85.2.5. Common Mode Rejection Enhancement

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. 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

This chunk introduces the basic circuit configuration used in the study of common mode rejection. It describes a common source amplifier where a diode-connected load is utilized. The circuit includes transistors where specific labels (M1, M7) and connections (to DC voltage VDD) are mentioned. The text emphasizes that at this point, a small signal is superimposed on a DC bias. It’s crucial to note that this setup allows us to analyze how the circuit responds to small input signals in addition to the constant DC voltage.

Think of this circuit like a water pipe system. Just as a water source provides a constant flow of water (DC bias) while also allowing for variations in water pressure from a hose connected (small signals), this circuit maintains a steady voltage while handling fluctuating input signals.

Now, if you analyse this circuit or if you draw the small signal equivalent circuit, you can find that the voltage here v = ‒ g × . So, this into whatever input signal we are applying v . In fact, as I said the voltage here also it will be same. So, we may say that v under common mode operation is also = v and that gives us v = v common mode.

This section explains the process of small signal analysis by transitioning to the small signal equivalent circuit. It states that under this analysis, the output voltage (vo) can be represented as a function of the small signal gain (g), depending on the input small signal (vin_c). The relationship implies that during common mode operation—where both inputs are treated the same—the output for both nodes is equal. This reinforces the concept of common mode signals and how they affect the circuit's response.

Imagine tuning a radio to hear a specific station (the common mode signal). In this analogy, the adjustments in volume can reflect how much of the input signal you're amplifying. Just as the radio adjusts its output for a steady stream of music, this circuit adjusts its output voltage based on the uniform input signals.

Now, again you can see here because the resistance now it is getting changed. So, this A it is much smaller than the previous common mode gain. So, to summarize what we can see the advantage of having this current mirror in the load part.

This chunk discusses the impact of changing resistance in the circuit. It indicates that the gain associated with common mode signals (denoted A) becomes smaller than what it was previously. The introduction of a current mirror in the load improves the circuit by allowing better control over the gain, resulting in enhanced performance in rejecting unwanted common mode signals.

Consider a speaker using equalization to lower background noise while maintaining sound quality. Similarly, by using a current mirror, the circuit is fine-tuning its response to lessen the effects of common mode signals, letting the desired signals stand out.

So, this is one consequence of having this active current mirror load and the gain at this point, gain at this point if I say that . So, that is becoming g ( ). In fact, you may say g or g both are same. So, we can say that differential input to single ended output gain it is same as whatever the gain earlier you obtained.

This portion explains a critical consequence of incorporating an active current mirror load—it provides a means of keeping the differential input to single ended output gain unchanged despite modifications in the circuit. The paragraph implies that while adjustments are made to the current mirror's configuration, the effective gain derived is still consistent with prior calculations, reinforcing the reliability of the current mirror setup.

Think of this as using a dependable translator who maintains the meaning of speech regardless of how fluently it is spoken. The current mirror acts as such a translator, keeping the core message (or gain) intact, despite various changes in the input's format.

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.

This final chunk reveals the importance of the current mirror in enhancing common mode rejection in the circuit. It emphasizes the significance of practical applications in circuit design, potentially leading to improved audio systems, communication equipment, and more, all requiring effective signal processing capabilities. The instructor suggests that further understanding will continue in a subsequent class with numerical examples to highlight the concepts discussed.

Visualize this as tuning out static noise while listening to a podcast during a morning commute. Just as filters help isolate the voice you want to hear, current mirrors in this circuit help to focus on the desired signal while minimizing background interference, greatly improving communication clarity.

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