66.2.5. Common Emitter and Common Source Amplifier

Today we are going to talk about Amplifier with active load. This is also a special kind of multi-transistor amplifiers, where the passive load is replaced by a load utilizing a MOS or BJT transistor.

The section introduces the concept of active load amplifiers, specifically focusing on common emitter and common source amplifiers. An active load amplifier uses transistors instead of passive components for loading, providing unique characteristics and improved performance.

Think of a passive load as a traditional water tank filling at a constant rate. Now imagine using a variable pump (like an active load) to control how fast the water flows out, allowing you to adjust based on needs. The pump (transistor) offers better control compared to static pipes (passive loads).

This is a recapitulation of the basic operation of the CE amplifier. The main amplifying transistor operates when the input signal is fed with a DC component, ensuring it stays in the active region. A resistor, referred to as the load (R_C), is connected to the collector.

In a common emitter (CE) amplifier, the transistor amplifies an input signal while remaining in the active region, thanks to the DC bias provided by a resistor connected to the supply voltage. The resistor collects and converts the varying current into a voltage output, essential for amplification.

Consider a dimmer switch in a room. Adjusting the dimmer (the input signal) allows you to control the brightness (output voltage) of the light. The dimmer serves as the input, while the lightbulb's voltage reflects the current's activity.

The gain of the CE amplifier is limited by the voltage drop across the resistor divided by the thermal equivalent voltage (V_T).

As we analyze the common emitter amplifier, the maximum gain is restricted by the voltage drop across the load resistor. If this voltage drop approaches the supply voltage, the gain can theoretically be maximized, but in practice, it often falls short due to various constraints.

Imagine trying to fill a glass with water, where the maximum water level represents the supply voltage. The more water you try to pour (voltage drop), the harder it becomes to avoid spilling. The limit at which you can fill the glass (gain) is determined by how much water you can safely add without overflowing.

To enhance the gain, we can replace the passive load with an active load. This modification helps maintain the operating point while allowing for a steeper load line, which increases gain.

Swapping a passive load for an active load allows us to achieve a greater voltage gain without changing the supply voltage. Active loads enable better linearity and higher effective resistance, which results in improved amplifier performance over passive loads.

Think of trading an old car for a high-performance vehicle. The old car (passive load) can only carry a certain amount of weight (voltage gain), but the new sports car (active load) offers enhanced capacity and speed, allowing you to travel faster and more efficiently.

For common source amplifiers, the gain is similarly limited and typically much lower than that of the common emitter amplifier. The gain will principally depend on the slope of the I-V characteristic curve.

In a common source (CS) amplifier, the gain is usually less than 10, in stark contrast to the higher gains achievable with common emitter amplifiers. The performance is determined by how effectively the transistor can convert input voltage variations into output current changes.

Think of two types of water fountains: one with powerful jets shooting water high (common emitter amplifier) and a small trickler (common source amplifier). The first creates a showy display, while the second offers a subtle, quieter output—the differences reflect the amplifiers’ capabilities.

Both common emitter and common source amplifiers face voltage gain limitations with passive loads, and thus, alternative solutions (like active loads) offer potential enhancements.

Both types of amplifiers confront similar challenges regarding voltage gain when using passive loads. However, switching to active loads provides a path forward, allowing for improved performance and efficiency, especially when power supply constraints exist.

Just as a computer can become slow with outdated hardware, amplifiers also suffer with passive loads. Upgrading components (like switching to active loads) can drastically improve your device's performance, akin to boosting a computer’s processing power.

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