76.1.6. Common Mode Operation

So, let us see for common mode operation. Again coming back to the same circuit, we are applying say v here and v here. So, v ; however, of course, both of them with DC voltage V and; however, in this case we consider signal here equals to v and also the signal here it is v which means that it is a stimulus in perfectly in common mode operation in other words, we are assuming that a differential input v = 0.

In common mode operation, two identical signals are applied to the inputs of the amplifier, both riding on the same DC level. This means that if we add the same signal to both inputs, it does not affect the output designed to respond to differences between the inputs. Essentially, in common mode, the differential input (the difference between the two signals) remains zero.

You can think of this like two friends talking in sync at a coffee shop. If both friends state their agreement on the same topic (input signals are the same), it’s like applying common mode operations. However, if only one friend adds a differing opinion (one input signal changes), then the conversation will shift, akin to differential mode operations.

So with this operation before we make this connection again, we can analyze the circuit and then we can see that the signal coming at this node it is ‒ v × . So, likewise at this point this signal it is ‒ v × . Now, both this signal and this signal they are identical, but of course, they do have a different magnitude and the signal coming at the at this point.

When the common mode signals are applied, the output at the circuit node will be -v times some gain. Since the signals are identical, both outputs reflect the same behavior when analyzed. This is important for understanding how the differential amplifier will manage signals that are the same across both inputs, as it should ideally not amplify common signals.

Imagine you and a partner are both raising your hands to reach the same height simultaneously. Your hands represent identical signals. When you both adjust to reach the same level, neither hand (signal) gets preferential treatment over the other, just like how a common mode operation seeks to ignore identical inputs.

Now if I say that a individual signal this signal and this signal they are identical. So, if I take the average called v . Let me use different color it is not so, visible. So, v = which is same as an individual namely ‒ v × assuming R and R they are same and from this one we can say that = ‒ , but this is the definition of the common mode gain.

When we connect the outputs of both identical signal outputs, we find that their average will cancel out any difference, creating a common mode gain value which represents how much the circuit amplifies signals that are the same. The goal of differential amplifiers is usually to have a high differential gain and a low common mode gain.

Think of this as a balance scale where both sides have the same weight. No matter how much weight you add equally to both sides, the scale doesn’t tip. In the circuit, this is akin to the common mode signal not causing any output—it’s balanced out.

So, the summary of that what do you see, that once you make this connection this circuit and this circuit they are equal and by considering a differential mode of operation, we obtained the expression of A which is equal to g whether it is g or g ; g × R and the common mode gain on the other hand common mode gain = ‒ . In fact, to be more precise this expression it is ‒ right.

The summary emphasizes that in common mode operation, applying the same input to both terminals results in a net output gain that can be defined and calculated. Hence, the relationship shows the circuit's design criteria for differential amplifiers, which aims to maximize differential gain while minimizing common mode gain.

Imagine a multi-channel speaker system that only plays the same song from two separate speakers. If equal play occurs, the sound waves coming from both speakers will simply reinforce each other without creating noticeable directional effects, much like how common mode signals should not impact the desired outcome from the amplifier.

--