85.1.3. Common Mode Gain

This circuit becomes like a common source amplifier, where the load is diode connected. The corresponding output here due to the small signal is v_in_c.

In this segment, we introduce a circuit that resembles a common source amplifier, a common configuration in analog electronics. The load in this case is a diode connected to act effectively as a current source. This means that when a small AC signal is applied alongside a DC bias, the output is affected by this small input signal. The notation v_in_c represents the small signal input applied to the system.

Think of a simple water pipe system where a steady flow of water (the DC bias) is running through the pipe, and then a small splash of water (the AC signal) is added. The splash will create ripples that can be observed at the other end of the pipe, much like how the small signal influences the output voltage.

We may say that v_o1 under common mode operation is also = v_o2, which gives us v_common_mode = v_o1 = v_o2.

Here, we establish an important relationship in common mode operation: the output voltages v_o1 and v_o2 are equal. This means that for the common mode input, both outputs respond similarly, leading to a common mode gain definition, denoted as A'_C. This relationship is crucial to understanding how the circuit behaves when the same signal is applied to both inputs.

Imagine two identical speakers (outputs) playing the same sound (input). If both speakers are connected to the same audio system, they will produce the same volume and tone. This illustrates how the outputs in a common mode situation behave alike.

The corresponding common mode gain A'_C is defined by A'_C = ‒g_m1 * v_in_c.

The common mode gain A'_C measures the ability of the circuit to amplify the common mode signal. In this formula, g_m1 represents the transconductance of the first transistor in the circuit, and it directly influences how much the input signal will be amplified. The negative sign indicates a phase inversion that is typical in amplifier configurations.

Think of a microphone that picks up sound waves. Here, the sound waves represent the input signal, and the microphone's sensitivity (analogous to transconductance) determines how loud the amplified output is. If the microphone picks up sound well, the output is loud; if its sensitivity is low, the output is weak.

The active current mirror load decreases the common mode gain, and it can be expressed as A'_C = ‒(1 / (1 + 2r_o3)).

Introducing an active current mirror as a load in the circuit significantly impacts the common mode gain. The formula shows that the gain is reduced because of the resistance in the circuit. This reduction in gain is advantageous as it helps suppress unwanted common mode signals, thus enhancing performance in differential applications.

Consider a water flow system with a valve (the active current mirror). When the valve restricts flow too much, it can dampen sudden changes in water pressure (common mode signals), ensuring that only desired flows pass through, much like how the circuit manages noise and enhances desired signals.

The DC voltage can be directly obtained and can be used to bias PMOS transistor in the subsequent stage.

In the final part, we discuss how the DC voltage derived from the previous circuit stage can effectively bias a PMOS transistor in the following circuitry. This is significant as it allows the circuit to maintain functionality and signal integrity across different stages of amplification.

Imagine a relay system where a small current activates a larger switch. This small controlling current (DC voltage) influences the ability of the larger switch (PMOS transistor) to conduct larger currents in the next stage of the operation, enabling the entire system to function efficiently.

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