77.3. Differential Amplifier Realization

So, dear students welcome back to our NPTEL online certification course on Analog Electronic Circuit. Today’s topic of discussion is: Differential Amplifier and we will primarily focus on Circuit Analysis.

A differential amplifier is a key component in analog circuits used to amplify the difference between two input signals while rejecting any signals that are common to both inputs. This section of the course reintroduces differential amplifiers with a focus on understanding their circuit analysis.

Imagine a referee in a sports match who only listens to the cheers and jeers of the crowd to make a call. If there’s a loud cheer from one side and silence from the other, the referee pays attention; however, if both sides cheer equally, the referee ignores it. This is similar to how a differential amplifier works.

Specifically, for two modes of operation: differential mode and common mode operation and their corresponding gain.

Differential amplifiers operate in two modes: differential mode, where the amplifier responds to the difference between the two input signals, and common mode, where the amplifier responds to inputs that are common to both inputs. The gain of a differential amplifier can vary significantly based on which mode it is operating in, hence understanding these modes is crucial for effective circuit design.

Think of a music mixer where one slider controls the volume for a specific track while another slider adjusts the overall level. Changing one track while keeping the others the same reflects the differential operation, while adjusting all tracks together reflects the common mode.

So, we shall start with small signal equivalent circuit of differential amplifier both; implemented by BJT as well as MOSFET.

The small signal equivalent circuit simplifies the analysis of the differential amplifier by focusing on the small variations around a DC operating point. This technique replaces the transistors in the circuit with their small signal models, making calculations of gain and response much more manageable.

Consider a car tire. When it is inflated to a certain pressure, it sits comfortably. Small changes in pressure represent small signal variations. The overall tire structure (equivalent circuit) can be analyzed without worrying about every little change, as long as we remain within that comfortable range.

And, then we may have a special case which is referred to as pseudo differential stimulus.

The pseudo differential stimulus involves applying inputs that simulate a differential input but may not account for all variables in the circuit. This method allows for a simplified analysis, especially in preliminary design phases, by assuming ideal conditions.

Think of rehearsal for a symphony orchestra. Musicians might practice together, adjusting their instruments to sound harmonious (the ideal conditions) without the pressure of a live performance. This allows them to identify adjustments needed before the actual performance, similar to preliminary tests in electronics.

We shall discuss about DC operating point analysis and then we shall talk about; the input DC voltage range over which devices are in proper region of operation.

Large signal analysis focuses on how the differential amplifier behaves when subjected to large input signals. This is crucial for understanding how real-world signals affect the circuit and determining the appropriate operating points to ensure linear operation without saturation or cutoff.

Imagine trying to measure the water level in a jug when it’s almost full. If you pour in too much water (large input), it overflows and you can’t accurately measure anymore. Conversely, with just the right amount of water, you can measure effectively without spilling.

We shall talk about the possible signal swing for a given circuit which referred as output signal swing.

The output signal swing of a differential amplifier determines the range of output voltages it can produce for a given input. It is important for ensuring that the amplifier operates effectively within specified limits, allowing for maximal output while avoiding distortion.

Think about driving a car on a racetrack. The track’s width defines how much you can maneuver (signal swing). If the track is too narrow, you can’t make sharp turns without leaving the path, similar to how an amplifier is limited in its range based on its design.

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