75.2.1. Basic Operation

So, to recapitulate whatever we have discussed in our previous lecture we do have say this is a main differential amplifier. Where we have the power supply DC supply and ground and then at the input we do have the differential input port having two terminals 1 and 2 and at this input we are given signal or I should say voltage signal and it is having a DC part meaningful DC part and on top of that we do have two signals. One is v in1 and v in2.

The differential amplifier consists of two input terminals (1 and 2) where voltage signals are applied. The signals at these terminals have two components: a meaningful DC voltage level and the actual signals (v in1 and v in2) that will be amplified. The purpose of having these two input terminals is to amplify the difference between the two input signals while rejecting any common signal that might be applied equally to both terminals.

Think of a differential amplifier like a pair of noise-canceling headphones. You have two microphones that pick up sounds (the input terminals), and the headphones amplify the sound coming from one microphone while canceling out the noise picked up by the other microphone. This way, you only hear what you want to, just like how the differential amplifier aims to amplify the difference between two signals.

So, these are the two signals, but we must be careful that while you are feeding the signal the voltage DC voltage at this point and DC voltage at this point should be appropriate. So, that this differential amplifier should be working properly. Since in this connection supply connection we do have only one supply namely, V cc and the common terminal is ground here invariably we need a +ve DC voltage at both terminal-1 and terminal-2.

For the differential amplifier to work effectively, it is essential to maintain appropriate DC voltage levels at both terminals. Both inputs must receive a positive DC voltage to ensure that the transistors within the amplifier are operating correctly. If the DC levels are too low or incorrect, the amplifier may not function as intended, and the output may become distorted or unusable.

Consider a car that needs a specific amount of fuel to run smoothly. If the fuel gauge (representing the DC level) is too low, the car will stall and not operate effectively. Similarly, if the DC levels at the input of the differential amplifier are not suitable, the amplifier won’t function correctly, just like the car won't run without proper fuel.

Once we are sure that all the transistors are in proper region of operation, then we may focus on linearized circuit or something called small signal equivalent circuit. And they are what they do the DC parts we will be considering it is 0.

After ensuring that all transistors in the differential amplifier are functioning properly, we can simplify the analysis by focusing on the small signal equivalent circuit. In this model, the DC components are set to zero, allowing us to concentrate on how the amplifier responds to small AC signals or variations around a bias point. This simplification makes it easier to analyze the amplifier's performance in practical scenarios.

This is similar to adjusting your music player to only focus on the treble and bass, while ignoring the static noise in the background. By zeroing in on the small signals (like treble and bass), you can fine-tune how your music sounds without interference from unwanted sounds (the DC levels).

Now once we are in small signal equivalent circuit, then it is basic parameters are differential mode gain and common mode gain as you can see here and we say that ideally we want this differential mode gain it should be as high as possible and this common mode gain on the other hand it should be as small as possible.

In small signal analysis of the differential amplifier, the two critical parameters are differential mode gain and common mode gain. The differential mode gain represents how well the amplifier amplifies the difference between the two input signals, while the common mode gain indicates how much of any input signal that is common to both terminals is amplified. Ideally, we want the differential mode gain to be high to ensure that the intended signal difference is amplified effectively, whereas the common mode gain should be minimized to avoid amplifying noise or interference that is the same on both inputs.

Imagine you are trying to listen to a conversation between two friends at a party. You want to hear their discussion (the differential signal), but there are many other conversations happening around you (common signals). A good differential amplifier is like having a super-sensitive earpiece that enhances only the discussion between your friends while filtering out all the background noise. This way, you focus on what matters most.

Suppose, we do have a circuit whether in this form or in the model form then, how do we find the value of this differential mode gain and common mode gain? For say analysis of a circuit or maybe in actual hardware, how do we how do you find this parameter? So, which is referred as characterization of the existing circuit to find the value of differential mode gain and common mode gain.

To characterize a differential amplifier, we can analyze either the physical circuit or its model to determine the differential mode gain and common mode gain. This process involves applying specific test inputs to the circuit and measuring the resulting outputs. By establishing these relationships, we can quantify how effectively the amplifier functions in amplifying desired signals and rejecting common noise.

Think about calibrating a scale before weighing objects. You would place known weights on it to see how accurately it measures. Similarly, characterizing a differential amplifier involves testing it with known input signals to determine how well it amplifies and how much common noise is suppressed, just like ensuring the scale gives accurate readings.

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