83.3.1. Common Emitter and Common Source Amplifier

In this section, we will explore the common emitter and common source amplifiers, including their configurations and how a current mirror can be applied as an active load in these circuits.

Common emitter amplifiers use a bipolar junction transistor (BJT) to amplify voltage, while common source amplifiers use a metal-oxide-semiconductor field-effect transistor (MOSFET) for the same purpose. The main difference lies in the types of transistors used, but both configurations aim to achieve similar amplification objectives. A current mirror can be used in these types of amplifiers as an active load, allowing for better performance compared to passive loads.

Think of a common emitter amplifier as a team of horses pulling a wagon (the signal) uphill. The BJT acts like a strong horse that can pull the wagon over difficulties; similarly, a common source amplifier works with a different kind of horse (MOSFET) for better efficiency. The current mirror serves as extra assistance that allows the horse to pull the load more effectively, much like how a team of horses can share the workload.

The common emitter amplifier includes a transistor (Q1) that serves as the main amplifying device. The circuit typically connects a bias resistor (R1) to provide the necessary bias current (IB). Instead of a passive load, an active load (transistor Q4) can be connected to improve output characteristics.

In a common emitter amplifier, the signal input is applied through a coupling capacitor while the bias resistor ensures that the transistor operates in its active region. The choice between a passive and an active load significantly impacts the amplifier's performance. Using a transistor as the load (Q4) enhances the gain and linearity of the amplifier setup compared to a resistor, as it can mirror the current more effectively.

Imagine cooking a meal with a single pot (passive load) versus using multiple pots that can heat food more thoroughly (active load). The common emitter amplifier with an active load works like multi-tasking in cooking, allowing for more efficient cooking (amplifying) without burning food.

When utilizing a current mirror in this circuit, if transistors Q3 and Q4 are identical, the current through Q4 is nearly equal to that through Q2, allowing for accurate current matching and improved performance.

In a well-designed common emitter amplifier, matching currents between transistors is crucial for stable and predictable performance. When transistors Q3 and Q4 are identical, the current flowing through them can be assumed equal. This is important because it minimizes discrepancies caused by variations in transistor characteristics, ensuring that the amplifier operates as intended without degradation in performance.

Think of identical twins working together on a project. If both twins have the same skills (identical transistors), they can coordinate their efforts easily, ensuring that tasks are completed efficiently and consistently without miscommunication or errors.

The voltage gain in a common emitter amplifier can be derived by analyzing its small signal equivalent circuit, ultimately leading to a relationship expressed as A = -gm (ro1 || ro4).

The small signal equivalent circuit helps analyze the behavior of the amplifier under small input signals. The transconductance gain (gm) describes how effectively input voltage changes produce output current changes, while the output resistances (ro1 and ro4) determine how much voltage the amplifier can deliver. By combining these elements, we derive the voltage gain relation of the common emitter amplifier.

Consider a speaker system where the gain is like the volume control. The more efficient the speakers are at converting electrical signals into sound, the louder the music plays (voltage gain). Higher efficiency (gm) along with better speaker design (ro) results in a louder output while ensuring clarity without distortion.

In a common source amplifier, a similar approach is used where the MOSFET acts as the amplifying element, typically configured with a biasing network for stable operation.

Similar to the common emitter amplifier, the common source configuration utilizes a MOSFET to amplify input signals. This design places importance on a biasing network that keeps the MOSFET in its active region. Ensuring proper biasing allows the common source amplifier to achieve optimal functionality, similar to a BJT version.

Think of the common source amplifier as a reliable waiter in a busy restaurant. The waiter (MOSFET) needs to operate efficiently (well-biased) to serve food (amplify signals) properly to avoid delays (signal loss). Proper training and experience allow the waiter to excel, ensuring the dining experience is enjoyable for all (consistent performance of the amplifier).

--