88.1.3. Indian Institute of Technology, Kharagpur

So, we are using current mirror and transistor-1; it is the amplifying device then we are assuming that Q and Q are identical and also we are assuming that whatever this Q and Q are also identical.

This chunk introduces the common emitter amplifier configuration using a current mirror. A current mirror is a circuit used to duplicate a current flowing in one active device into another, maintaining a constant current in application circuits. It is essential to assume that transistors Q1 and Q2 are identical, which simplifies the analysis as their electrical characteristics, including current gain (beta), are the same. This assumption allows for equal currents in the related parts of the circuit, facilitating calculations.

Think of a current mirror like a photocopier that makes identical copies of a document. If you use the same settings, every copy will look exactly like the original, just as the identical transistors will behave the same way when current is applied.

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

Here, the section discusses how to determine the bias resistance required to achieve a collector current (Ic) of 2 mA in the amplifier. The early voltage for all devices is given as 100 V, which is used in transistor biasing calculations. The relationship between base current (Ib) and collector current is also highlighted, showing that if the transistor beta (β) is 100, then the base current needs to be calculated for accurate biasing.

Imagine trying to fill a water tank to a specific level (2 mA) using different pipe sizes (resistances). To achieve the right level, you need to find the correct size of the pipe that allows just the right amount of water to flow in at any given time.

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

In this part, the output resistance of the amplifier is calculated using the small-signal model. The output resistance is defined as the resistance looking into the output of the amplifier when it is operating. Additionally, the voltage gain of the amplifier is derived from the relationship between the load resistance and the transconductance of the transistor, giving an expected voltage gain close to 2000.

Imagine using a loudspeaker to amplify a sound. The speaker's output resistance is like how well it can push air against the surrounding environment. The greater the speaker's power (voltage gain), the louder the sound it produces. Thus, just like in electronics, optimizing the speaker allows for better sound amplification.

As I said that the current flow here and here should be equal and if you see it carefully the DC voltage here it is defined by this V – V drop.

This chunk describes how to calculate the DC output voltage of the amplifier. The DC voltage is influenced by the voltage drop across the transistor junctions and must be carefully evaluated to ensure proper circuit function. The relationship between the voltage at the output and the current flowing through the transistor is also emphasized, reaffirming the importance of matching currents in transistors.

Think of balancing a seesaw with two children of equal weight on either side. For the seesaw to remain level, the weights (currents) must match on both sides (transistors), which impacts the overall height (voltage) of the seesaw (output voltage).

In fact, this is the non-ideality factor is . And then the corresponding DC voltage it is given here.

In this segment, the speaker discusses what happens when there is a mismatch in the currents due to transistor non-ideality, which can lead to inaccuracies in the performance of the amplifier. Variations in the transistor characteristics can lead to significant changes in the output voltage that must be accounted for in practical applications. This relates to understanding real-life circuit behavior rather than just theoretical outcomes.

This can be likened to a team of people working on a project where not everyone performs at the same level. If one or two team members underperform (like a transistor not working correctly), it will impact the team's overall outcome (the amplifier’s output), making it essential to address these differences for success.

--