 the session on a stable multivibrator using transistor. Learning outcomes are at the end of session students will be able to explain the working of multiple multivibrator using transistor as well as they can draw the output waveforms and calculate the value of time period frequency of oscillations. Contents are like this before moving towards a stable multivibrator configuration we will recall some building blocks nothing but basic building blocks of multivibrator. It consists of amplifier. So, there are two stages of amplifier which provides 180 degree phase shift that is why total 360 degree phase shift will be generated as well as gain of this amplifier will be greater than unity. So, that it will satisfy the criteria for oscillations nothing but Barkhausen's criteria and that is why it is possible to get the oscillations at the output and output of this amplifier is coupled through this coupling network. Here we are using the resistor and capacitor for coupling as well as positive feedback is maintained. Here we are using the transistor as a switching element and that is configured as RC coupled amplifier. So, we will see the circuit diagram for a stable configuration using transistor. Recall the definition it is a two stage amplifier with the coupling it was positive feedback is there which generates a non-sonosodal waveforms and very important thing in the optimal multivibrator is it has two quasi stable states nothing but two temporary states that is why it has no stable state means transistor will switch in the either state depending on the time constant of base circuit elements. So, transistor remains in the either state depending on the time constant of base circuit elements. The a stable multivibrator circuit where transistor Q1 and Q2 are used which are identical, but although these two are identical these two will have different doping levels that is why these two will not have the same condition at the same time instant means as we are giving the VCC to the circuit, but both transistor will not be on at the same time. Collector is connected to RC1 and which is connected to VCC same RC2 is there at the collector side of Q2. Register R1 and R2 are used for the biasing purpose. So, R1 is used to bias Q2 and R2 is used to bias Q1. Collector of transistor Q1 is connected to the base of transistor Q2 same collector of Q2 is connected to base of transistor Q1 through capacitor C2. So, here AC coupling is provided through this coupling capacitor and 180 degree phase shift is provided through each of this amplifier configuration. Now, we will see the circuit operation as you know that transistor Q1 and Q2 cannot be on at the same time instant. Therefore, we assume transistor Q1 is in on condition. When transistor Q1 is on operating point will be in the saturation region. Therefore, it will act as a short circuit that is collector and emitter terminals will be short and as it is short VCE nothing, but output voltage at the collector of transistor Q1 will be ideally equal to 0 and practically it will be VCE sat nothing, but very very less value. It may be between 0.1 to 0.3 volt. So, as this is on condition at the same time this is output voltage which is given to the transistor Q2. So, this is 0 volt as VCE equal to 0 and therefore, low signal will be there at the base of Q2. So, Q2 will remain in the off state. So, operating point of Q2 will be in the cutoff region. Now, see while these two conditions are satisfied capacitor C1 gets charged through VCC that is capacitor C1 will be charged through register R1. Now, capacitor C1 gets charged up to its maximum value. As you know that path of this charging is like this. So, this is at the positive peak. So, capacitor C1 will be at plus VCC level. Now, this voltage is applied to transistor Q2. So, what is this voltage? This is greater than 0.7 volt. Voltage appeared at the base of transistor Q2 is greater than 0.7 volt. Therefore, transistor Q2 will now be in the on condition. So, operating point will be in the saturation region. So, it will act as a short circuit at the collector and emitter terminals. So, when this is at the short circuit condition, then you can identify the same voltage that is VCE equal to 0 and practically it varies from 0.1 to 0.3 volt. This voltage is given to the base of transistor Q1. So, what is this voltage? This voltage is very very less. So, this voltage is applied to Q1. So, at that time Q1 will be at its off condition that is 0. So, when it is off, it will be act as a open circuit. When it act as open circuit, it will be giving the voltage as VCE equal to VCC. So, here positive level of the signal will be obtained. So, this is your positive level. Now, what we have obtained over here? This is on. So, negative will be obtained at the output of transistor Q2. You can obtain the output from either step amplifier. So, at Q2 or at Q1. So, if you assume output you are considering at Q2. So, this is obtained that is negative level or 0 volt is obtained at the transistor Q2. Now, same way as capacitor and C2 instantly this cannot be charged. So, it will take some time. Therefore, the voltage at the base terminal of these two transistors will be not sharp. It will be varying slowly. As these are varying slowly you know that it is through capacitor. So, it will be like a step signal. Now, the other condition see here we have assumed transistor Q2 is now on and therefore, we have obtained 0 volt over here and while this is on at the same time capacitor C2 gets charged through resistor R2 and same voltage is provided to transistor Q2. So, initial voltage to base of transistor Q1 will be 0 due to the Q2 or collector voltage at Q2 which is 0, but now this capacitor will get charged slowly and the same voltage is applied over here and which is greater than 0.7. As this voltage is greater than 0.7 it will be obtained as the on condition operator. Output waveforms across the collector terminal of Q1 which switches between 0 to VCC level and vice versa at the collector of transistor Q2. So, as I told you capacitor will charge slowly that is why increase in the voltage at the base will be like this and which is reverse for base of transistor Q2. So, these are the waveforms at the collector and base terminal of transistor Q1 and transistor Q2. So, let us see the formula. If you derive the formula for base voltage and as you know base voltage depends on the capacitor charging and discharging. So, T1 can be calculated as 0.693 into R1 into C1. So, R1 into C1 are the base circuit elements which decides the T1 thing, but output across Q2 we are considering and that is why T1 is your time period to get the output voltage that is on state that is VCC level and this is off state. So, in this way we have T1 and T2 are same when we are selecting the same values that is R1 and R2 are identical. So, if we add these two values when R1 and R2 are same C1 and C2 are same therefore, frequency of oscillations can be calculated with the help of 0.72 divided by R into C. So, we can calculate the frequency of oscillations of a stable multivibrator. So, recall the formula and find the value of time period and frequency of oscillations for a stable multivibrator. Carefully see what are the components at base circuit of amplifier. So, answer is 0.14 millisecond and frequencies 7.2 kilohertz. Applications of stable multivibrators are like this. It is a free running multivibrator as we are not providing any trigger powers to get the output across the output terminals. So, it is called as free running multivibrator. It will continuously switch between its 0 volt and 1 volt level nothing, but VCC. For digital circuit we are considering the level as 0 and 1. So, that is why it is also called as square wave frequency generator. So, it is also used as a timing oscillator. It is used as a clock to a computer system. It is used for flashing lights, switching and power supply circuit. So, with the help of a stable multivibrator we can have various fancy kind of applications so that no need to give any trigger pulse over here, but for that you must select the value of R and C carefully that is R 1, C 1 or R 2 and C. It is also used for switching and power supply circuits. So, references are like this.