85.1.5. DC Voltage Consideration

this part and if we analyse 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. It is just a just a matter of representation and at this point we are giving the small signal along with the DC.

The section begins by introducing a circuit resembling a common source amplifier. In this configuration, one transistor (M) is stressed between its gate and source (denoted as g), indicating that it is biasing the circuit. The load of the circuit is specified as diode-connected, which allows for current mirroring. By connecting these transistors in this way, the circuit can handle both small signal variations on top of a steady DC voltage, making it versatile for operations requiring amplification.

Think of a common source amplifier as a restaurant's ordering system. The DC signal (the established food orders) can be likened to regular customers placing their orders at the counter. The small signals represent the occasional last-minute special orders from customers who want something extra. The ordering system (the amplifier) efficiently processes both regular and special requests simultaneously, ensuring all orders are served promptly.

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

In this chunk, the text dives into the analysis of how the circuit behaves under small signal conditions. It states that there is a biasing condition at a particular node in the circuit contributing to the output, which is affected by the small input signal (v_in_c). By drawing the small-signal equivalent circuit representation, one can find that the output voltage (v_o) is related to the input signal by a negative gain (‒g), demonstrating that for inputs changes, the output reacts inversely to them, a characteristic of amplifiers.

This can be likened to a volume control on a radio. When you turn the volume knob slightly (small signal), the speaker's output (sound) can react significantly in the opposite way, either amplifying or decreasing the sound based on the negative feedback mechanism established within the system.

So, we may say that v_1 under common mode operation is also = v_2 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 = ‒ g ×.

The discussion shifts to common mode operation, where both outputs (v_1 and v_2) change simultaneously, thus behaving equivalently. This equivalence yields a common mode output voltage (v_common_mode). The corresponding common mode gain, denoted as A''_o_c, shows how the circuit's response to common mode inputs leads to amplification that is again inversely related to the input through the factor ‒g. This concept is essential for designing circuits that effectively reject common noise present in both signals.

Imagine two friends on a video call. If both friends speak at the same time (common mode), the software may struggle to filter their voices, leading to confusion. Similarly, common mode gain in electronics refers to a circuit's ability to amplify both signals equally, often making it necessary to improve its ability to focus only on the essential signals by rejecting these common disturbances.

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 it = ‒ divided by 1 plus sorry, one part we can remove so, × 2r.

In summary, the presence of a current mirror in the circuit acts as an advantageous element. The effect of the current mirror significantly reduces the common mode gain, which aids in distinguishing useful signals from noise. The mathematical relationship described indicates how the gain loss is compensated by certain factors related to the circuit's resistance, thus providing a better operational efficiency overall.

Imagine a teacher using a microphone in a noisy classroom with many students talking. By using a special filtering device (analogous to a current mirror), the teacher can reduce the background noise (common mode gain) and focus on the primary speaker (the desired signal), creating a clearer communication environment.

So, this helps to improve the common mode rejection drastically. I think that is all to cover. In the next class we will be talking about numerical examples.

Finally, it is emphasized that the improved common mode rejection rate enhances the overall performance of amplifiers in circuit design. This element of current mirrors is integrated into various amplifier stages to optimize their functioning in practice, making designs more reliable and efficient. The section concludes with a brief on what to expect in upcoming discussions, hinting at a transition towards applied numerical examples to solidify understanding.

Think of when you put on noise-canceling headphones to better focus on your music while ignoring external noises. Similarly, when designing amplifiers using current mirrors, the ability to ignore unwanted signals significantly enhances the clarity and quality of the intended audio output.

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