- A circuit that creates a stronger version of its input signal is referred to as an amplifier. However, as we will see in this introduction to the amplifier tutorial, there are different types of amplifier circuits since they are categorized based on their operational modes and circuit designs.
- Operational amplifiers, small signal amplifiers, large signal amplifiers, and power amplifiers are only a few of the different types of electronic circuits that fall under the category of amplifiers. The classification of an amplifier is based on the magnitude of the input signal, whether it is large or little, the amplifier’s physical design, how the input signal is processed, or the relationship between the input signal and the current flowing through the load.
Amplifier Circuit Symbol
- The typical amplifier symbol is a triangle, which is frequently enclosed within a square in general block diagrams as illustrated below.
Design of a Basic Single-Stage Amplifier
- A single-stage amplifier is a circuit that amplifies a weak signal using just one transistor and any necessary circuitry.
- Understanding the construction and operation of multi-stage amplifier circuits is made simple by analyzing the operation of a single-stage amplifier circuit. One transistor, a bias circuit, and other auxiliary parts make up a single-stage transistor amplifier. An illustration of a single-stage transistor amplifier is provided in the circuit diagram above.
- A little base current flows when a weak input signal is applied to the transistor base as illustrated in the image. A greater current flows in the transistor’s collector as a result of the transistor’s operation. (Because IC = IB because the collector current is times the base current.) The voltage drop across the resistor RC now rises along with the collector current and is gathered as the output.
- As a result, a weak signal at the base is amplified so that it has a stronger and larger magnitude at the collector output. This transistor, therefore, functions as an amplifier.
Practical Circuit of a Transistor Amplifier
- The circuit of a real-world transistor amplifier, which serves as a voltage divider biasing circuit, is depicted below.
- The functions of all the components are explained below.
Circuit for Bias
- The biasing and stabilization circuit, which aids in setting a correct operating point, is made up of the resistors R1, R2, and RE.
Cin Input Capacitor
- This capacitor connects the transistor’s base to the input signal. Although it enables AC signals, the input capacitor Cin separates the signal source from resistor R2. Without this capacitor, the input signal is delivered directly, which alters the bias at R2.
CC, or Coupling Capacitor
- This capacitor joins the first stage to the second stage at its conclusion. It is known as a coupling capacitor because it couples two stages. This capacitor permits AC to flow but prevents DC from one stage from entering the next. As a result, it is also known as a blocking capacitor.
- The output across the resistor RL is free from the collector’s DC voltage since coupling capacitor CC is present. If not, the shunting effect of RC, which would come in parallel to R2 of the following stage, would significantly alter the bias conditions of the subsequent stage.
Capacitor Emitter By-Pass CE
- Utilized in parallel with the emitter resistor RE is this capacitor. This is used to feed the boosted AC signal through. Without it, the signal will travel through RE, causing a voltage drop across RE that will cause the input signal to feedback, lowering the output voltage.
RL, or load Resistor
- The term “load resistor” refers to the resistance RL connected to the output. When multiple stages are employed, RL stands for the input resistance of the following stage.
Various Circuit Currents
- Let’s examine various circuit currents throughout the entire amplifier circuit. These are mentioned in the figure above.
1. Base Current
- Due to the biasing circuit, DC base current IB flows when there is no signal applied to the base circuit. The AC base current, ab, also flows when the AC signal is applied. As a result, when the signal is applied, the total base current, iB, is provided by,
2. Current Collector
- Due to the biasing circuit, a DC collector current flows even in the absence of a signal. The AC collector current also flows when the AC signal is applied. The total collector current, iC, is therefore given by,
IC = βIBIC = βIB = zero signal collector current.
Ic = βibic = βib = signal-driven collector current.
3. Emitter Current
- There is a DC emitter current IE flowing when no signal is applied. Total emitter current iE is determined by the signal application and is provided by,
It should be remembered that,
- As base current is usually small, it is to be noted that
IE ≅ IC and
ie ≅ ic
- These are the important considerations for the practical circuit of a transistor amplifier. Now let us know about the classification of Amplifiers.
Amplifier Power Gain
- Gain is the difference between an amplifier’s input and output voltage,
- where the two inputs VIN1 and VIN2 are subtracted. Although the gain in a real circuit will vary on frequency, let’s start by thinking about the gain in a hypothetical amplifier.
Power Efficiency of Amplifier
- Depending on how the RF input power is handled, there are various ways to describe amplifier efficiency. The useable output power divided by the input power, which takes into account the contribution of the RF input power, is one way to assess a circuit’s efficiency. Power-added efficiency is the name of this efficiency metric (PAE). The most typical definition of PAE used with power amplifiers at RF and microwave frequencies concentrates on the added RF power divided by the DC input power. Thus,
ηPAE=PRF, out−PRF, in/PDC