Active RF circuits are RF circuits that require an external power source to function properly. Such circuits usually contain components such as amplifiers, mixers, and voltage-controlled oscillators, and are capable of increasing the gain of the signal, modulating the frequency and amplitude of the signal, and are highly resistant to interference.
Compared to passive circuits, active rf circuits increase the gain of the signal and are able to modulate the frequency and amplitude of the signal and have a high level of immunity to interference. RF active circuits are widely used in receivers, transmitters, modems and other parts of the signal can be achieved through voltage changes to regulate the signal.
RF circuit principle:
RF signal generation mechanism: RF signals can be generated through the operation of an oscillator, which is capable of outputting RF signals steadily at a constant frequency. In the RF field, the common types of oscillators used to generate RF signals are crystal oscillators and voltage controlled oscillators (VCOs).
Enhancement of RF signals: In order to enhance the transmission of RF signals in wireless communications, it is often necessary to amplify them in terms of amplitude or power. This process relies on RF amplifiers, which use specialised designs and techniques such as power amplifiers and broadband amplifiers.
Codeconversion of RF signals: Modulation techniques are the process of converting an information signal (e.g. audio or data) into an RF signal for transmission of the information. This conversion can be done through different modulation modes such as Amplitude Modulation (AM), Frequency Modulation (FM), Phase Modulation (PM), etc., so that the RF signal can carry the required information.
Information reduction of RF signals: Demodulation is the process of extracting and reducing the information signals carried in the RF signals. The demodulator applies demodulation techniques such as synchronous demodulation, frequency demodulation, coherent demodulation, etc. to ensure that the original information signal can be accurately restored.
Frequency filtering of RF signals: In RF circuits, filters play a key role in removing unwanted frequency components or reducing interference. RF filters are capable of selecting signals within a specific frequency range as required, while weakening or excluding signals at other frequencies.
Common Active Electronic Devices
Active components are the core components in electronic circuits,which can be divided into two categories:discrete components and integrated circuits,based on their physical structure, circuit role and engineering characteristics.
Discrete components include:
Bipolar transistors (usually referred to as triodes, abbreviated BJT)
Field Effect Transistors (FETs)
Thyristors (also known as SCRs)
Semiconductor resistors and capacitors made by integrated technology (these components are usually used inside integrated circuits)
Analogue IC components are integrated circuits designed to process continuously varying analogue voltage or current signals.These components mainly include:
Integrated operational amplifiers (referred to as integrated op amps)
Comparators
Logarithmic and exponential amplifiers
Analogue multipliers and dividers
Analogue switching circuits
Phase-Locked Loop Circuits (PLLs)
Integrated Voltage Regulators
Reference power supplies
Waveform Generators
Power amplifiers
Digital integrated circuit components are then used to process digital signals and mainly include:
Basic logic gate circuits
Flip-flops
Registers
Decoders
Data comparators (note that these are different from comparators in analogue integrated circuits and have different uses and characteristics)
Drivers
Counters
Shaping circuits
Programmable Logic Devices (PLDs)
Microprocessors (MPUs)
Microcontroller (MCU)
Digital Signal Processors (DSP)
Active components play a vital role in electronic circuits and are essential for all functions involving oscillation, amplification, modulation, demodulation, and current conversion.
Applications and Challenges of Active RF Circuits
Active RF circuits are used in a wide range of applications in the field of wireless communications, including mobile communications, satellite communications, radar systems, wireless local area networks (WLANs), and so on. All of these systems require active RF circuits to provide stable and efficient RF signal transmission.
However, with the continuous development of wireless communication technology, active RF circuits are facing more and more challenges. For example, as communication frequencies continue to increase, the performance and stability requirements for RF circuits are becoming higher and higher; at the same time, in order to meet the growing demand for communication, RF circuits also need to have a higher degree of integration and lower power consumption.
To meet these challenges, researchers are constantly exploring new materials, processes and design methods to improve the performance and stability of active RF circuits. For example, the use of advanced semiconductor materials, the development of new manufacturing processes, and the optimisation of circuit design are all hot directions of current research.
RF passive circuits are circuits composed of passive devices that do not require an external power supply for normal operation. Such circuits are characterised by almost no noise in the signal transmission process, and are therefore widely used in RF front-end antenna matching and filters and other parts. The advantages of RF passive circuits are that its loss is smaller than active circuits, does not require feedback circuits can be self oscillating, easy to achieve broadband characteristics, and therefore can be used to achieve some special communication modes.
Differences between active and passive circuits
Difference in Output Power
Because active rf circuits have built-in gain components, they are able to enhance the original signal to achieve higher output power. Therefore, in the field of RF circuits, active circuits have a more powerful power output capability compared to passive circuits. Conversely, passive circuits are often used for attenuation and distribution of RF signals.
Difference in Tuning Flexibility
Passive circuits tend to have a wider tuning range when it comes to RF circuit design.The reason for this is that passive circuits do not contain a gain component and therefore operate without distortion or saturation, giving them greater tuning flexibility.
Design and Manufacturing Cost Differences
Active circuits are generally more expensive to design and manufacture than passive circuits.This is because active circuits require the incorporation of gain components, which are more complex to design and manufacture. In contrast,passive circuits tend to have a simpler design approach, which significantly reduces their cost.
Active RF circuits play a pivotal role in the field of wireless communications with their powerful signal processing capability, high output power and flexible tunability. Facing the challenges of technological development, researchers are constantly exploring innovations to improve the performance and stability of active RF circuits. Meanwhile, passive circuits provide a useful complement to RF systems with their advantages of low loss and broadband characteristics.
