86.7. Improvised Current Mirror

Now we can go for more precision current mirror and as we can see here, if I consider finite values of β’s and then early voltage, the ratio instead of 1∶3, it is becoming different, slightly different though. There may be some precision cases, precision applications where this much of difference may not be still acceptable.

In this chunk, we introduce the concept of improvised current mirrors, emphasizing that they are designed to provide greater precision than standard current mirrors. The precision differences arise from accounting for variables like the transistor’s beta (β), which represents current gain, and the early voltage, which affects current flow in the saturation region. Instead of maintaining a current ratio of 1:3, adjustments can lead to slightly varying ratios, making the circuit more adaptable to specific precision requirements.

Think of a standard current mirror as a recipe for cookies that yields a specific taste. If you want the cookies to taste slightly different without completely changing the recipe, you might adjust the amount of sugar or vanilla. Similarly, improvised current mirrors tweak technical parameters like the transistor’s beta to achieve a more precise performance tailored to specific applications.

to take care of the base current loss, as we have said that we can have a Beta-helper circuit here, so that the current loss to the base of this third transistor which is much smaller than whatever the base current is going to Q and Q which is referred as Beta-helper.

The Beta-helper circuit is an innovative solution to address current loss in the base of the transistors within the current mirror. When transistors like Q1 and Q2 are used, small currents may escape into their bases, reducing the delivered output current. By using an additional transistor (the Beta-helper), which has a high current gain, the losses at the bases can be significantly minimized. This setup helps ensure more consistent performance from the current mirror, leading to precision without substantial current loss.

Imagine a water distribution system where some water leaks through small holes in the pipes. The Beta-helper acts like a repair team that efficiently seals the leaks, ensuring most of the water reaches its destination. In electronic circuits, it does this by preventing base current loss, thereby securing the total output current.

So, in the next slide we do have the Beta-helper circuit drawn for you. We do have Q here and as I said there this is approximately 0.6 and here also, it is approximately 0.6. So, we do have 1.2 V. So what we are doing here, to retain this current of 0.5 mA in this numerical example, we are readjusting this R to 21.6 kΩ. And that gives us the I current, it is the same as 0.5.

In this part, the Beta-helper circuit is illustrated, showing how it effectively maintains a consistent reference current of 0.5 mA. To achieve this, a resistor (R) is adjusted to a new value of 21.6 kΩ to accommodate the characteristics of the circuit with the Beta-helper included. This reconfiguration demonstrates a practical way to enhance the performance of the current mirror while still meeting design specifications.

Consider fine-tuning a musical instrument. Just like you might adjust the tension of guitar strings to achieve the perfect pitch, tweaking the resistor value in the Beta-helper circuit optimizes the flow of current, ensuring the electrical output matches what is desired in a precise manner.

So, you may recall the non-ideality factor in presence of the β the Beta-helper is.

This chunk discusses the concept of non-ideality factors when using a Beta-helper in the current mirror configuration. The non-ideality factor is crucial in evaluating how effective the Beta-helper circuit is in minimizing losses. It considers the ratios of current gains and other circuit parameters, allowing designers to understand how closely the behavior of the improvised current mirror can approximate the ideal conditions.

Think of a team preparing for a presentation. If everyone contributes their best quality, you get a flawless performance (ideal case). If one member struggles, the presentation suffers. A Beta-helper boosts performance by ensuring all team members stay engaged and effective, thus improving the overall output of the team, akin to refining the non-ideality factor.

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