87.2.2. Improvisation using Cascode Transistor

So in the next example, we are starting with MOSFET version and then after that we will go to the BJT version. And so, here transistor-3 it has been added on the main current mirror. So, the main current mirror it is constructed by M and M and whatever the reference current it is coming, it is mirroring here.

In this section, we introduce the concept of using a cascode transistor in a current mirror circuit. The example begins with a MOSFET configuration, where an additional transistor (transistor-3) is incorporated into the current mirror setup. A current mirror is an important component in analog circuits that duplicates the input reference current, providing a constant output current regardless of the variations in supply voltage or load conditions. By adding a cascode transistor, we improve the performance of the current mirror.

Think of a current mirror like a photocopier that needs to make many copies (outputs) of a document (input current). If the copier has a powerful assistant (the cascode transistor), it can make high-quality copies despite variations in paper quality (changes in output voltage), ensuring that every copy looks just right.

And then we do have this transistor M , to have meaningful operation of this circuit, we require its gate voltage V , denoted here. V it should be sufficiently high. So, that transistor-2 and maybe 3 also are in saturation region of operation.

For the cascode transistor setup to function properly, the gate voltage (V_BIAS) applied to transistor M must be high enough to ensure that both transistors-2 and -3 are working in their saturation region. The saturation region is crucial for transistors to operate efficiently as amplifiers or current sources, allowing them to maintain a constant current output. If the gate voltage isn't high enough, the transistors may enter the cutoff region, which can lead to poor performance of the current mirror.

Imagine trying to lift weights; you need to push your body hard enough (high gate voltage) to effectively lift the weights (ensure transistors stay in saturation). If your energy isn’t sufficient, you won't be able to lift those weights effectively, just as transistors become ineffective if they fall out of saturation.

Now coming to the different sizes of different transistors given here, we do have for transistor-1, we do have the it is 1 mA/V2. On the other hand for transistor-2 and transistor-3, we do have = 4 mA/V2. This is just for a change, we are using different values of the aspect ratio.

The efficiency of the current mirror can also depend on the sizing of the transistors involved. In this case, transistor-1 is set at a transconductance value of 1 mA/V², while both transistors-2 and -3 are set at a higher value of 4 mA/V². The aspect ratio, which relates the width and length of the FET, influences the current capabilities of each transistor. Different sizing allows for optimized operation under specific conditions, adjusting the electrical characteristics as needed to minimize discrepancies in mirrored current.

Think of transistors like vehicles; if you need to transport heavy loads (large currents), you would opt for bigger trucks (larger transistors) for efficiency, while lighter tasks can be managed with smaller cars (smaller transistors). Each choice of vehicle (transistor size) affects how well you can deliver those loads (current).

This gives us solution of this x which = −2. We have to consider only + term because, we are considering V GS − V th should be +ve.

The analysis of the current flowing from the circuit leads us to establish parameters for the operation of the system, particularly focusing on output resistance. Utilizing the cascode configuration generally leads to enhanced output resistance, which is crucial in maintaining terminal characteristics and ensuring good performance from the current mirror, keeping the mirrored output stable under load or supply variations.

Imagine a team of workers at a busy intersection managing traffic (current). If the signals (output resistance) are clear and well-coordinated, the traffic flows smoothly even when many cars come through (output variations). Using a cascode transistor helps ensure these signals remain uninterrupted, maintaining an effective flow of current.

So that means, we can say that this is equal to 1. So, that gives us V GS = V th + 1. ... this high output resistance. In other words, what should be the minimum value of this voltage so, that transistor-3 it is remaining in saturation region of operation?

Finally, the benefits of incorporating a cascode transistor include achieving higher output resistance and improved stability for the mirrored output. The example demonstrates how these configurations lead to reliable and consistent performance in current mirroring applications. On ensuring transistor-3 remains in saturation, the analysis highlights the threshold voltage and necessary conditions that preserve optimal operation, ultimately enhancing the performance of the current mirror.

Think of a restaurant that offers high-quality meals. If the kitchen (current mirror) has the right equipment (cascode transistor), it can consistently deliver delicious meals (stable current) even when the number of orders fluctuates. Keeping certain conditions intact ensures customers get the same great service no matter how busy the restaurant gets.

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