60.2.1. Usefulness of Common Collector and Common Drain Stage

So, now, we do have another example, where we do have the CC followed by CE amplifier. And what we have here it is, the CC stage it is directly getting coupled to CE stage. And you can see here the in the CC stage, basically this part is the CC stage and normally we do have a current sink here for proper biasing; but here we assume that whatever the emitter current we do have out of Q1 that is entirely getting consumed to the base or base terminal of Q2.

This chunk introduces the concept of coupling a Common Collector (CC) stage with a Common Emitter (CE) stage in amplifiers. The CC stage is responsible for providing the necessary input to the CE stage. In typical configuration, a current sink is used for biasing; however, in this example, the current from transistor Q1 is being directly utilized to bias transistor Q2. This implies that the setup is simplified by eliminating the need for a separate bias circuit.

Think of the CC stage as a water reservoir that fills a second tank (the CE stage), supplying it with just the right amount of water (current) needed to function. The reservoir doesn't need a pump (current sink) if the water level (emitter current) from the first tank (Q1) is enough to fill the second tank (Q2).

On the other hand for Q2 which is forming the CE amplifier, so we do have CE amplifier here and for the CE amplifier while we are feeding the signal at the base, so along with the signal we also require meaningful DC voltage at the base of Q2. Now again here we assume that DC voltage of Q2, it is sufficient to feed the signal at the base of Q1; or you may say that, whatever the emitter current we do have out of Q1 that is good enough to make a bias of the Q2.

In this chunk, the mutual biasing between the two transistors is explained. The CE amplifier needs a proper DC voltage at its base for stable operation. Thanks to the setup, the emitter current from Q1 not only provides the necessary bias for Q2 but also facilitates how these two transistors work together to stabilize and strengthen the overall circuit's performance.

Imagine two connected batteries in a circuit; battery one (Q1) provides the power (emitter current) needed for battery two (Q2) to operate efficiently. Without support from the first battery, the second battery would struggle to maintain its performance.

Now, here for comparison with our previous circuits, we are setting the value of this R1 such that the current flowing through this Q2 we are expecting that it will be say 1 mA... And if I say that this 10 µA current of transistor Q1 based base current of transistor Q2 = the emitter current of Q1.

This chunk discusses the calculations involved in ensuring the correct current flows through Q2, which is set at 1 mA. By determining the base current through Q1 and factoring in the current gain (B2), the circuit designer ensures that all components work harmoniously. Careful resistance calculations are crucial for maintaining circuit integrity across transistors.

Think about setting up a line of dominoes. If the first domino (Q1) has enough force to knock down the next (Q2), it ensures the entire sequence flows smoothly. The calculations ensure that each domino receives the perfect push to keep the chain reaction going.

So, the input resistance here, R_in, it is parallel connection of R1 and whatever the resistance we do have... So, roughly you can say that this is 100 and 101 and 2.6 kΩ it is getting multiplied by that factor and then we do have 265.

In this chunk, the concept of input resistance is explored. By incorporating the CC stage, the overall input resistance of the circuit is enhanced. The theoretic combinations of resistance values are evaluated to illustrate how the configuration increases the input resistance significantly when compared to a simple CE stage. This feature is vital in reducing signal loss and improving performance in practical amplifier circuits.

Imagine you have multiple power strips connected in parallel to several devices; this arrangement allows for a greater number of devices to be powered effectively without diminishing the power each one receives. In electronics, increasing input resistance allows the amplifier to effectively deal with higher source resistance from signal sources.

So, C of the circuit it is we do have C of transistor-1... So, definitely if you compare this value and this value, this is much smaller. So, again putting the CC stage, it is helping us to reduce the input capacitance and to increase the input resistance; that may be useful in case if you are feeding the signal from a signal source having very high source resistance Rs.

This section focuses on how the CC stage not only increases input resistance but also helps to decrease input capacitance. This is instrumental when interfacing with sources that present high resistance, as lower capacitance allows for better performance, minimizing phase shift and signal distortion.

Consider a sponge soaking up water. The less porous the sponge (lower capacitance), the more effective it is at maintaining its structure while holding the water (signal). The CC stage acts like a better sponge, allowing better and more reliable signal transmission without losing quality.

In compare to CC-CE amplifier, there is another configuration something called Darlington pair... So, the C here it is C1 (1 + g m R) which leads to a bigger amount of input capacitance.

In the final chunk, the text compares the advantages of using the CC stage with Darlington pairs. While both configurations achieve similar gains, the notable difference lies in the performance of input capacitance. The Darlington pair tends to have a higher effective input capacitance due to its unique design, making the choice between these configurations context-dependent based on the application's requirements.

Think of the Darlington pair as a double-layered cake; although it looks impressive and is rich in flavor (gain), it can become quite heavy (higher capacitance). If you need something lighter (better input capacitance), the CC stage configuration might be your best option.

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