83.1. Analog Electronic Circuits

Dear students welcome back to our online certification course on Analog Electronic Circuits, myself, Pradip Mandal from E and EC department of IIT Kharagpur. Today’s topic of discussion is Usage of Current Mirror. So, we have started the current mirror circuit and today we will be talking about its application.

In this introduction, Professor Pradip Mandal welcomes students back to the course. He emphasizes the focus for the day, which is the application of current mirrors—a fundamental component in analog electronic circuits. Understanding how current mirrors work and their applications is crucial for students to enhance their knowledge in electronics.

Think of a current mirror like a photocopy machine that can produce duplicates of the same document. Just as a photocopier takes one original document and creates many copies, a current mirror takes an original current and produces an identical current in another part of the circuit.

Today we are going to discuss more detail of the current mirror specifically its application and we do have different applications for this current mirror. One is in common emitter and common source amplifier based on whether it is BJT or MOSFET based implementation.

In this chunk, Professor Mandal highlights various applications of current mirrors in amplifiers such as common emitter and common source configurations. The implication is that these configurations can either use Bipolar Junction Transistors (BJT) or Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFET), illustrating the versatility of current mirrors. This versatility enables engineers to implement current mirrors in diverse scenarios where maintaining consistent current is critical.

Imagine a traffic light system that manages traffic flow in different lanes. Depending on whether you are driving a bicycle (common emitter) or a car (common source), the traffic lights adjust to ensure smooth transitions. Similarly, current mirrors adapt to various types of transistors to ensure optimal current regulation.

To understand or to appreciate the effect of common current mirror in amplifier standard amplifier where, we normally talk about the linearized circuit whether it is common emitter or common source or common collector or common drain or for that matter even for differential amplifier.

Professor Mandal introduces the small signal model, emphasizing its importance in analyzing how common current mirrors function within various amplifier configurations. The small signal model allows for simplification during calculations, which aids in determining the behavior of the circuit under small perturbations (or variations). This is a critical step in designing efficient amplifiers that utilize current mirrors.

Consider tuning a musical instrument like a guitar. Initially, the guitar is out of tune, but when you make small adjustments to the tension of each string, it slowly begins to sound just right. Similarly, the small signal model helps engineers make tiny adjustments to the current mirrors, allowing circuits to operate optimally.

We do have two possible situation. One is the current mirror may not be carrying any signal namely under DC condition what is the small signal equivalent circuit.

Here, the discussion shifts to two scenarios regarding the operational state of the current mirror. The first case is when the current mirror does not carry any signal, primarily existing under Direct Current (DC) conditions. In such situations, the small signal equivalent circuit simplifies to an open circuit, which means it does not influence the performance of the main circuit significantly. Understanding this helps in analyzing circuits more easily when no variations occur.

Think of an air conditioner that operates only when the temperature reaches a certain threshold. If the temperature doesn’t change, the air conditioner remains idle and doesn’t impact the overall climate of the room. In similar fashion, when the current mirror operates under DC conditions without signal flow, its effect is negligible.

On the other hand the small signal model for transistor-2 M , we do have g v and v as I said that it is same as v .

This chunk introduces a more complex scenario where the current mirror does carry an alternating current (AC) signal. In this state, the small signal model becomes relevant and analyzes how these signals affect the overall circuit performance. The circuit's response under AC conditions is crucial for various applications as it allows for amplification and processing of small voltage changes.

Imagine a dial on a radio that adjusts the music volume. When the dial is turned slightly, it changes the sound we hear. In electronic circuits, small variations in AC signals affect the output just like slight adjustments in the dial affect music volume.

So, let us see that. So, here we do have the current mirror circuit, we do have transistor-1 it is diode connected and the we do have reference current DC current that is getting mirrored into this branch through this transistor-2 and here we do have the application circuit.

Professor Mandal discusses the architecture of the current mirror circuit. In this architecture, a diode-connected transistor is used to establish a reference current that gets replicated in another branch of the circuit. This mirroring effect is pivotal for stabilizing the amplification process in the application circuits, whether those circuits are based on BJTs or MOSFETs. Understanding this configuration is essential in designing circuits that facilitate effective amplification.

Consider a puppet show, where one puppet master controls a string of puppets to make them all move in sync. The master (reference current) gets replicated through the strings (transistors), ensuring that all puppets (current flows) behave similarly and harmoniously.

So, we can say that whatever the current we do have if it is signal current. So, its effect in this application circuit can be considered by considering this voltage dependent current source which is the input signal here multiplied by.

In this concluding discussion, current mirrors' practical applications in circuits handling signal currents are further explored. The circuits need to consider the voltage-dependent characteristics of the current mirrors to analyze how they react to changes in input signals. This understanding allows engineers to optimize their designs for desired outcomes, such as improved gain and efficiency in amplifiers.

Think of a water fountain where the height of the water spout depends on how much water is pushed into it. Similarly, the performance of a current mirror in a circuit depends on the input signal, with the voltage source dictating the 'height' or output current seen at the output.

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