88.1.2. Department of Electronics and Electrical Communication Engineering

So, we are going through different numerical examples and now we are going to talk about one common emitter amplifier which is using current mirror and particularly to bias the active load say Q.

In this initial segment, we are being introduced to the concept of a common emitter amplifier using a current mirror. The common emitter amplifier is a basic but important circuit configuration that allows for voltage amplification. Here, the active load is indicated as transistor Q, which will be explained further in the context of the current mirror's functionality.

Think of the common emitter amplifier like a public announcement system. If you want to amplify a speaker's voice (input), the system uses various electronic components to enhance the sound (amplify) before it comes out of the loudspeakers (output).

We are using current mirror and transistor-1; it is the amplifying device then we are assuming that Q1 and Q2 are identical and also we are assuming that whatever this Q3 and Q4 are also identical.

This chunk discusses the assumptions made about the identical nature of the transistors in the circuit. To ensure uniform performance, it's critical that Q1, Q2, Q3, and Q4 share similar characteristics. This helps in maintaining consistent current flow and amplification properties in the circuit. The idea is that identical components behave predictably under similar conditions.

Imagine we are assembling a sports team. If all players (transistors in this case) have the same training and skills, the team works effectively together. However, if one player is significantly stronger or weaker, it could disrupt the game (circuit performance).

So, to get the I current of transistor-1 and collector current or transistor-4 equal. We want the current flow through transistor-2 should be equal to current flow through transistor-1.

Here, the focus shifts to ensuring that specific current levels are adequately matched across the transistors. A key intention is to keep the collector current of transistor-1 (I_C1) same as that of transistor-4. This balance is essential for the effective operation of the current mirror, which stabilizes and controls the overall current flow in the circuit.

Consider a relay race. For the team to perform well, each runner (transistor) must pass the baton (current) smoothly and consistently to the next runner. If one runner is faster, it could disrupt the race.

So, also we do have othered information namely all the devices are having early voltage 100 V and with this information let we try to find what will be the value of this resistance to get the collector current I_C = 2 mA.

At this point, the focus is on calculating the bias resistance required to achieve the desired collector current of 2 mA. The Early voltage is a critical factor that influences how current behaves in a transistor, impacting the output characteristics and helping in fine-tuning the circuit parameters to achieve optimal performance.

Imagine setting a thermostat in a house. Just like how we adjust the thermostat to maintain a comfortable room temperature (current), here we adjust the resistance to achieve a desired current through the transistors.

Now with this information let us try to find the small signal output resistance and voltage gain of the amplifier.

This part deals with calculating the small signal output resistance and the voltage gain of the amplifier circuit. The output resistance must be calculated accurately, as it significantly impacts the overall efficiency and gain of the amplifier. Understanding these parameters is crucial for predicting how the amplifier will respond to varying input signals.

Think about how an amplifier works much like an amplifier in a sound system: the stronger the amplifier's signal processing (output resistance and gain), the better the sound quality. If the amplifier has low gain, you’ll barely hear the sound despite the volume being up.

Now we like to get what will be the gain of this amplifier. So, the gain of the amplifier of course, the voltage gain of this amplifier it is g_m R_out with a ‒ sign.

In this chunk, the process of determining the voltage gain is discussed in terms of transconductance (g_m) and output resistance (R_out). The gain itself plays a pivotal role in amplifying the signal, and a negative sign indicates a phase inversion typical of a common emitter configuration, where the output signal is inverted relative to the input signal.

When you turn the volume knob on a radio, a little twist results in a much louder sound; similarly, adjusting the parameters in this amplifier circuit allows for significant amplification of the input signal.

--