75.1. Differential Amplifier: Basic Structure and Principle of Operation

Dear students, welcome back to our NPTEL online certification course on Analog Electronic Circuit; myself Pradip Mandal from E and EC department of IIT, Kharagpur. Today’s topic of discussion is Differential Amplifier and we will be talking primarily the Basic Structure and Operating Principle.

This introduction sets the stage for understanding differential amplifiers. It acknowledges the returning students and briefly states the focus of the lecture, which is essential to appreciate the relevance of differential amplifiers in the field of analog electronics.

Think of a differential amplifier as a pair of ears that can focus on sounds coming from two different directions. Just like how you might ignore background noise and concentrate on a friend’s voice amidst a crowd, differential amplifiers focus on the difference in voltage between two inputs, allowing for clearer signal amplification.

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 and v . in1 in2

A differential amplifier comprises two input terminals: terminal 1 and terminal 2. These terminals receive voltage signals that often have a direct current (DC) component and an alternating current (AC) signal. The DC part provides a baseline, while the AC signals are the variations that we want to amplify.

Imagine you are trying to hear a conversation while also listening to background music. The DC voltage acts like the steady rhythm of the music, giving you a reference point, while the two signals, v_in1 and v_in2, represent the conversation, which you want to amplify above the music noise.

So, 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, unless otherwise it is stated we assume that the DC voltage at terminal-1 and terminal-2, they are equal and we refer this DC voltage at input common mode DC voltage V_INC.

Feeding the correct DC levels to both terminals ensures that the differential amplifier operates correctly. If the DC voltages at both terminals are not properly aligned, the amplifier may not function as intended. Typically, we refer to this common voltage as V_INC, ensuring both inputs start from the same baseline.

Consider a balanced see-saw: if both sides start at the same height (like having equal DC voltages), it operates smoothly. If one side is lower or higher (unequal voltages), the see-saw won't behave as intended, much like a differential amplifier would fail to amplify signals accurately.

Now, so these are the two inputs, v and then v . [...] As a result if we see the individual output see v is ‧v on the other hand v = ‒v.

From the differential signals applied, the outputs generated at the terminals also reflect this difference. For example, if the input to terminal 1 produces a certain amplified signal at output 1 (V_o1), the signal at output 2 (V_o2) will be the negative or complementary version of that signal if all conditions are ideal.

Think of a pair of dancers who mirror each other's movements. If one dancer moves up, the other moves down. In this context, the 'up' and 'down' represent the amplified outputs of the differential amplifier functioning with respect to its inputs.

So, 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.

Characterization involves measuring key parameters of differential amplifiers: the differential mode gain (how well the amplifier responds to the difference in signals) should be high, while the common mode gain (how well it responds to the same signals applied to both terminals) should be low. This minimizes interference from noise.

Imagine you are trying to listen to your friend talking (the differential signal) while ignoring a loud television playing in the background (the common mode signal). If you can drown out the TV noise and amplify your friend's voice well, the amplifier's effectiveness is similar to achieving a high differential gain with low common mode gain.

Now let us see the realization of this circuit. There may be different possible realization of differential amplifier, but here we do have a very basic realization of differential amplifier I should say it is basic, but still it is I should say practical circuit.

Implementing a differential amplifier can be done using various approaches. Basic realizations might involve transistors and resistors, but practical designs can employ either BJTs (Bipolar Junction Transistors) or MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors) depending on the desired performance characteristics.

Just as building a house can start from different foundations - like wood, concrete, or steel - designing a differential amplifier can also start with different base components depending on the desired stability, efficiency, and cost.

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