49.1. Analog Electronic Circuits

Today we are going to discuss about the common base and common gate amplifiers. Under that we will be discussing about basic operation biasing, analysis and design and numerical examples will be covered will be covering later in the next class.

In this chunk, the focus is on introducing the primary topic of the discussion: common base and common gate amplifiers. These amplifiers are critical components in analog circuits. The professor informs the students that the session will cover the basic operations, biasing techniques, analysis, design, and that numerical examples will follow in the next class. The expectation is to build upon previously discussed material while exploring new configurations.

Think of learning to cook. Initially, you may start with basic recipes (like common emitter or common source amplifiers), but as you gain confidence, you progress to more complex dishes (common base and common gate amplifiers), which require understanding new techniques and combinations.

Coming to the motivation of these two new configurations; as we have discussed already about the common emitter and common emitter cascaded it is having some cascading effect. Namely, the output impedance of the previous stage and the input impedance of the subsequent stages they are creating potential division.

This chunk addresses the reasons for incorporating common base and common gate amplifiers into circuit design. It underscores issues such as output impedance from one stage interacting with input impedance of the next stage, leading to a phenomenon known as 'loading effect.' This interaction can reduce overall gain and affect frequency response. By employing common base or common gate configurations, these issues can be mitigated, improving amplifier performance.

Imagine a relay race where one runner's speed affects the next runner's start. If the first runner slows down (due to high resistance), the second runner will also slow down, ultimately affecting the race outcome. The common base and gate configurations act as efficient transitions between runners, ensuring optimal performance.

The basic operation with ideal bias at the emitter of this BJT we require some biasing arrangement. So, that the DC current need to be supported at the base we require a DC voltage.

This section explains the fundamental operating principles of both common base (BJT) and common gate (MOSFET) amplifiers. It discusses the importance of proper biasing to maintain the operating point and allow for effective signal amplification. The input signal is applied at the emitter for BJT and at the source for MOSFET while the output is taken from the collector or drain respectively. It also emphasizes the concept of DC and AC interactions, noting how coupling capacitors allow AC signals to pass through while blocking DC.

Think of biasing like adjusting the volume of a radio to a comfortable level before tuning into your favorite station. If the volume is too low (poor biasing), you won’t hear anything; too high, and the sound might distort. The ideal bias point ensures you can hear the signal clearly without any distortion.

Here how do you practically make this bias. Suppose you do have the BJT main BJT is here and here either we can put another transistor say NPN transistor having a meaningful bias at its base.

This segment discusses how to effectively bias a common base amplifier using additional circuits or transistors to create a stable operating point. It outlines designs where another transistor is used for biasing, explaining how these devices support necessary DC levels without affecting AC signal paths. The importance of ensuring high output impedance while providing sufficient current for the main transistor is also stressed.

Consider your car’s navigation system. Just like the system needs a good signal (biasing) to provide accurate directions, the common base amplifier requires proper biasing to function effectively. If the navigation system isn’t calibrated correctly, you may find yourself lost—this is similar to how poor biasing can affect circuit performance.

The biasing scheme of the common gate on the other hand here we are having the main transistor. So, we do have the main transistor and then at the source side we do have.

In this part, similar to the common base amplifier, the common gate amplifier is discussed regarding effective biasing solutions. The focus is on ensuring the gate voltage is appropriately set while providing stable operating conditions for the MOSFET. The text again emphasizes the need for matching currents and maintaining balance between source and drain biasing, reiterating that incorrect biasing can lead to operational issues.

Think of setting up a new thermostat in your home. If the sensors (bias conditions) are set wrong, your house may become too hot or too cold (operational issues). Just as you want your thermostat to function properly to maintain a comfortable environment, ensuring correct biasing of your common gate amplifier is crucial for optimal performance.

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