 Welcome to the session on multivibrator and its basic configuration. Learning outcomes are at the end of session students will be able to identify components of multivibrator nothing but various blocks of multivibrator as well as they can list out the various states and types of multivibrator. Components are like this. So, what is a multivibrator? Multivibrator is nothing but a switching circuit which generates non-sinusoidal waveforms means multivibrator is nothing but oscillator which generates the non-sinusoidal waveforms nothing but rectangular waveforms then sawtooth waveforms as well as square waveforms. Figure 1 shows the basic building blocks of a multivibrator where it consists of two stages of amplifier which are connected to each other with the help of coupling network and output of one amplifier is feedback to the another amplifier. So, it is mandatory to have two stages of amplifier and the coupling network. Coupling networks can be register and capacitors. Amplifier A1 and A2 can be of any kind of component nothing but it can be a transistor, it can be a IC or it can be free flops or any kind of switching element also. So, why we say the multivibrator as a switching circuit that we will see, but before to that definition of multivibrator is nothing but it is a two stage RC coupled amplifier with coupling network and the feedback is positive. Means when positive feedback is there then and then it is possible to generate nothing but your circuit will oscillate and generate the waveforms. So, positive feedback is necessary to generate. Now moving towards the very important part of multivibrator which is nothing but a switching will apply any source to the load and in between source to load you connect a switch. The switch can be in the on or off condition when switch is on. So, that provides the voltage to load. So, output at the load will be at its positive level, but after particular time period when you make this switch off it will be as a open condition and therefore output across the load will be at its negative level. So, in this way output across the load can be switching between positive and negative level. So, this is the basic waveform generation nothing but square wave generation. So, any kind of square wave generation is nothing but it must carry some kind of switching action. This switch can be the mechanical switch or electrical switch, but as you know that when you go for mechanical switch then there will be the losses, wear and tear losses. So, switching can or component that may be your transistor that may be any flip flop that may be any kind of another kind of a provision to make the switching action. So, by turning switch on and off square waves can be generated and our aim is to see the multivibrator with the help of transistor. So, we are concentrating on the transistor as a switch which is basic switching element of your multivibrator. So, how can the transistor used as a switch? Figure 3 shows the load line on characteristic curve with the two extreme states. What are the two extreme states? As you know transistor can be used as a switch when operating point nothing but Q point is in the cut off region and in the saturation region. When the operating point moves through this load line very fast between this cut off region to the saturation region we can operate the transistor as a switch. Now, we see the two condition and how this can be obtained in the transistor. So, on bi-axis maximum current is shown here VCC upon RL. So, at this time transistor will be in the saturation region which gives the condition of a transistor as on and when IC is 0 nothing but collector current is 0 at that time output across collector to emitter terminal is nothing but PCC that gives the off condition of a switch. Now we will see how this transistor will act as a switch. So, figure 4 shows the transistor in the off biased condition means when input is less than 0.7 volt or 0 volt at that time you know that now the input junction nothing but base emitter junction will be reverse biased and the base collector junction will be also reverse biased. At that time switch or transistor will act as a open circuit condition IC will be 0 nothing but collector current will be 0 and transistor will be in it off condition at that time operating point will be in the cutoff region. So, transistor will be act as a off switch condition nothing but transistor operates as an open switch condition. So, keep in mind at that time IC is equal to 0 nothing but collector current is 0 and output voltage is equal to VCE and that is equal to VCC. So, whatever is the VCC value will appear at the output and we can treat it as the positive voltage level at the output side. So, in this way when no input is applied output is 1 means for input is 0 output is 1 and that says a closed switch when input is greater than 0.7 volt nothing but input junction is now forward bias output junction is also forward biased. Now transistor is on and that gives the operating point in the saturation region maximum current will flow through the collector nothing but VCC on RL at that time output voltage is 0 and ideally it is 0 but practically it is 0.1 to 0.3 volt and that gives the low voltage level at the output. So, in this way positive and negative nothing but 0 is off cross the transistor nothing but output. So, here transistor acts as a closed switch. Now see the classification here three types of multivibrators are there where input is not applied then you will get the square wave at the output that is that is why a stable multivibrator also called as free running square wave generator. Second type of multivibrator is mono stable where trigger pulse is applied at once and when trigger pulse is applied your level or stable state will be from one state to other state. Now here you can see first we have the 0 volt level once you apply the trigger at time period T1 then output switches from its low level to the high level and it will be maintained at high level for the predetermined time and then again it switches to its low level. So, here next type of multivibrator is biostable multivibrator and for that we are applying the two trigger pulses at T1 you are switching from low to high level and at T2 again you will be switched back from high level to low level. So, for any transition you must apply the trigger pulse and this is biostable multivibrator. Now recall a multivibrator is an electronic circuit which is used to implement which of the following. So, correct answer is D. Now pause this video for a minute and find which of the components are required to get the multivibrator, registers, capacitors, transistors or all of the above mentioned yes and the correct answer. So, these are the appliances of frequency divider, sawtooth generators, wave and pulse generator as a frequency source for the testing instrument it is used in radar and television circuits also and very important thing in the digital systems it is used as a memory element and these are the references used.