49.1.1. Introduction to Common Base and Common Gate Amplifiers

So, today we are going to discuss about the common base and common gate amplifiers. Under that we will be discussing about basic operation biasing, analysis and design and numerical examples will be covered later in the next class.

In this introduction, we focus on two important types of amplifiers: common base (CB) and common gate (CG) amplifiers. Both are configurations commonly used in BJT (for CB) and MOSFET (for CG) technologies. The session will cover fundamental concepts such as how these amplifiers operate, how they are biased, their analysis, design considerations, and will also include numerical examples in future discussions.

Think of common base and common gate amplifiers as different types of bridges that help connect two roads (input and output), allowing traffic (signals) to flow smoothly without interruptions. Each type of bridge has its own design (configuration), which is suited for specific types of vehicles (signal types).

Now, we are entering into the third configurations. So, let us see what the basic motivation is as I said... the loading effect degrades the overall gain starting from the primary input to primary output.

The motivation for using common base and common gate amplifiers arises from limitations observed in previous amplifier configurations, like common emitter and common source. When these amplifiers are cascaded, the interaction between their output and input impedances (loading effects) can degrade the amplifier's performance by lowering gain and bandwidth. Common base and gate configurations serve as effective buffers that minimize these loading effects by providing high output impedance and low input impedance.

Imagine a series of water pipes (representing cascaded amplifiers) where each pipe connects to the next. If one pipe is too narrow (high loading effect), it restricts water flow (signal), leading to lower pressure (gain) in the following pipes. A wider section (common base/gate) is added to ensure the water flows smoothly and pressure is maintained across the system.

So, let us see what the requirement there is... the requirement here it will be complementary in nature.

Current mode buffers, specifically common base and common gate amplifiers, have unique operational requirements compared to voltage mode buffers. These include needing high output resistance and low input resistance to ensure optimal current throughput and minimal signal attenuation. Where voltage mode demands high input impedance, current mode typically prefers low input impedance to avoid excessive loading on the preceding stages.

Consider a sports relay team where the incoming runner (signal) must pass the baton (signal) smoothly to the next runner. If they hold on too tightly (high input impedance), it slows down the process, whereas a more relaxed grip (low input impedance) facilitates faster transitions. Both runners need to understand the optimal way to pass while maintaining speed (current flow) effectively.

The input here it is the emitter... the voltage here at the input node or at the source node it is having say sinusoidal form like this.

In elucidating the basic operation, the common base amplifier takes its input at the emitter. If a small AC signal is applied while maintaining a DC bias, the amplifier can produce an amplified version of the input signal at the collector. An important characteristic of this configuration is that it offers low input impedance and good voltage gain, often resulting in the output being in phase with the input.

Visualize this configuration as a conversation between two friends where one person (input) shares a story (signal) quietly in a crowded room. As this friend speaks, they adjust their volume (small AC signal) based on the room's noise, and the second friend (output) can hear and amplify that voice for everyone to listen (output amplification).

To start with the practical bias of the common base amplifier... if we consider input resistance of this circuit ok.

Biasing in the common base amplifier is crucial as it helps establish the DC operating point necessary for efficient signal amplification. Proper biasing ensures that the active devices operate within their linear region, which is essential for minimizing distortion and ensuring maximum signal fidelity. Both active and passive biasing schemes can be applied, each with its characteristics and implications on circuit performance.

Just like a gardener (biasing) carefully waters (provides DC supply to the amplifier) a plant (amplifier) to ensure optimal growth, improper watering (biasing) can lead to stunted growth or even wilting (poor amplifier performance). A gardener must know the right amount to provide.

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