51.1.11. Effect of Source Resistance on Voltage Gain

So, now the next thing what we have to see that, even though we have seen that the voltage gain it is good, but input impedance is not so good. Now, if I compare again the CE amplifier and CB, then and if I consider practical value of the source resistance signal source resistance.

This chunk sets the stage for discussing the effect of source resistance on voltage gain. It highlights that while the voltage gain of the common base (CB) amplifier is good, its input impedance is comparatively poor. The next step involves evaluating the impact of practical source resistance, which is often an overlooked factor in amplifier design.

Think about a water pipe system where voltage gain is related to the amount of water flowing out at the end (voltage output), and the input impedance is like the size of the pipe at the initial point (input side). Even if the output is strong, if the input pipe is too small (high input impedance), it restricts the flow of water into the system, leading to inefficiency. Similarly, in amplifiers, high source resistance can limit the effectiveness of the input.

Because of the input impedance is low we are expecting that primary input to emitter node there will be large attenuation. Namely if I consider say this is the input signal v having a finite value of the source resistance followed by the input impedance of the amplifier and since this R is small so we are expecting there will be big attenuation.

In this part, the concept of attenuation is introduced. Due to the low input impedance, when a signal with a finite source resistance is fed into the amplifier, a significant portion of the voltage may be 'lost' or attenuated before it even reaches the amplifier's input. This results in less voltage gain than expected, emphasizing the importance of considering source resistance in practical scenarios.

Imagine filling a wine glass with a small funnel (representing low input impedance). If the funnel is small (high resistance), not all the wine (signal) can flow into the glass effectively, causing waste. Thus, even if there’s a lot of wine available (high voltage gain), you only get a little in the glass (actual signal), demonstrating the effect of resistance on gain.

So, 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.

Here, the author is calculating the actual voltage gain considering the voltage gain from the emitter to the collector as well as the combined effect of source resistance and input impedance. If the source resistance is high, the attenuation factor becomes significant, which directly impacts the overall voltage gain.

Consider trying to inflate a balloon (representing voltage gain) using a thin straw (high source resistance). The thinner the straw, the harder it is to get enough air into the balloon. Thus, no matter how much effort you apply, the balloon will not inflate as expected due to the resistance presented by the straw, resulting in low effectiveness (low voltage gain).

So, naturally if we consider practical value of the source resistance we can see the effect. In fact, if I consider R it is a lower say R = 1 kΩ, then also this attenuation it will be quite significant.

This chunk illustrates the consequences of increasing the source resistance. The attenuation caused by a higher source resistance results in an overall significant reduction in voltage gain, demonstrating how practical component values can drastically affect performance.

Picture a hose with a tap at the end. If the tap is partially closed (high resistance), even if the water pressure in the hose is high, only a trickle comes out at the end. This makes it clear that resistance (the partially closed tap) can significantly reduce the effective flow rate (voltage gain) to a point where it may be inadequate.

What is the effect on the output impedance on the other hand, since we are considering the finite resistance R. So, I let me use different color to explain that, output resistance if I see it is having 2 components one is R and the resistors coming from the active part.

In this segment, it discusses how output impedance is influenced by the source resistance. The output impedance is considered, taking into account both the resistance from the source and the active components within the amplifier. The interaction of these resistances plays a crucial role in determining the overall impedance seen at the output.

Think of a network of roads leading to a town (output resistance). If one major road is under construction (source resistance), it slows down traffic (current), affecting overall accessibility. Similarly, in an amplifier, the source resistance can significantly impact the effective output resistance, ultimately influencing circuit performance and functionality.

If I consider R it is higher and higher then this may also create the bandwidth limitation. So, ultimately, even though R it is say 10 kΩ. Since the input capacitance it is remaining low unlike common emitter amplifier, where the input capacitance it is large.

This chunk discusses how increasing the source resistance can create bandwidth limitations. However, in this amplifier configuration, the input capacitance remains low, which is a distinctive advantage over common emitter amplifiers that tend to have larger input capacitances. This allows for wider bandwidth performance despite the presence of source resistance.

Imagine trying to tune a radio to a specific station. If your receiver has a tight bandwidth (narrow frequency range due to high input capacitance), it becomes difficult to pick out the station. Conversely, a wideband receiver (low input capacitance) allows for a clearer reception over a broader range, demonstrating the relationship between bandwidth and capacitance in amplifiers.

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