Astable Multivibrator Definition and its Working:

The multivibrator circuit which has no stable state is called the astable multivibrator. The two states of operation of astable multivibrators are quasi-stable (temporary) states. The astable multivibrator, therefore, makes successive transitions from one quasi-stable state to the other one after a predetermined time interval, without the help of an external triggering circuit. The periodic time depends upon the circuit time constants and parameters. The output of the circuit is a square wave with two time periods T₁ and T₂. If T₁ = T₂= T/2, the circuit is a symmetric astable multivibrator. If T₁ # T₂ it is called unsymmetric astable multivibrator. Since the output of an astable multivibrator oscillates in between off and on states freely, it is called a free-running multivibrator, The main application of an astable multivibrator is as a clock in digital circuits.

The astable multivibrator is essentially a square-wave gen­erator. We consider three circuits: the collector-coupled astable multivibrator, the emitter-coupled astable multivibrator and com­plementary transistor astable multivibrator.

Collector Coupled Astable Multivibrator:

The circuit arrangement of Collector Coupled Astable Multivibrator is shown in Fig. 31.19. It is essentially a two-stage R-C coupled amplifier with the output of first stage coupled to the input of second stage; and the output of second stage coupled to the input of the first stage. As the phase of a signal is reversed when amplified by a single stage common emitter amplifier, it comes back to its original phase when passed through two stages. Thus, the signal fed back to the base of either transistor is in the same phase as the original signal at its output. It amounts to a positive feedback. The amount of feedback is so large that the transistors are driven between cut-off and saturation almost instantaneously. A transistor remains in either saturation or cut-off for a period depending on the time constant of the base-circuit elements.

Operation:

The operation of the Collector Coupled Astable Multivibrator circuit illustrated in Fig. 31.19 is as follows :

When power is first applied to the circuit, both transistors start conducting. Because of small differences in their operating characteristics, one of the transistors will conduct slightly more than the other. This starts a series of events. Assume arbitrarily that Q₁ initially conducts more than Q₂. This causes the collector voltage of Q₁ to drop more rapidly than that of Q₂. The resulting negative signal is fed to the base of Q₂ through capacitor C₁ and drives it towards cut-off. As a result the collector voltage of Q₂ rises towards V_CC.

This change in collector voltage of Q₂ is fed to the base of Q₁ through capacitor C₂. It causes transistor Q₁ to go into saturation. This happens so quickly that capacitor C₁ does not get a chance to discharge, and the reduced voltage at the collector of Q₁ appears across resistor R₂.

The switching action now begins, capacitor C₁ begins to discharge exponentially through R₂. When the voltage on C₁ reaches zero volt, C₁ attempts to charge up to the value of + V_BB, the base supply voltage. But this action immediately places a forward bias on Q₂ and thus transistor Q₂ begins to conduct. As soon as Q₂ starts conducting, its collector current causes collector voltage to fall. This negative-going voltage signal is fed to the base of transistor Q₁ through capacitor C₂ which begins to conduct less i.e., it comes out of saturation. This resulting increased voltage at the collector of Q₁ is coupled through C₁ and appears across R₂. The collector current of Q₂, therefore, increases. This process continues rapidly until transistor Q₁ finally cuts off and transistor Q₂ conducts heavily. At this instant, the collector voltage of transistor Q₁ attains its maximum value of V_CC, capacitor C₁ charges to the full value of V_CC, and a full cycle of operation has been completed. The cycle then repeats itself. The output of the multivibrator can be taken from the collector of either transistor. The output is a square wave, as shown in Fig. 31.20 and whose frequency is determined by R₁, R₂, C₁ and C₂ in the circuit.

Frequency of Oscillation:

From the above discussion it is found that as soon as astable multivibrator is switched on, transistor Q₁ goes on while transistor Q₂ goes off. Let this instant be t = 0.

At time = 0 For Transistor Q₁ (in On-state)

Collector voltage,

Collector current,

Base voltage,

Base current,

For Transistor Q₂ (in off state)

Collector voltage,

Collector current,

At the instant of switching on the power supply but before the commencement of regenerative feedback sequence, the voltage on capacitor C₁ is V_CC. This implies that the collector side voltage of the capacitor C₁ is V_CC and base side voltage is V_B2(on). At the end of the regenerative cycle, the collector side voltage of the capacitor C₁ has decreased from V_CC to V_{CE (sat)} (almost zero). But the capacitor C₁ cannot discharge instantaneously. Thus the voltage at the base of the transistor Q₂ is given as

After commencement of regenerative feedback but before Q₁ goes off (i.e., 0 < t < T₁), voltage across capacitor C₁ increases from V_B2(off) (t = 0) towards V_BB. Charging path is provided by capacitor C₁ and resistor R₂ assuming that V_C1 = 0. At any instant t, voltage V_B2 is given as

Thus, the base voltage of transistor Q₂, V_B2(t) rises exponentially with time constant C₁R₂. However, as soon as V_B2 becomes equal to V_B2(on) at time T₁, the transistor Q₂ commences conducting and V_B2(t) remains constant at V_B2(on) .

At time T₁

Solving Eqs. (27.8) and (27.10) we get

If V_BB >> V_B2(on), Eq. (31.11) becomes

and if V_BB = V_CC, then Eq. (31.12) reduces to

From Eq. (31.13) the on-time of transistor Q₁ or the off-time of transistor Q₂ is given as

Similarly, the off-time of transistor Q₁ or the on-time of tran­sistor Q₂ is given as

Hence, total time period of square wave

For a symmetrical multivibrator R₁ = R₂ = R (say) and C₁ = C₂ = C (say) then total time period is given as

Frequency of the square wave,

Minimum Values of β: For sustained oscillations, the tran­sistors must saturate for which minimum values of β are as be­low

Emitter Coupled Astable Multivibrator:

In an Emitter Coupled Astable Multivibrator circuit capacitor C is connected between emitters of two transistors, as shown in Fig. 31.21. Transistor Q₁ remains either in saturation or in cut­off while transistor Q₂ is either in cut-off region or in active region. Capacitor C is a storage element. Let the transistor Q₂ start conducting first.

As soon as Q₂ starts conducting, a current flows through R_C2 making its collector voltage to fall and a voltage drop across R_E2 too. This drop is followed by R_E1 as voltage across capacitor C cannot change instantaneously. This emitter voltage keeps tran­sistor Q₁ off. Now the capacitor C begins charging in the direc­tion shown in Fig. 31.21 and as a result emitter voltage of tran­sistor Q₁ starts falling.

When capacitor C charges to a level so that emitter voltage of transistor Q₁ falls to a level to make it conducting. When transistor Q₁ starts conducting, its collector voltage drops causing base voltage of transistor Q₂ to fall. Thus transistor Q₂ goes into cutoff region. Now capacitor C starts charg­ing in reverse direction and when it goes to a level so that emitter voltage of transistor Q₂ falls to force it to conduct. When transis­tor Q₂ begins conducting and voltage drop across R_E2 rises cou­pling to emitter of Q₁. Thus Q₁ is cutoff. Thus one cycle of op­eration is completed.

An emitter coupled astable multivibrator has the following advantages over a collector-coupled astable multivibrator.

1. Collector of transistor Q₂ is not connected to any part of the circuit, so it can be used to provide an output terminal.
2. Frequency adjustment is easy because of use of only one capacitor in the circuit.
3. Synchronization is easily possible at base of transistor Q₁ as input at base is isolated.
4. There are no recovery transients (such as overshoot voltages) in the output.

However, the emitter coupled astable multivibrator has the drawbacks of using more components and difficulty in obtaining proper operating conditions.

Complementary Transistor Astable Multivibrator:

An astable multivibrator can be realized by using a comple­mentary pair of transistors. In the Complementary Transistor Astable Multivibrator both the transistors are either on or off si­multaneously when switched on. This is contrary to two transis­tor astable multivibrator, where, at any instant of time, one of the transistor is on and the other is off, and these states change sequentially. The noteworthy point is that a complementary transistor astable multivibrator consumes very little power, since the power is drawn from the supply for a very short time duration when both the transistors are on simultaneously.

The circuit diagram of a complementary transistor astable multivibrator is shown in Fig. 31.23 (a). Here Q₁ is PNP transistor and Q₂ is NPN transistor.

Operation:

When the power supply is switched on at time t = 0, the voltage across capacitor, which was initially zero, rises exponentially towards V_CC. When the instantaneous voltage across capacitor C attains the voltage level of (V_z + V_γ), the emitter-base junction of NPN transistor Q₂ is at the edge of forward bias, V_z being zener voltage and V_γ is the cut-in voltage of the transistor.

Now when the NPN transistor Q₂ starts conducting, its collector current flows via the emitter-base junction of PNP transistor Q₁. Here the collector current of NPN transistor is the base current of PNP transistor. Further the collector current of PNP transistor is also in part of the base current of NPN transistor. Thus, any increase in base current of NPN transistor via the zener diode will abruptly bring both the transistors in saturation and the capacitor C is instantaneously discharged. As the capacitor C discharges, the base currents of both the transistors Q₁ and Q₂ are reduced and consequently both the transistors are off.  Again the capacitor C starts charging and the process repeats itself. Resistance R₂ limits the capacitor discharge current. If V₁ is the residual voltage across the capacitor, when the switch is off, then the repetition period T is given by expression