Multivibrators
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This document discusses three types of multivibrator circuits: 1. Astable multivibrators continually switch between two quasi-stable states without any external input. They are used as oscillators to produce a square wave output. 2. Monostable multivibrators have one stable state but can be triggered externally to another temporary state, producing a single output pulse before returning to the stable state. 3. Bistable multivibrators have two stable states and can be switched between them by external trigger pulses, functioning like a flip-flop.
Multivibrators
- 1. Multivibrators (MV) A multivibrator is an electronic circuit used to implement a variety of simple two-state systems such as oscillators, timers and flip-flops. It is characterized by two amplifying devices (transistors, electron tubes or other devices) cross-coupled by resistors or capacitors. The name "multivibrator" was initially applied to the free-running oscillator version of the circuit because its output waveform was rich in harmonics. There are three types of multivibrator circuits depending on the circuit operation: Astable: in which the circuit is not stable in either state —it continually switches from one state to the other; Monostable: in which one of the states is stable, but the other state is unstable; Bistable: in which the circuit is stable in either state 1. Astable Multivibrator (AMV) It is also called free-running relaxation oscillator. It has no stable state but only two quasi stable (half-stable) states between which it keeps oscillating continuously of its own accord without any external excitation. In this circuit, neither of the two transistors reaches a stable state. When one is ON, the other is OFF and they continuously switch back and forth at a rate depending on the RC time constant in the circuit. Hence, it oscillates and produces pulses of certain mark-to-space ratio. Moreover, two outputs (180° out of phase with each other) are available. It has two energy-storing elements i.e. two capacitors. The figure below shows the circuit of a symmetrical collector-coupled AMV using two similar transistors. It, in fact, consists of two CE amplifier stages, each providing a feedback to the other. The feedback ratio is unity and positive because of 180° phase shift in each stage. Hence, the circuit Multivibrators 1
- 2. oscillates. Because of the very strong feedback signal, the transistors are driven either to saturation or to cut-off (they do not work on the linear region of their characteristics). The transistor Q1 is forward-biased by VCC and R1 whereas Q2 is forward-biased by VCCand R2. The collector-emitter voltages of Q1 and Q2 are determined respectively by RL1 and RL2 together with VCC. The output of Q1 is coupled to the input of Q2 by C2 whereas output of Q2 is coupled to Q1 by C1. The output can be taken either from point A or B though these would be phase-reversed with respect to each. Circuit Operation The circuit operation would be easy to understand if it is remembered that due to feedback (i) When Q1 is ON, Q2 is OFF; and (ii) When Q2 is ON, Q1 is OFF. When the power is switched on by closing S, one of the transistors will start conducting before the other does (or slightly faster than the other). It is so because characteristics of no two seemingly similar transistors can be exactly alike. Suppose that Q1 starts conducting before Q2 does. The feedback system is such that Q1 will be very rapidly driven to saturation and Q2 to cut-off. The following sequence of events will occur: 1. Since Q1 is in saturation, whole of VCC drops across RL 1. Hence, VC1 = 0 and point A is at zero or ground potential. 2. Since Q2 is in cut-off i.e. it conducts no current, there is no drop across R L2. Hence, point B is at VCC. 3. Since A is at 0 V, C2 starts to charge through R2 towards VCC. 4. When voltage across C2 rises sufficiently (i.e. more than 0.7 V), it biases Q2 in the forward direction so that it starts conducting and is soon driven to saturation. 5. VC2 decreases and becomes almost zero when Q2 gets saturated. The potential of point B decreases from VCC to almost 0 V. This potential decrease (negative swing) is applied to the base of Q1 through C1. Consequently, Q1 is pulled out of saturation and is soon driven to cut- off. 6. Since, now, point B is at 0 V, C1 starts charging through R1 towards the target voltage VCC. 7. When voltage of C1 increases sufficiently, Q1 becomes forward-biased and starts conducting. In this way, the whole cycle is repeated. It is seen that the circuit alternates between a state in which Q1 is ON and Q2 is OFF and a state in which Q1 is OFF and Q2 is ON. The time in each state depends on RC values. Since each transistor is driven alternately into saturation and cut-off Multivibrators 2
- 3. the voltage wavefrom at either collector (points A and B in Fig. above) is essentially a square waveform with a peak amplitude equal to VCC(figure below). Switching Times It can be proved that off-time for Q1 is T1 = 0.69 R1C1 and that for Q2 is T2 = 0.69 R2C2. Hence, total time-period of the wave is T = T1 + T2= 0.69 (R1C1 + R2 C2) If R1 = R2 = R and C1 = C2 = C i.e. the two stages are symmetrical, then T = 1.38 RC Frequency of Oscillation It is given by the reciprocal of time period, Minimum Values of β To ensure oscillations, the transistors must saturate for which minimum values of β are as under: Multivibrators 3
- 4. 2. Monostable Multivibrator (MMV) It is also called a single-shot or single swing or a one-shot multivibrator. It has (i) One absolutely stable (stand-by) state; and (ii) One quasi stable state. It can be switched to the quasi-stable state by an external trigger pulse but it returns to the stable condition after a time delay determined by the value of circuit components. It supplies a single output pulse of a desired duration for every input trigger pulse. It has one energy-storing element i.e. one-capacitor A typical MMV circuit is shown in Fig. below. Here, Q1 is coupled to Q2 base. In this multivibrator, a single narrow input trigger pulse produces single rectangular pulse whose amplitude, pulse width and wave shape depend upon the values of circuit components rather than upon the trigger pulse. Initial Condition In the absence of a triggering pulse at C2 and with S closed, 1. VCC provides reverse bias for C/B junctions of Q1 and Q2 but forward-bias for E/B junction of Q2 only. Hence, Q2 conducts at saturation. 2. VBB and R3 reverse bias Q1 and keep it cut off. 3. C1 charges to nearly VCC through RL1 to ground by the low-resistance path provided by saturated Q2. As seen, the initial stable state is represented by (i) Q2 conducting at saturation and (ii) Q1 cut-off Multivibrators 4
- 5. When Trigger Pulse is Applied When a trigger pulse is applied to Q1 through C2, MMV will switch to its opposite unstable state where Q2 is cut-off and Q1 conducts at saturation. The chain of circuit actions is as under: 1. If positive trigger pulse is of sufficient amplitude, it will override the reverse bias of the E/B junction of Q1 and give it a forward bias. Hence, Q1 will start conducting. 2. As Q1 conducts, its collector voltage falls due to voltage drop across RL1. It means that potential of point A falls (negative-going signal). This negative-going voltage is fed to Q2 via C1 where it decreases its forward bias. 3. As collector current of Q1 starts decreasing, potential of point B increases (positive-going signal) due to lesser drop over RL2. Soon, Q2 comes out of conduction. 4. The positive-going signal at B is fed via R1 to the base of Q1 where it increases its forward bias further. As Q1 conducts more, potential of point A approaches 0 V. 5. This action is cumulative and ends with Q1 conducting at saturation and Q2 cut-off. Return to Initial Stable State 1. As point A is at almost 0 V, C1 starts to discharge through saturated Q1 to ground. 2. As C1 discharges, the negative potential at the base of Q2 is decreased. As C1 discharges further, Q2 is pulled out of cut-off. 3. As Q2 conducts further, a negative-going signal from point B via R1 drives Q1 into cut-off. Hence, the circuit reverts to its original state with Q2 conducting at saturation and Q1 cut-off. It remains in this state till another trigger pulse comes along when the entire cycle repeats itself. As shown in Fig. 65.30, the output is taken from the collector of Q2 though it can also be taken from point A of Q1. The width of this pulse is determined by the time constant of C1 R2. Since this MV produces one output pulse for every input trigger pulse it receives, it is called mono or one-shot multivibrator. The width or duration of the pulse is given by T = 0.69 C1R2 It is also known as the one-shot period. Multivibrators 5
- 6. 3. Bistable Multivibrator (BMV) It is also called flip-flop multivibrator. It has two absolutely stable states. It can remain in either of these two states unless an external trigger pulse switches it from one state to the other. Obviously, it does not oscillate. It has no energy storage element. 1. The base resistors are not joined to VCC but to a common source–VBB, 2. The feedback is coupled through two resistors (not capacitors). Circuit Action If Q1 is conducting, then the fact that point A is at nearly 0 V makes the base of Q2 negative (by the potential divider R2 – R4) and holds Q2 off. Similarly, with Q2 OFF, the potential divider from VCC to –VBB (RL2, R1, R3) is designed to keep base of Q1 at about 0.7 V ensuring that Q1 conducts. It is seen that Q1 holds Q2 OFF and Q3 holds Q1 ON. Suppose, now, a positive pulse is applied momentarily to R, it will cause Q2 to conduct. As collector of Q2 falls to zero, it cuts Q1 OFF and, consequently, the BMV switches over to its other state. Similarly, a positive trigger pulse applied to S will switch the BMV back to its original state. Multivibrators 6
- 7. SATURATION CUT-OFF IC is maximum IB is roughly zero Vc=Icx(Rc +RE) IC is also proportional to IB VCE=0 VCE is maximum = VCC IB is greater than zero The transistor is working like an OFF-Switch The transistor is working like a ON-Switch Multivibrators 7