89.2.1. Example of Differential Amplifier with Current Mirror

Yeah. Welcome back to our next example. So here we do have the differential amplifier and the load of course, it is active load, but internally we do have current mirror. So, what we are expecting here is transistor-7 DC wise it will be mirroring its own DC current into transistor-8 and that is good.

In this introduction, we learn about a differential amplifier that has an active load and uses a current mirror. The key feature is that transistor-7 keeps its DC current consistent, which is mirrored to transistor-8. This mirroring is important for the balance and efficiency of the circuit.

Think of a group of people running a relay race. Each runner (transistor) has to pass the baton (current) to the next runner while maintaining their speed (current value). The better they mirror each other's speeds, the smoother the race will be.

In fact, we want this current should also be the same as the one from a balance point of view. So, we do not have to put any external circuit to bias here and in addition to that, this transistor-7 may also mirror signal current, in case if we have say g_v current coming here at the collector.

The goal of current mirroring is to maintain a stable operating point without needing additional circuitry for biasing. Furthermore, transistor-7 does not only mirror the DC current, it can also adapt to signal currents, which enables the amplifier to function dynamically.

Imagine two musicians playing in harmony. If one adjusts their volume based on the music's dynamics (like the signal current), the other musician can adjust similarly, creating a balanced performance without additional adjustments.

So to start with, we do have to find the DC currents through all the transistors. Again the value of this resistor is 11.4 k, so that makes this current = 1 mA.

Calculating DC currents involves analyzing resistor values and applying Ohm's law. With a resistor valued at 11.4 kΩ, the current flowing through the circuit is determined to be 1 mA. This current level is crucial for the functionality of the amplifier.

Think of a water system where the resistor acts like a valve controlling water flow. By adjusting the valve (resistor), you can ensure just the right amount of water (current) flows through to keep everything running smoothly.

So, the voltage here is decided by the 12 V ‒ 0.6 V, so the voltage here is 11.4 V. In fact, it can be shown that the DC voltage at this point, assuming Q1, Q2, Q7, Q8 are identical, is also 11.4 V.

The effective voltage at certain points in the circuit is determined by the supply voltage minus any drops due to components (like the 0.6 V drop across a transistor). Assuming all transistors are identical, the same voltage will be present across them, allowing uniform performance.

Consider how a team uniform affects the players' performance on a field. If everyone wears the same uniform (identical transistors), they can work together seamlessly, resulting in better teamwork and efficiency.

So now our next calculation is to find differential mode gain and common mode gain.

Differential mode gain is a measure of how well the amplifier reacts to signals differing between its two inputs. This calculation helps evaluate how effectively the circuit amplifies the desired signals compared to any unwanted noise.

Imagine a microphone that captures sound from two different instruments. The differential mode gain helps the sound engineer determine how well they can amplify the desired music signal while minimizing background noise.

So, in summary, now to analyze this circuit we can just simplify and say that this resistance is r_o8. The common mode gain essentially is very small.

This part summarizes that the common mode gain is kept very low compared to the differential gain, thus ensuring better performance of the amplifier in processing desired signals while reducing the impact of noise and other unwanted signals.

Think of how a spotlight focuses on a performer on stage (differential gain) while the rest of the stage is dim (common mode gain). By minimizing the unnecessary light on the background, the audience focuses exclusively on the performance.

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