51.1.10. Comparison with Common Emitter Amplifier

In this example, since we are considering R = 0 that is that is why the effect of the C is not coming in the frequency response. But if we consider in general so the upper cutoff frequency defined by R and C , so that is the other candidate of the upper cutoff frequency that may using this expression. And if we are having R it can be easily prominent that the common base circuit. Since it is input capacitance is low it is useful for some high bandwidth application.

This chunk highlights the importance of input resistance and bandwidth in comparing common base amplifiers to common emitter amplifiers. When R (source resistance) is considered very low (approaching zero), the capacitance C has minimal effect on the frequency response. In scenarios where the source resistance is higher, the upper cutoff frequency of the common base amplifier becomes significant. The low input capacitance of the common base amplifier makes it favorable for high bandwidth applications, as it allows faster signal processing.

Think of a common base amplifier like a highway with wide lanes (low input capacitance) where cars (signals) can travel quickly. If the highway has low traffic (low source resistance), cars can zoom through easily, reflecting how well the circuit can transmit signals. However, if more cars show up (higher source resistance), the efficiency of the highway decreases, affecting how well it can handle increased traffic without slowdowns.

So yes, so this is this is what I was telling, that all the parameters here we are taking same except the source resistance. So, we are considering the source resistance it is 10 kΩ. Now, let us see what will be the corresponding voltage gain. So, the voltage gain from this point to this point namely emitter to collector we already have seen that voltage gain it is g (r ⫽ R ). And then if I consider from this point to this point primary input to the emitter node it is having attenuation and that attenuation it is .

In this chunk, the focus shifts to real-world applications where source resistance is not negligible, such as 10 kΩ. It discusses how this higher source resistance affects voltage gain by introducing attenuation from the input signal to the emitter of the amplifier. The voltage gain from the primary application signal at the input to the output at the collector is now subject to this source resistance, which can significantly decrease the overall effectiveness of the amplifier.

Imagine trying to fill a swimming pool (the amplifier) with water (the input signal) through a thin hose (the high source resistance). If the hose is wider (low source resistance), water flows in quickly, and the pool fills efficiently (high gain). But if the hose is narrow (high source resistance), the water flow slows down significantly (attenuation), making it harder to fill the pool, similar to how voltage gain is affected in the circuit.

Now, what is the effect on the output impedance on the other hand, since we are considering the finite resistance R. So, the output resistance if I see it is having 2 components one is R and the resistors coming from the active part. So, the resistance coming from this active part it is basically it is you may recall if I call this is R′, so that is the impedance of the active device.

This chunk addresses the output impedance of the circuit when incorporating practical resistance values. The output impedance is influenced by two components: the external resistance (R) and the intrinsic resistance of the active device (R′). The calculation combines these two resistances to find the effective output resistance of the amplifier, which can significantly affect performance metrics such as gain and bandwidth.

Think of the output impedance as a faucet in your bathroom (the amplifier's output). The water flow (output current) comes from two sources: the tap's maximum flow rate (active resistance) and the pipe leading to the faucet (external resistance). If the pipe is thin (high external resistance), the overall flow to the faucet will be greatly reduced, affecting your ability to fill a bathtub or sink efficiently.

So the voltage gain in summary the voltage gains it is very bad particularly if I start considering practical value of the source impedance. Of course, it depends on the source impedance. But then even if you consider say 1 k which may be in many of the application, then also the this attenuation will be quite big.

This chunk concludes that the common base amplifier's performance, especially its voltage gain, diminishes when realistic levels of source impedance are applied. This effect is exacerbated as the source impedance increases, leading to greater signal attenuation. It emphasizes the importance of considering source impedance in real-life applications when designing and analyzing amplifiers.

Imagine trying to listen to music on a phone placed inside a noisy car (high source impedance). As the engine roars and the music plays, the sound from the car reduces music clarity (attenuation). If you were to listen in a quieter setting (lower source impedance), the music would come through much clearer, much like how a circuit performs better with appropriate source resistance.

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