44.1.3. Practical Circuit Considerations

Now, if we rearrange this equation what we can get is ratio of that is . So, that is the output resistance. In fact, you can further simplify this is we can say that this is . And this is you can see it is a normal approximation is . So, the output resistance it is which is quite low.

In this chunk, we discuss how to derive the output resistance of an amplifier. The output resistance is defined as the resistance seen by the load connected to the amplifier’s output. By rearranging certain equations related to the amplifier operation, we conclude that the output resistance can be approximated as being low. This low output resistance is desirable because it allows maximum power transfer to the load.

Imagine a garden hose. If the hose has a small diameter (representing low resistance), it allows water to flow freely to the plants (the load). However, if the hose has a large diameter (representing high resistance), less water may reach the plants. Similarly, in electronic circuits, a low output resistance ensures that more signal gets delivered to the next stage without much loss.

Now, coming to the input capacitance. So, we already got the expression of the voltage gain and its magnitude is very close to 1. So, let we use that information and let we draw the small signal equivalent circuit now we are including the parasitic components namely the C and C for the common drain, likewise for common collector C and C we are including.

This section focuses on input capacitance in amplifiers. Input capacitance refers to the capacitance seen by the input signal at the amplifier's input. The text suggests that the equivalent circuit includes parasitic capacitances that can affect performance. When we evaluate the effects of these capacitances, we find that for a common drain and common collector configurations, the input capacitance remains comparatively low, primarily because of a voltage gain that approaches 1.

Think of input capacitance like a sponge soaking up water. A larger sponge (higher capacitance) can hold more water (current), but if the sponge is too big, it can take longer to soak up enough water when you need it quickly. In electronic terms, a low input capacitance means the amplifier can respond quickly to input signals, which is beneficial for high-speed applications.

So, if you see here the small signal equivalents circuit earlier this part it was open, now we do have R. And if you see this R its connection, R it is connected from this emitter terminal to the AC ground. In fact, this R it is coming in parallel with r.

In practical circuits, when we introduce a load resistance (R), it affects our previous calculations about input resistance. The load resistance connects the emitter to the ground and is in parallel with intrinsic resistances. This addition influences the overall input resistance, which can still remain high as long as the values are appropriately chosen. The key takeaway is that load resistance can modify circuit characteristics, but under certain conditions, high input resistance can still be achieved.

Consider a crowded room where more people (higher load resistance) can make entry challenging. If everyone stands too close together, it can be difficult to move around quickly (analogous to reduced responsiveness of a circuit). However, if these individuals agree to spread out, or if only a few people remain, the empty space allows for easy movement. This example highlights that adding a load can change dynamics in a circuit.

So, this is the conclusion of today’s discussion. What we have seen in our discussion that common collector and common drain amplifier they are really working as a buffer in voltage mode amplification.

The concluding remarks reiterate the findings about common collector and common drain amplifiers functioning effectively as buffers in voltage mode amplification. They highlight the key characteristics: a voltage gain close to 1, high input impedance, and low output impedance, which are beneficial for many circuit applications.

Think of a buffer as a go-between in a conversation. For instance, in a situation where someone is passing a message through several people, a skilled communicator (the buffer) ensures that the message remains clear and unchanged. Similarly, common collector and common drain configurations maintain the integrity of the input signal while effectively transferring it to the output.

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