52.2.1. Example of Common Base Circuit

Welcome back after the short break. What we are discussing is Common Base Amplifier and we will be going for Common Gate amplifier, but before going I have another example based on the common base where we are talking about practical circuit of this base bias.

This section serves as an introduction to the common base amplifier, highlighting its importance in practical circuit designs. The speaker mentions transitioning to discussing a common gate amplifier but stresses the need to understand an example involving the common base first.

Think of an amplifier like a speaker. A common base amplifier amplifies a signal at a certain input level, much like how a speaker boosts the sound level of your voice when you speak into it.

Instead of having an ideal separate voltage source here along with the Thevenin equivalent resistance R , practical circuit wise we may have only one supply voltage and from that we need to generate whatever the voltage we like to generate here.

The speaker discusses how practical circuits often differ from ideal models. Instead of multiple voltage sources, a single supply voltage is utilized alongside a potential divider to generate the desired voltage for the circuit, making the design simpler.

Imagine you need to charge multiple devices using just one power bank. You would need a smart way to distribute that single charge to multiple outputs rather than relying on having a separate charger for each device.

So, we can have a potential divider constructed by say R and R connected to ground which generates the voltage here...

The use of a voltage divider, created by resistors R_A and R_B, is explained. This setup helps to create the necessary base voltage for the transistor in a common base configuration. The values of these resistors are crucial for ensuring the correct voltage is generated.

Consider how a teacher adjusts the volume of a classroom speaker to ensure all students can hear clearly. The resistors in the potential divider adjust the 'volume' of the voltage reaching the transistor’s base.

In fact, we are considering 10.3 kΩ so that with a β of 100, we can get the emitter current or rather collector current very close to 0.5 mA.

The FET transistor parameters, specifically the current gain 'β' and the resistance values, are explained in the context of calculating collector current. It illustrates how biasing impacts operational parameter expectations.

Think about a water flow system: if you increase the pressure (or voltage), more water (or current) will flow through the system. Similarly, with the right biasing, more current can 'flow' through the transistor, producing a stronger output.

Now, once you obtain the operating point of the transistor, then again we can go for calculating the value of the small signal parameters namely g_m.

After establishing the right resistor values and collector current, students are guided on how to calculate the operating point of the transistor, which is crucial for determining the small signal parameters that characterize the amplifier's performance.

Finding the operating point is like getting the settings right on an espresso machine. You need to adjust various knobs and dials to ensure that every cup of coffee made is consistent and perfect.

So, we can say that the collector voltage it can go as high as 12 V and the corresponding signal may get distorted like this...

This section discusses the analysis of the signal swing capability of the common base amplifier, detailing its limits in the positive and negative directions as well as distortion issues.

Imagine a bridge that allows cars to pass under it. If too many heavy trucks travel over, the bridge might sway o distort, limiting what can cross. Similarly, the amplifier has limits to how much signal it can handle before distortion occurs.

If the source resistance is significant, then the input signal will be experiencing significant amount of attenuation...

The section explains how source resistance can affect the input signal, emphasizing the importance of keeping impedance low in common base amplifier applications to prevent signal loss.

Like trying to pour syrup from a bottle with a narrow opening, if resistance is high (like the opening), it’ll slow your pouring (the signal), illustrating the need to manage impedance effectively in circuits.

If I say that we are shorting this node here; the corresponding v = 0. And if you recall the small signal model here...

This chunk discusses the concept of current gain in a common base amplifier circuit, highlighting that for practical designs, the current gain is often close to 1 throughout most operations.

Think of a relay that activates a light: while the relay simply activates the light when current passes through, the light stays bright and consistent, akin to how the output maintains a steady current gain in a circuit.

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