Dear students, welcome back to this NPTEL course on Analog Electronic Circuits and today we are going to cover the topic of Linear Models of Amplifiers. So, in fact if you see, we already have covered few topics and today actually we are in the first day of 3rd week. So, let see what where do we stand today, compared to our overall plan. This 3rd week we are planning to cover amplifier models and followed by cascading multiple amplifier stages; and then followed by CE amplifier, common emitter amplifier and common source amplifier. And under this topic of amplifier models what you are going to cover it is; the voltage amplifier, current amplifier and trans-conductance amplifier and trans-resistance or trans-impedance amplifier. So, these are essentially simplified model of the different types of voltage amplifier. Today we are going to discuss with simple example and we will cover the purpose and the little detail of the model different, these four models.

Now, what are the concepts we are going to cover today? It is, basically as I said that the model of voltage amplifier, then model of current amplifier, model of trans-conductance amplifier and trans-impedance amplifier. So, primarily we will be discussing more detail of these two topics and whatever the idea we will be gaining from that, we will be extending to the other two types of amplifiers quickly.

Say for example, the first one it is; it consist of BJT, second one it is having a MOS transistor. And for each of the cases what we have here it is, the DC voltage source and then of course the ground and along with the biasing element resister. The transistor it is in this case the BJT is kept in the active region of operation with the help of V_BE, the DC voltage here. And then on top of the DC voltage we do have the small signal called v_be.

Whenever we are going for say model or linearization basically what you do; as I said that the transistor we are keeping in the appropriate region of operation, and then after that we try to find what is the relationship between the applied voltage here to whatever the corresponding output we are getting. So, in our main operation the DC conditions are kept intact, only the small signal part changes with time and then the corresponding effect we observe here.

What we mean by the voltage amplifier? It is an equivalent linear circuit and it is main purpose is to provide the dependency of output signal on the input signal. The DC part we are not changing. So, we can probably eliminate the DC part in the simplified model to make the circuit simple enough or probably the analysis we can make it simpler.

So, we may say that to again to summarize what we said is; that we like to get the input to output signal relationship primarily. So, this is the say, you may say that input signal and this is the corresponding output. So, the input signal of course, it may be coming from the signal source it need not be same. And since we are expressing this output in terms of the input signal, we the simplified model must capture one basic element called voltage dependent voltage source.