69.1.1. Multi-Transistor Amplifiers: Amplifier with Active Load (Contd.) – Numerical Examples (Part B)

We are talking about CE amplifier with active load and passive load. We discussed and compared their performance. Before we go to CS amplifier, we must make a note of the CE amplifier and the circuit we have discussed particularly its stability issue of its operating point.

The Common Emitter (CE) amplifier is widely used in electronic circuits for amplifying signals. It is important to understand the difference between amplifiers with active loads (like transistors that provide gain) versus passive loads (like resistors). Before diving into common-source (CS) amplifiers, we first revisit the stability of the operating point in CE amplifiers. Stability here refers to how well the amplifier maintains its performance (like output voltage and gain) despite changes in component values or environmental conditions.

Imagine you have a delicate balance scale. If you place various weights on one side, it can get unbalanced. Similarly, if the components in a CE amplifier change (like due to temperature variations), it can affect the balance of the amplification, potentially leading to distortion or unwanted noise.

In case say the early voltage of the two transistors is not consistent, it will directly affect the operating point here. For example, if the early voltage of transistor-2 got changed from 100 to 200, the DC output voltage will also change.

The Early voltage is a key parameter in transistors that impacts their performance, specifically how they behave under different operating conditions. If the Early voltage changes, it alters the relationships between the currents and voltages in the circuit. In this case, increasing the Early voltage of one transistor affects the overall output voltage of the amplifier. If transistor-2's Early voltage increases, both devices can still operate in the active region, but the output voltage level will shift.

Consider a water reservoir system where the water inflow rate changes due to a valve adjustment. Although the system can still function with the new inflow rate, the water level (akin to voltage output) will change depending on how the valve is set. Similarly, the Early voltage adjustment changes how the CE amplifier 'handles' its output.

If β is getting changed, the output voltage will be heavily affected. For example, if β changes from 200 to 180, the output operating conditions such as voltage will experience a significant drop.

Beta (β) is the current gain of a transistor, and variations in β can severely impact the performance of the CE amplifier. If the assumed β drops, it means that the transistors cannot amplify the input currents as effectively, leading to a decrease in the output voltage. This shift can push the transistor out of its operating region, leading to saturation or cutoff conditions, where the amplifier stops functioning properly.

Think of a team of runners where each person has a different ability (like their racing skill). If the fastest runner (the one with a high β) pulls out of the race (let’s say their performance decreases), the overall team performance suffers significantly. Similarly, in electronics, if the β of a transistor decreases, the amplifier's overall performance is impacted.

To achieve a stable bias in the circuit, instead of connecting the bias resistor to ground, we are connecting it to the output node, creating a feedback loop.

Stability can be enhanced in amplifier circuits by utilizing negative feedback, where a portion of the output voltage is fed back to adjust the input. In this case, connecting the bias resistor to the output helps to stabilize the output voltage against variations in other components. If the output voltage tends to rise, the feedback can reduce the base current, helping to regulate the transistor's operation and maintain a steady output.

Imagine a temperature-controlled heating system. If the room gets too warm, the thermostat reduces the heat output. This feedback mechanism ensures the room temperature remains stable, similar to how this circuit design ensures the amplifier output stays within desired limits despite varying component parameters.

In conclusion, we have extensively discussed the performance differences between CE amplifiers with active versus passive loads. The discussion also touched on stability solutions addressing how output remains insensitive to process variations.

The section wraps up by highlighting the importance of active load in enhancing amplifier performance through better gain characteristics and reduced sensitivity to variations in component properties. Understanding the inherent stability issues in CE amplifiers with active loads compared to passive loads is key for designing reliable circuits.

Consider the difference between a modern car and an older model. Modern cars have various automated systems that ensure they adjust to road conditions and maintain performance efficiently—akin to how active loads and feedback ensure amplifier stability and performance. In contrast, older models might struggle with these conditions, similar to how passive loads lack the adaptability to varied conditions.

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