87.1.3. Indian Institute of Technology, Kharagpur

So dear students welcome back after the break. So, before the break we were talking about the numerical examples of the current mirror. As you can see here, we have used Beta-helper to improve the non-ideality factor due to the base current loss, namely, this base current loss.

In this chunk, the lecturer welcomes students back and summarizes what was covered prior to the break. Specifically, the discussion revolves around the numerical examples of the current mirror, which is an essential electronic circuit used to duplicate the current flowing in one branch of a circuit as a controlled output in another branch. The lecturer mentions the challenges of non-ideality factors in a current mirror, particularly the base current loss. This means that the ideal behavior, where the output current perfectly mirrors the input current, can be compromised by factors like the load, transistor characteristics, and current leakage.

Think of a current mirror like a printer that prints the same document multiple times. If the printer has an issue and doesn’t print perfectly, each copy may come out slightly different, similar to how the output current from a current mirror may vary due to various non-ideal conditions.

So in the next example, we will see the improvisation of the numeric current mirror circuit to take care of the non-ideality factor due to early voltage. To be more precise, we like to place one cascode transistor here, and that improves the output resistance of the current mirror.

In this step, the lecturer discusses the upcoming example, which aims to enhance the current mirror circuit. The primary improvement involves the use of a cascode transistor. The cascode configuration is designed to increase the output resistance of the current mirror, effectively reducing the output current variation in response to changes in output voltage. By placing a cascode transistor, the circuit can maintain a more stable current copy, even when the output load varies, thus addressing issues from the earlier example.

Imagine a water fountain that is supposed to maintain a steady flow of water regardless of wind effects. By adding a protective cover (similar to a cascode transistor), you ensure that the water flow remains consistent, much like how a cascode transistor helps stabilize the output current in a current mirror.

In the next example, we are starting with the MOSFET version and then after that we will go to the BJT version. So, here transistor-3 has been added to the main current mirror.

The lecturer specifies that the following example will switch focus from a MOSFET configuration of the current mirror to a BJT (Bipolar Junction Transistor) configuration. This shift allows students to understand how the same principles apply across different types of transistors while examining the effects of transistor characteristics on current mirroring. By adding transistor-3 to the main current mirror, the circuit in the MOSFET version gains additional functionality, making it easier to explain concepts like output stability and performance.

Consider how switching from one type of bicycle to another (like a mountain bike to a road bike) allows someone to adapt their riding style to different terrains while retaining the fundamental mechanics of riding. Similarly, understanding both MOSFET and BJT versions of current mirrors helps students apply their foundational knowledge to varying components.

To have meaningful operation of this circuit, we require its gate voltage V_BIAS to be sufficiently high so that transistors-2 and maybe 3 also are in the saturation region of operation.

This section highlights the importance of biasing in transistor circuits, especially for transistors that need to operate in the saturation region to perform effectively. The gate voltage, denoted as V_BIAS, must be controlled to ensure that the transistors function properly. If the voltage is too low, the transistors may not conduct as required, leading to poor circuit performance. Understanding these voltage requirements is critical for students designing and troubleshooting various electronic circuits.

Think of a light switch that needs to be at a certain level to turn on a light bulb properly. If the switch doesn’t provide enough power (or voltage), the light wouldn’t shine at its intended brightness. Similarly, providing the correct bias voltage ensures that the transistors remain 'on' and work effectively in the current mirror circuit.

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