24.1. Analog Electronic Circuits

So, today’s topic of our discussion it is Common Emitter Amplifier. So, this is a basic amplifier and many of the concepts need to be getting cleared in this amplifier. Some of the prerequisites we already have covered, which are necessary to understand and appreciate the operation of the common emitter amplifier.

The Common Emitter Amplifier (CE amplifier) is one of the fundamental amplifiers in analog electronics. It's especially notable for its voltage amplification capabilities. Before diving into the workings of the CE amplifier, it's important to have a solid grasp of earlier concepts covered in the course, such as the characteristics of bipolar junction transistors (BJTs) and different amplifier configurations. Understanding these concepts helps us appreciate how the Common Emitter Amplifier operates.

Imagine you are at a concert where a singer performs live. The microphone captures the singer's voice, but the sound is too soft for the audience to hear clearly. A sound engineer uses an amplifier to boost the sound of the microphone so everyone can hear the singer clearly. Similarly, the CE amplifier takes a weak input signal and amplifies it to a higher voltage level.

So, what are the concepts we are going to cover today? It is the we will start with the operating principle of CE amplifier, but again as I said that we will not be going very deep into that. And, then main thing is that the biasing of CE amplifier.

In our discussion, we will highlight the basic operating principle of the CE amplifier. Instead of going deep into the theory, the focus will shift to the practical aspects, particularly biasing. Biasing is crucial as it sets the operating point of the transistor, which directly affects its performance. A properly biased transistor ensures that it operates efficiently without distortion.

Consider a car that runs best when the engine is tuned correctly; if it runs too fast or too slow, it might break down or not function properly. In the same way, biasing ensures that the BJT in a CE amplifier is tuned to the right operating point, allowing it to function efficiently without signal distortion.

In this course primarily we will be covering 2 types of biasing of BJT amplifier; one is fixed bias and, then subsequently you will see that, what may be a better option.

The discussion will include two major methods of biasing BJT amplifiers: fixed bias and an alternative method that is typically more effective. Fixed bias is a simple method where a single resistor is used to set the quiescent current through the transistor, but it can be sensitive to variations in transistor properties. We will explore alternative biasing techniques that provide greater stability and reliability, thus allowing the CE amplifier to maintain its performance across different conditions.

Think of setting your home thermostat. A fixed setting might keep the house warm, but if it is really cold outside, that fixed temperature might not be enough. A more advanced thermostat, which adjusts based on current conditions, ensures comfort at all times. Similarly, alternative biasing techniques adjust the operating conditions of the CE amplifier, ensuring it works effectively under various circumstances.

So, as a result in case if you are replacing a transistor by another one having different beta then it is operating point completely gets shifted elsewhere, or in if the beta may not be changing due to replacing the device it may be due to temperature effect.

One critical aspect we will discuss is the sensitivity of the DC operating point of the CE amplifier to changes in beta (β), which is the current gain of the transistor. If a transistor with a different beta value replaces the one in the circuit, the operating point can shift significantly, resulting in poor performance. Additionally, temperature changes can affect beta, leading to similar issues. It's vital for engineers to carefully consider these factors when designing a circuit to ensure consistent performance.

Imagine trying to bake a cake and you change the main ingredient based on what's available. If you use a different flour that reacts differently at various temperatures, the cake may not rise as expected. This analogy serves to explain how changing the transistor or its conditions can drastically affect the amplifier's output.

Whenever we are calling the common emitter amplifier, the input it is fed at the base of the BJT and then output it is observed at the collector node.

In the Common Emitter Amplifier configuration, the input signal is introduced at the base terminal of the BJT while the output is taken from the collector. This essential configuration supports voltage amplification, where a small input voltage can result in a larger output voltage. This section also elaborates on the forms of signals used—mostly voltage signals— for analysis.

Think of a microphone as the input to the amplifier system—capturing a soft sound and feeding it to the amplifier, which then produces loud sound through speakers. The microphone represents the base input, and the loudspeaker represents the output from the collector.

While we are feeding the signal, it is also accompanying a DC voltage. The instantaneous output voltage should be sufficiently high even in this critical case, when the signal it is going to the minima.

When feeding an input signal into the Common Emitter Amplifier, it's important to remember that this signal is superimposed on a DC voltage level. Proper design ensures that even during the minimum moments of the input signal, the instantaneous output voltage remains high enough to keep the transistor in its active region, thus preventing distortion. Capacitors may be used to block DC components from the output signal, allowing only the AC signal to pass through.

Consider a wave in the ocean with the tide (DC voltage) at the surface of the water. Even if a small wave (the input signal) comes, the tide needs to be high enough to keep the small wave noticeable. If the tide goes too low, smaller waves may become indistinguishable and lost.

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