68.1.4. Example Circuit Analysis

Today we are going to continue Amplifiers with Active Load we have started this topic and today primarily we will be discussing about Numerical Examples.

In this section, we are focusing on the discussion of numerical examples related to amplifiers with an active load, which is a crucial aspect of circuit design. The aim is to understand how active loads can be utilized to enhance the performance of amplifiers, particularly in the context of practical applications. By engaging with numerical examples, students can apply theoretical concepts and gain hands-on experience in circuit analysis.

Think of this analysis like learning to cook. In cooking classes, you start by studying recipes (theory) and then move on to practical cooking (examples). In the same way, this lesson will allow you to apply your understanding of circuit theory in real-life scenarios.

The parameters of the two transistors for Q1 β is 100, for Q2 β is 200, just for a change we are taking different values.

In this example, the two transistors used in the amplifier circuit differ in their current gain (β), which is a critical parameter in determining their performance. Transistor Q1 has a β of 100, while Q2 has a β of 200. This difference can affect how current flows through the circuit and highlights the need to balance the collector currents of both transistors for optimal performance.

Imagine a team where each member has different strengths (like different β values). If one person (transistor) is stronger (higher β), they might need to take on less work to ensure the entire team functions effectively. Balancing the workload ensures the team performs at its best.

Instead of having a passive load, we like to use active load, and the purpose of this one we have discussed is to enhance the voltage gain.

The main idea of using an active load instead of a passive one is that active loads can significantly increase the voltage gain of an amplifier circuit. This occurs because active loads dynamically adjust to changes in the circuit, providing better performance compared to static passive loads, which can limit gain. The enhancement in voltage gain is crucial for applications where signal amplification is needed.

Think of the difference between a standard microphone and a powered microphone. The powered microphone uses its own power source (active load) to amplify sound, helping it capture audio better than a standard microphone that may not have any amplification (passive load).

To achieve balanced collector currents, we make the base current of transistor-1 and base current of transistor-2 different by adjusting resistors.

In this analysis, balancing the collector currents of both transistors is crucial for maintaining stability in the circuit. By adjusting the base currents through specific resistor configurations, we ensure that even with different β values, both transistors draw equal collector currents. This adjustment is vital for the proper functioning of the circuit, as mismatched currents can lead to distortion or failure in amplification.

Consider a relay team where each runner (transistor) needs to pass the baton (current) equally for the team to win. If one runner (transistor) is faster (higher β), the coach (designer) might adjust their handing of the baton (base current) to keep everyone synchronized.

Note that in this equation we have not considered the effect of early voltage.

When analyzing transistor circuits, the Early voltage is a significant concept that helps in understanding how the output characteristics of a transistor change with varying collector voltage. Though not included in the initial calculations, it’s crucial to factor it into the more detailed analysis of transistors, as it can impact output voltage and current characteristics significantly.

Think of the early voltage like the height of a tree. Just as a tall tree can influence how high birds can fly (current), the Early voltage affects the performance of transistors, as it defines their behavior under different operating conditions.

To find the DC voltage at the output node VOUT, we can compare β I × (VCE1) = β I × (VCE2).

This calculation is performed to derive the output voltage by comparing the effects of both transistors in the active circuit. By equating the currents influenced by their respective Early voltages, we can accurately determine the output voltage, which informs us of the operational state of the transistors within the circuit.

This process resembles balancing two scales. If both sides have equal weights (currents), they will remain balanced (stable voltage). If one side is heavier, adjustments must be made to keep things on an even keel.

From that, we can calculate the small signal parameters of the transistors namely g and r.

Calculating the small-signal parameters allows for understanding how the transistors will respond to slight variations in input signals. These parameters are essential in predicting the performance under transient conditions, particularly in amplifier design, where small signals are frequent.

Imagine being on a tightrope. The small signal parameters help us understand how even tiny shifts in balance can impact our stability. By calculating these parameters, we can predict how the amplifier will behave when it encounters small fluctuations in the input signal.

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