This article covers the principles of signal amplification in bipolar junction transistors (BJTs), including voltage, current, and power amplification. It also explores the key differences between amplification and reproduction, along with their importance in various applications.
Electronic systems must perform a variety of basic functions to accomplish a particular operation. Understanding how these functions are performed is vital to becoming a successful electronics technician or electronics engineer. In this article, you will study the amplification function. Amplification is achieved by devices that produce an increase in signal amplitude. Bipolar junction transistors (BJTs) are frequently used as an amplifier.
To understand the operation of amplifiers and the purpose of various amplifier configurations, you need to be familiar with some basic amplification principles. Amplification is similar to the step-up capability of a transformer in that the input signal is increased by a certain ratio. In both cases, the amplification of voltage, current, and power can be calculated through a series of ratio formulas. In this article, you will learn about the differences between reproduction and amplification and will use several ratio formulas to determine the voltage, current, and power amplification of an amplifier.
Figure 1. Basic Amplifier Circuit Using BJT
Reproduction Vs. Amplification
Amplification is achieved by an amplifying device and its associated components. As a general rule, the amplifying device is placed in a circuit. The components of the circuit usually have about as much influence on amplification as the amplifying device. The circuit and the amplifying device must be supplied electrical energy to function. Typically, amplifiers are energized by direct current. This may be derived from a battery or an electronic power supply. The amplifier then processes a signal of some type when it is placed in operation. The signal may be either AC or DC, depending on the application. The signal to be processed by an amplifier is first applied to the input part of the circuit. After being processed, the signal appears in the output circuitry. The output signal may be reproduced in its exact form, amplified, or both amplified and reproduced. The value and type of input signal, operating source energy, device characteristics, and circuit components influence the output signal.
Figure 2. Signal (a) Reproduction, (b) Amplification, (c) Combined Action
Figure 2 illustrates the process of reproduction, amplification, and the combined amplification-reproduction functions of an amplifier. The amplifier at the top of Figure 2 performs only the reproduction function. Note that the input and output signals have the same size and shape. The amplifier in the middle shows only the amplification function. In this case, the input signal is increased in amplitude. The output signal is amplified but does not necessarily resemble the input signal. In many applications, amplification and reproduction must be achieved simultaneously. This function is shown by the amplifier at the bottom of Figure 2.
Types of Signal Amplifications
Depending on the application, an amplifier must be capable of developing any of these three output signals.
Voltage Amplification
A system designed to develop an output voltage that is greater than its input voltage is called a voltage amplifier. A voltage amplifier has voltage gain, or voltage amplification, which is defined as the ratio of the output signal voltage to the input signal voltage. In equation form, voltage amplification ($A_{V}$) is expressed as:
$$A_{V} = \frac{V_{out}}{V_{in}} \quad \text{or} \quad A_{V} = \frac{\Delta V_{out}}{\Delta V_{in}} $$
The uppercase letters in $V_{out}$ and $V_{in}$ denote DC voltage values. AC voltages are often expressed by lowercase letters. The Greek capital letter delta ($\Delta$) indicates a changing value. Voltage amplifiers are capable of a wide range of amplification values.
Current Amplification
An amplifying system designed to develop an output current that is greater than the input current is called a current amplifier. A current amplifier has current gain, which is defined as the ratio of output current to input current. In equation form, current gain, or current amplification ($A_{i}$) is expressed as:
$$A_{i} = \frac{I_{out}}{I_{in}} \quad \text{or} \quad A_{i} = \frac{\Delta I_{out}}{\Delta I_{in}} $$
Current gain takes into account the beta of a transistor and all the associated components that make the device operational.
Power Amplification
Power gain, or power amplification, is the ratio of the developed output signal power to the input signal power. Note that this refers only to the power gain of the signal. All amplifiers consume a certain amount of power from the energy source during operation. This is usually not included in an expression of power gain. An equation of power gain shows that power amplification ($A_{P}$) equals output signal power ($P_{out}$) divided by the input signal power ($P_{in}$):
$$A_{P} = \frac{P_{out}}{P_{in}}$$
Power signal gain can also be expressed as the product of voltage gain and current gain. In this regard, the equation is:
$$A_{P} = A_{V} \times A_{i}$$
It should be apparent from the $A_{P}$ equation that $A_{V}$ and $A_{i}$ do not both need to be large to have power gain. Typical power amplifiers may have a large current gain and a voltage gain less than 1. Power gain must take into account both signal voltage and current gain to be meaningful.
Review Questions
- The signal to be processed by an amplifier is first applied to the __________ part of the circuit.
- After an input signal has been processed by an amplifier, it appears in the __________ circuitry.
- The __________ occurs when the input signal retains its size and shape in the output circuitry.
- The __________ occurs when the input signal is increased in amplitude in the output circuitry.
- An expression of output voltage divided by input voltage is __________ amplification.
- An expression of output current divided by input current is __________ amplification.
- An expression of output signal power divided by input signal power is __________ amplification.
Answers
- input
- output
- Reproduction
- Amplification
- Voltage
- Current
- Power
Key Takeaways
Bipolar junction transistors (BJTs) offer versatile amplification capabilities, where voltage amplification increases signal strength without altering its form, current amplification enhances current levels, and power amplification boosts both voltage and current for efficient energy transfer. These principles are essential in applications like audio amplification, signal processing, and power regulation, where precise control of signal parameters ensures reliable and effective operation of electronic systems.