55.2. Introduction to Multi Transistor Amplifiers

Yeah, dear students welcome back to the online certification course of Analog Electronic Circuit. Myself Pradip Mandal from E and EC Department of IIT, Kharagpur, today’s discussion it is Multi Transistor Amplifiers. So far we have discussed about different amplifier configurations, so where one transistor primarily you know single transistor it was doing the amplification and other activities. So, the transistor it was either BJT or MOS transistor and for different configurations we have seen the merits and demerits of the different configurations.

This introduction provides context for the lecture on multi-transistor amplifiers, indicating that previous discussions focused on single transistor amplifiers. The lecturer will guide students through various configurations of amplifiers, specifically how combining them can lead to improved performance. Essentially, we started from understanding basic amplifying components (BJTs and MOSFETs) and their uses in circuits before we jump into the complexities of using multiple transistors together.

Imagine if you are cooking a dish; using a single spice might enhance flavor, but combining multiple spices could create a more rich and layered flavor profile. Similarly, in electronics, while single transistors can amplify signals, using combinations can yield better overall circuit performance.

Based on that merits and demerits we shall try to combine different configurations together, so that we can get overall better performance. So, let us see what is the overall plan in the next slide.

The aim here is to take the strengths of various amplifier configurations and blend them effectively, which can lead to better performance metrics like gain, input, and output impedance, etc. This section emphasizes that understanding the advantages and disadvantages allows us to mix different types to achieve enhanced results in circuits.

Think of a sports team comprising players with different specialties: a strong defender, a skillful attacker, and a quick midfielder. When they work together, the team's performance improves significantly. Likewise, combinations of different amplifier configurations can cater to diverse signal amplification needs.

If we say that common emitter is the main amplifier followed by a common collector, its purpose it is to decrease the output impedance. So, we can say that conceptually we can decrease the output impedance of existing amplifier by simply cascading one common collector stage.

This piece discusses the cascading of amplifier stages. For instance, a common emitter amplifier can be combined with a common collector (also referred to as an emitter follower) to reduce the overall output impedance. The cascading helps manage impedance levels better, which is critical for matching large signal inputs and maintaining signal integrity through the circuit.

Imagine a relay team in a race where one runner passes the baton to another. The second runner may not be as fast, but they ensure the baton (signal) is quickly and smoothly passed without losing pace. This smooth transition ensures better overall performance, similar to how cascading different amplifiers decreases impedance and boosts overall efficiency.

So, likewise the if we precede common emitter or common collector stage by another one common collector configuration we can increase the input impedance. Conceptually again we can increase the input impedance of an existing amplifier by connecting one common collector stage at the input side.

The input impedance of amplifiers is vital for how they interface with preceding stages. By placing a common collector stage at the input, we enhance the input impedance, which prevents signal attenuation and allows for more efficient transfer of signal. Proper impedance matching is crucial in ensuring that signals are not lost before being amplified.

Think about plugging a thin straw into a thick drink—it might be harder for the drink to flow through a small straw. If you switch to a large straw, the flow (or signal) is much easier. Similarly, increasing impedance using a common collector stage ensures better signal integrity at the input.

In fact, we do also have a special popular configuration called Darlington pair we shall see. In fact, this is similar to common collector common emitter configuration, but we will also see what are the difference and pros and cons.

The Darlington pair is introduced as a specific type of multi-transistor configuration that combines two BJTs (or more) to produce very high current gain. This configuration amplifies current more effectively than a single transistor could, making it useful in applications where higher amplification is desired. The discussion will later focus on the distinctions, advantages, and drawbacks of employing this setup.

Consider the motion of a seesaw. If one person pushes down, the other side goes up. If we have multiple people on one side, their combined push makes it much easier to lift more weight on the other side. Similarly, in a Darlington pair, the output benefit amplifies due to the cascading effect of two transistors working together.

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