83.5.1. Common Emitter Amplifier with Active Load

So, to start with let me consider common emitter or common source amplifier let me see which one I do have yeah and to start with I do have common emitter amplifier with active load. So, this is the main circuit where transistor-1 it is working as amplifying device, input signal we are feeding through the coupling capacitor and then R it is providing bias current I . So, this is more like a fixed bias common emitter amplifier.

The common emitter amplifier is a basic amplifier circuit commonly used in electronics. It utilizes a transistor (transistor-1 in this case) as the main amplifying device. The input signal is fed through a coupling capacitor, ensuring that any DC component of the signal is blocked while allowing AC signals to pass through. Bias current, provided by resistor R, is essential for the transistor to operate in the active region, where it can amplify the input signal effectively. This setup is typical in fixed bias configurations, ensuring consistent performance.

Think of the common emitter amplifier as a microphone at a concert. Just as the microphone picks up sound (the input signal) and sends it to an amplifier (the transistor), which then boosts the sound for a large audience, the common emitter amplifier takes a small electrical signal and makes it larger for further processing.

At the collector instead of connecting the passive load here we do have Q4 which is working as active load. In fact, we have discussed about the active load circuit considering an independent bias here, but instead of having a bias this may be better one particularly if we have say 2 NPN transistors they are identical.

An active load, implemented through another transistor (Q4 here), replaces a traditional resistive load. This approach increases the output impedance of the amplifier, which enhances the gain and efficiency of the amplifier. If both transistors (Q3 and Q4) are identical, they can mirror currents accurately, ensuring consistent operation. The use of active loads is particularly advantageous because it allows for better integration within integrated circuits as compared to passive loads.

Imagine a team where each member has a specific skill set. If two engineers (transistors) are identical in skills and work together (as active loads), their productivity (output impedance and gain) will be higher than if each was working independently. This collaboration creates a more powerful outcome, just like the active load enhances the performance of our amplifier.

Now, by the virtue of this current mirror and if I assume that this Q3 and Q4 they are identical then this current I it is getting mirrored here. So, the current flowing through transistor-4 which is also equals to I . I should say approximately equal to because as you may recall that because of the base current loss there will be a small difference between this current and this current.

In circuits with active loads, current mirroring plays a key role. It involves another transistor (Q4) taking the reference current set by transistor Q3 and providing an identical current flow. This mirroring process allows for precise control of the currents in the circuit. However, it's essential to note that due to base current losses (the small current required to keep a transistor 'on'), the mirrored current will not be exactly equal, but very close. This information is critical when designing high-precision circuits.

Think of a coach signaling two players to mimic each other's moves in a choreographed dance. Although they might not always move exactly the same due to slight differences in their positioning (base current losses), their overall movements are closely aligned, allowing for a synchronized performance, just like the mirrored currents in the amplifier circuit.

Here you might have observed one important changes in the current mirror, if you see this Q3 and Q4 they are forming a current mirror, but type of transistors here we have used it is PNP.

In this circuit, PNP transistors are employed for the current mirror, landing on the principle that both the transistor types (NPN and PNP) can be used based on the design requirements. The essential condition for effective current mirroring is that the two transistors (Q3 and Q4) operate under similar conditions. Ensuring that they are identical in characteristics is crucial, especially in integrated circuits where component matching can become a significant challenge.

Consider identical twins who work together in a business. For optimal performance, both twins should have the same skills and qualities. If one has been trained differently, the cohesiveness of their work may falter. Similarly, ensuring that PNP transistors are identical in properties is crucial for the current mirror to function effectively in enhancing performance.

So, once we have these two currents are equal, then to get the gain you can. So, the voltage gain which is defined by . So, that can be obtained by considering its small signal equivalent circuit.

The gain of the common emitter amplifier can be determined through a small signal equivalent model, which simplifies the analysis. The voltage gain is represented as the ratio of the output signal change to the input signal change. By analyzing the small signal parameters and equivalent resistances in the circuit, one can calculate how much the amplifier boosts the input signal, allowing for efficient and effective design and use.

Imagine a magnifying glass that can make tiny text appear larger. The lens does this by focusing and amplifying the light in a specific way. Similarly, in our amplifier, the small signal equivalent circuit helps us understand how the input signal can be magnified into a larger output signal, enabling clearer communication of information.

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