87.5. BJT Version of Current Mirror

Finally, the BJT current mirror is enhanced by adding a cascode transistor, which increases the output resistance and improves the circuit performance.

In this step, we introduce the concept of a cascode transistor. The cascode transistor is added to the current mirror, which helps to increase the output resistance of the current mirror configuration. By doing so, the circuit becomes less sensitive to variations in the output voltage, thus ensuring that the output current remains stable.

Think of the cascode transistor as a bouncer at a concert. Just like a bouncer can control and stabilize the crowd inside, ensuring that the concert runs smoothly, the cascode transistor helps stabilize the current mirror’s output.

To operate effectively, it's important to ensure that the bias voltage V_BIAS is maintained at a sufficient level, typically higher than the gate-source voltages of the transistors in the current mirror setup.

Here, the focus is on determining the appropriate bias voltage V_BIAS for the current mirror. If the bias voltage is insufficient, the transistors may not attain saturation, which can lead to non-ideal operation. Therefore, calculating the appropriate value of the bias resistor ensures that the transistors function properly, allowing the circuit to mirror the reference current accurately.

Imagine a plant that needs sunlight to grow. If it doesn’t get enough sunlight (analogous to bias voltage), its growth will be stunted. Likewise, the transistors need enough bias voltage to operate effectively.

The current flowing through each branch needs to be calculated based on the input conditions and the configurations of the transistors involved.

In this portion of the content, we calculate the current flowing through the branches of the BJT current mirror. Using parameters such as transistor characteristics and bias conditions, we derive the current flowing through the output transistor. It's important because a precise control of this current ensures that the mirror behaves as expected under different conditions.

Think of it like water flowing through pipes in a home. If we know the layout (the transistor configuration) and the pressure (the bias conditions), we can accurately predict how much water (or current, in this case) flows through each pipe.

For the BJT current mirror, output resistance can be calculated based on the transistor properties, indicating how well the current mirror can maintain output current despite changes in voltage.

Output resistance in the BJT current mirror is crucial because it determines how stable the output current is under varying output voltages. High output resistance means that changes in voltage at the output will not significantly affect the output current, a desirable trait for current sources in circuit design. We calculate this by taking into account the transconductance and output resistances of the transistors.

It's similar to a stabilizing weight attached to a seesaw. If one end goes up (voltage change), the weight helps to keep the other end (current) from following suit. This keeps things balanced!

Using a cascode configuration in the BJT current mirror significantly enhances its performance and reliability for real-world applications.

The inclusion of a cascode transistor amplifies the advantages of the BJT current mirror by improving its output resistance and making it less sensitive to variations in supply voltage. This makes the circuit more robust in practical applications, allowing for consistent performance in different operating conditions.

Think of the cascode BJT current mirror as a high-performance sports car. The added engineering (cascode transistor) makes it faster and more reliable under various driving conditions, similar to how the cascode improves the current mirror’s reliability.

--