative process continues and as a result transistor T2 is in saturation region and T1 is in the off region instantaneously.
b. Circuit behaviour during Quasi-State (t1 < t < t2):
During this time period, capacitor C2 charges from (VB2)off/t=0, towards VBB. At the time t = t2, the instantaneous base voltage is (VB) on which brings the transistor T1 into conduction. The time interval (t2– t1) may then be expressed as-
Then equation (3.50) reduces to the form-
Equation (3.51) shows that to the first approximation, the time period of astable multivibrator is independent of supply voltage, temperature and junction voltages.
Voltage-Controlled Astable Multivibrator:
When the supply voltage VBB is not same as VCC, then with equations (3.45) and (3.48), the period T for symmetrical multivibrator can be expressed as-
From equation (3.52) it can be inferred that time period T can be varied by varying VBB. However, this variation is not linear.
Dual Power Supply Monostable Multivibrator:
In a monostable multivibrator, one of the state is permanent, i.e., stable and the other is temporary, i.e., quasi-stable. When an external trigger is applied to the monostable at appropriate point the monostable changes its state from stable to quasi-stable. It stays in the quasi-stable state for a predetermined length of time and returns to stable state automatically. Monostable multivibrator can also be realized by BJT, JFET and a negative resistance device.
A dual PS monostable multivibrator is indicated in Fig. 3.21(a). Here the negative supply VBB through resistors R1 and R2 keeps the transistor T1 in reverse- biased condition in the stable state. Fig. 3.21 (b) shows the equivalent circuit in the stable state, i.e., the transistor T1 in the OFF and the transistor T2 in the ON is states.
Dual Power Supply BJT Bistable Multivibrator:
Fig 3.22 shows the circuit diagram of dual power supply bistable multivibrator. The dual power supply bistable has its output swing from VCC volts to zero neglecting the collector-emitter saturation voltage (VCE) of the transistor. This output swing is more compared to that in self-biased bistable multivibrator, where the output swing is from VCC to VEN only.
The stable states of the circuits are:
i. T1 OFF and T2 ON
ii. T2 OFF and T1 ON