44.6. Small Signal Equivalent Circuits

We are looking for a buffer circuit particularly for voltage mode amplification. The important performance matrices we are looking for are: output resistance should be low, input resistance should be high, input capacitance should be as small as possible, and voltage attenuation should not be very high.

The focus behind designing common collector and common drain amplifiers is to create a buffer circuit that maintains signal integrity when connecting two amplifier stages. In particular, we want to achieve a low output resistance, a high input resistance, and minimal input capacitance to prevent signal degradation. Additionally, we want the voltage gain around 1, meaning that the amplifier does not significantly decrease the input voltage.

Imagine trying to fill a bucket (amplifier) with water (signal) from a hose (previous stage of amplification). If the hose (output of the first amplifier) has a small opening (high output resistance), it becomes difficult for water to flow into the bucket, resulting in a slow fill (low voltage gain). A good buffer lets water flow freely without affecting the size of the hose, ensuring efficient filling.

The common drain configuration consists of a MOSFET with the gate receiving an input signal, the source as the output terminal, and the drain generally connected to DC voltage. The output at the source follows the input signal at the gate, making the voltage gain approximately 1.

In the common drain amplifier, the input signal is fed to the gate while the output is taken from the source. Because of the configuration, whatever signal is input to the gate, the source voltage closely matches it. This means the voltage gain of the common drain amplifier is effectively very close to 1. This operation is crucial for creating a buffer that doesn't distort the signal.

Think about a rubber band stretched between two fingers—one finger represents the input signal (gate) and the other the output (source). No matter how you pull the rubber band (input signal), the other finger’s height (output voltage) changes in almost the same way. This characteristic ensures that the output closely follows the input, thereby functioning effectively as a signal buffer.

Even with practical implementations having some conductance, the voltage gain remains approximately equal to 1. Both configurations retain the main advantage of not significantly attenuating the signal.

Despite real-world components introducing some resistance and conductance, the overall structure allows both common collector and common drain circuits to preserve the signal strength. The goal is not to amplify the signal but rather to ensure it remains unchanged, hence the emphasis on achieving a gain close to 1.

Think of these configurations like a professional phone call, where the call (signal) is transmitted clearly without distortion. Even if there are background noises (conductance), the person on the other side still hears you clearly, akin to how the output follows the input effectively in these amplifier configurations.

Using small-signal equivalent circuits helps to analyze input and output resistance in both common collector and common drain configurations. The expected behaviors are high input resistance and low output resistance.

By applying techniques such as Kirchhoff's Current Law (KCL) to small-signal models, we can derive expressions for input and output resistances. For common collector, high input resistance is achieved due to the transistor's characteristics, while low output resistance enhances efficiency in transferring signals.

Imagine a strong person lifting weights (input resistance) against a light weighted bar (output resistance). If the person is strong, they can handle more weights without straining, similar to how high input resistance allows better handling of incoming signals. Meanwhile, using a lighter bar lets other weights stack up easily without falling (efficient signal transfer from output).

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