85.1. Common Source Amplifier Analysis

This part and if we analyze this circuit it becomes like a common source amplifier, where the load it is diode connected. So, the circuit becomes like this. We do have g, we do have g connected to V and m1 m7 DD then this transistor half of the transistor we may call say M by 2.

In a common source amplifier configuration, the circuit has a specific structure where the output is taken from the source terminal of the transistor. In this analyzed circuit, the load is noted to be diode-connected, which influences how the amplifier behaves. The transistors mentioned (M1, M7) are part of this configuration, enabling signal amplification. Additionally, the reference to 'M by 2' suggests that we're simplifying our representation for better understanding.

Think of a transistor in a common source amplifier like a speaker cone. Just as a speaker cone moves back and forth to amplify sound, the transistor allows input signals to create larger variations in voltage output, thus amplifying the signal.

At this point we are giving the small signal along with the DC. Of course, this node it is getting the bias similar to this point and we like to know what will be the corresponding output here due to the small signal and this small signal it is v_in_c.

In amplifier circuits, small signal analysis is crucial for understanding how the circuit responds to small variations in input. Here, a small AC signal, denoted as v_in_c, is superposed on a DC bias. This DC bias ensures that the transistor operates in its active region, allowing it to amplify the small AC signal effectively. The output will reflect these small input variations or changes.

Imagine adjusting the volume on a radio. The station broadcasting at a specific frequency is akin to the DC bias, while the new music or announcements you hear at different volumes are like the small signals added to that baseline noise.

Now, if you analyze this circuit or if you draw the small signal equivalent circuit, you can find that the voltage here v_o2 = ‒ g × v_in_c.

When analyzing small signal response, we can derive relationships such as the output voltage (v_o2) in relation to the transconductance (g) and the input signal (v_in_c). This formula indicates that the output voltage is inversely proportional to the input signal multiplied by the transconductance, highlighting how effectively the amplifier translates input changes into output voltage.

Consider a dimmer switch in a room. The current input (the dimmer's settings) determines how bright the lights (the output) become. Just as the dimmer adjusts the voltage to the lights, the amplifier adjusts the output voltage based on input variations.

So, we may say that v under common mode operation is also = v and that gives us v_o = v_common mode. And hence the corresponding common mode gain if I say A''_o_c which is defined by A''_o_c = ‒ g × .

In common mode operation, where both inputs have the same signal, the output voltage maintains its relationship with the input. The common mode gain, A''_o_c, expresses how much of the common signal is amplified. This gain is crucial in understanding how well the amplifier can reject signals that are common to both inputs, ensuring that only differential signals are amplified.

Imagine you're in a crowded room trying to have a conversation. The common background noise (the common signals) can drown out your voice (the desired signal). A good amplifier can filter out that background noise and focus on the conversation, similar to how common mode gain works.

So, to summarize what we can see, the advantage of having this current mirror in the load part. What do we obtain? It is the common mode gain got drastically decreased and the gain at this point becomes g × .

Incorporating a current mirror in the load configuration of a common source amplifier improves performance significantly. It decreases the common mode gain, allowing the amplifier to better differentiate and amplify only the desired signals while suppressing unwanted noise. This enhancement in gain also leads to better overall amplification capabilities.

Think of a bouncer at a club who checks IDs before letting people in. The bouncer only allows valid patrons (desired signals) and keeps the unwanted guests (common mode noise) out, ensuring a more enjoyable experience for everyone.

Hence this voltage can directly be used to bias PMOS transistor in the subsequent stage. So, we can have this is V_DD and let you call this is transistor-9.

The DC voltage obtained from the common source amplifier can be used to bias subsequent stages in amplifier design, such as PMOS transistors. This allows for a seamless flow of signals, where one stage’s output helps in driving the next stage effectively and maintains amplification consistency across the circuit.

This is similar to passing a baton in a relay race. The successful runner (the first amplifier stage) hands off the baton (the DC bias voltage) to the next runner (the PMOS transistor in the next stage), ensuring the race continues smoothly and efficiently.

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