Malakus The emitter of the second transistor is met, on the one hand, to the cathode of a zener diode whose anode is connected to the primary mass 8 and, secondly, to the second power supply terminal through a fifth resistor which allows biasing the emitter of the second transistor to a fixed voltage V Z between 2 and 3 volts. A third input 3 thereof is connected to the output SP of the BS power device and a fourth input axtable is connected to the positive pole P of the source of input voltage VE. Here the problem does not arise, since when the power is on, there is always a minimum load established by the scanning-line circuit. The change in this relative phase shift or this delay interval gives rise to a variation of the power supplied, and therefore, the voltage across the reservoir capacitor.

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In that case , the circuit is known as symmetrical astable multivibrator. The transistor Q1 is forward biased by the Vcc supply through resistor R2. Similarly the transistor Q2is forward biased by the Vcc supply through resistor R1. The output of transistor Q1is coupled to the input of transistor Q2through the capacitor C2. Similarly the output of transistor Q2is coupled to the input of transistor Q1 through the capacitor C1.

It consists of two common emitter amplifying stages. Each stage provides a feedback through a capacitor at the input of the other. Since the amplifying stage introduces a o phase shift and another o phase shift is introduced by a capacitor , therefore the feedback signal and the circuit works as an oscillator.

In other words because of capacitive coupling none of the transistor can remain permanently out-off or saturated, instead of circuit has two quasi-stable states ON and OFF and it makes periodic transition between these two states.

The output of an astable multivibrator is available at the collector terminal of the either transistors as shown in figure a. However, the two outputs are o out of phase with each other. Therefore one of the output is said to be the complement of the other. C power supply is switched ON by closing S, one of the transistors will start conducting before the other or slightly faster then the other.

The feedback system is such that Q1will be very rapidly driven ton saturation and Q2to cut-off. The circuit operation may be explained as follows. Since Q2is in cut-off i. Hence point B is at VCC. When voltage across C2rises sufficiently i. VCC decreases and becomes almost zero when Q2gets saturated. This potential decrease negative swing is applied to the base of Q1through C1.

Consequently, Q1is pulled out of saturation and is soon driven to cut-off. When voltage of C1increases sufficiently. Q1becomes forward-biased and starts conducting. In this way the whole cycle is repeated. This time in each states depends on RC values.

Since each transistor is driven alternately into saturation and cut-off. The voltage waveform at either collector points A and B in figure b is essentially a square waveform with a peak amplitude equal to VCC. Emitter - Coupled Astable Multivibrator Figure c shows the circuit diagram of an emitter coupled asable multivibrator. In a collector coupled symmetrical astable multivibrator if it is desired to vary the frequency.

Thus it is difficult to achieve frequency control in collector coupled astable multivibrator, not an emitter coupled multivibrator, to be described here, has a single timing capacitor connected across the emitter.

This capacitor can be varied easily. In order to explain the operation of the circuit, ti is necessary that the following conditions must be satisfied. Under D. In the active region, the loop gain must be greater than unity at some non-zero frequency.

Bias conditions ar3e so adjusted that with the capacitor C concerned, during normal operation, transistor C1operates between cut-off and saturation while transistor C2operates at the same time between its active region and the off region. This transistor Q1operates in saturated mode and transistor Q1operates in saturated mode and transistor Q2operates in unsaturated mode. Astable multivibrator using OP-AMP Figure d shows differential input operational amplifier acting as a free running symmetrical multivibrator.

The two states of the circuit between which it switches are those in which the amplifier output is at positive and negative saturation. R1and R2provide a fixed level of positive feedback and R and C provide a frequency dependent level of negative feedback.

At high frequencies the negative feedback is reduced and the circuit becomes unstable. The circuit can not hang up in either output stage and is self starting.

As terminal B is positive with respect to terminal A and its potential is decreasing as C charges down through R. When the potential difference between the two input terminals approached zero the amplifier comes out of saturation. The positive feedback from the output to terminal A causes a regenerative switching which drives the amplifier to positive saturation. The voltage across a capacitor in series with a resistor can not change instantaneously, the potential at the terminal B, therefore, remain substantially constant during this rapid transition.

The duty cycle may be controlled, therefore, by the values of RA and RS. In order to understand the operation of both astable and monostable multivibrator circuit using timer, functional diagram of timer is shown in figure e.

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