How do diodes achieve current rectification in communication devices?

1, Working principle of diode
The diode is composed of N-type and P-type materials, forming a PN junction. Its core characteristic is unidirectional conductivity: when a forward voltage is applied to the diode (with the P terminal connected to a positive voltage and the N terminal connected to a negative voltage), the PN junction narrows, electrons flow from the N-type to the P-type, holes flow in the opposite direction, the diode conducts, and current can pass through; When a reverse voltage is applied, the depletion region of the PN junction widens, suppressing the flow of electrons and holes, causing the diode to turn off and almost no current to flow. This characteristic enables diodes to effectively control the direction of current flow, providing a foundation for the implementation of rectifier circuits.
2, Types and principles of diode rectifier circuits
(1) Half wave rectifier circuit
Half wave rectification circuit is the simplest form of rectification, consisting only of one diode. At the secondary of the power transformer, the sine wave voltage varies over time. During the positive half cycle of the voltage, the diode conducts in the forward direction, and the power supply voltage is applied to the load, driving the load current to flow; In the negative half cycle of the voltage, the diode is in a reverse cutoff state, and there is no voltage or current on the load. This process is repeated in each cycle, resulting in the waveform of the negative half cycle of the power supply being 'clipped', leaving only a single direction of voltage output. However, due to the fact that the magnitude of this voltage waveform still varies over time, it is referred to as pulsating DC voltage. For example, in an ideal situation, if the secondary voltage of the power transformer is E, the voltage across load R1 can be calculated using relevant formulas, and the reverse peak voltage that the rectifier diode can withstand also has a specific value. However, the half wave rectification circuit only utilizes the positive half of the power supply, resulting in lower power utilization efficiency.
(2) Full wave rectification circuit
The full wave rectification circuit has been improved on the basis of half wave rectification, adding a rectifier diode D2 and adding a center tap to the secondary of transformer B1. During the period of 0 to π, the upper end of the B1 secondary is positive and the lower end is negative, and D1 is conducting in the positive direction. The power supply voltage is applied to R1, and the voltage at both ends of R1 shows a waveform of positive up and negative down; During the period of π~2 π, the upper end of the B1 secondary becomes negative and the lower end becomes positive, and D2 begins to conduct positively. The power supply voltage is continued to be applied to R1, and the polarity of the voltage at both ends of R1 remains unchanged. Afterwards, in each subsequent cycle, the full wave rectification circuit will repeat the above process. The voltage of the positive and negative two and a half cycles of the power supply is rectified by D1 and D2, and then sequentially applied to the two ends of R1. The voltage obtained on R1 always maintains a waveform of positive top and negative bottom. The full wave rectification circuit can fully utilize the positive and negative two and a half cycles of the power supply, significantly improving rectification efficiency, but the production is relatively complex and requires a specially designed transformer.
(3) Bridge rectifier circuit
In order to simplify the manufacturing process, bridge rectifier circuits have emerged. It uses four rectifier diodes to form a bridge circuit and does not require a center tap transformer. When the power supply is in the positive half cycle, the upper end of the B1 secondary presents a positive potential and the lower end presents a negative potential. The rectifier diodes D4 and D2 conduct, and the current starts from the upper end of the B1 secondary of the transformer and passes through D4 R1,D2, Finally returning to the lower end of B1 secondary; In the negative half cycle of the power supply, the lower end of the B1 secondary becomes a positive potential and the upper end becomes a negative potential. D1 and D3 conduct, and the current returns to the upper end of the B1 secondary through D1, R1, and D3 from the lower end of the B1 secondary. Throughout the entire process, the voltage at both ends of R1 always maintains a positive polarity and a negative polarity, and the waveform is consistent with the condition during full wave rectification. The bridge rectifier circuit achieves effective rectification of the AC power supply through the clever arrangement of four diodes, with each rectifier diode carrying half of the load current.
3, The specific application of diodes in communication equipment
(1) Signal detection
In communication systems, diodes are commonly used for signal detection. For example, in a radio receiving circuit, the received signal is usually an amplitude modulated (AM) signal, and diodes convert the AM signal into an audio signal through rectification. When the positive half of the amplitude modulation signal is applied to the diode, the diode conducts and the current passes through the load to generate the corresponding voltage; When the signal is negative half cycle, the diode is turned off and there is no current on the load. In this way, the information in the amplitude modulated signal is extracted, achieving signal detection.
(2) Modulation and demodulation
Diodes also play an important role in the modulation and demodulation process of communication equipment. In terms of modulation, diodes can be used to modulate carrier signals. For example, in a simple amplitude modulation (AM) circuit, the audio signal modulates the amplitude of the carrier signal through a diode, causing the amplitude of the carrier signal to change with the variation of the audio signal. In terms of demodulation, as mentioned earlier, diodes can restore amplitude modulated signals to audio signals, achieving signal demodulation. In addition, in circuits such as frequency modulation (FM) and phase modulation (PM), diodes also participate in the modulation and demodulation process of signals through their nonlinear characteristics.
(3) Frequency conversion
In communication systems, frequency conversion is an important step. Diodes can be used to achieve frequency conversion. For example, in some frequency synthesizers, diodes work in conjunction with other components to generate new frequency components through nonlinear effects, thereby achieving frequency transformation. In addition, in the mixing circuit, diodes can mix two signals of different frequencies to generate sum and difference frequency signals, providing support for frequency conversion in communication systems.
(4) Overvoltage protection
Communication equipment may be subject to various voltage surges during operation, such as lightning, power fluctuations, etc. A diode can serve as an overvoltage protection component, conducting when the voltage exceeds a certain threshold, bypassing the overvoltage current to ground and protecting other components from damage. For example, at the power input of communication equipment, a voltage regulator diode is usually connected in parallel. When the input voltage exceeds the breakdown voltage of the voltage regulator diode, the diode conducts and releases the excess voltage to ground, thereby protecting the normal operation of the subsequent circuit.
4, Performance indicators and selection of diode rectifier circuits
When selecting rectifier diodes, multiple key parameters need to be considered comprehensively. The maximum rectified current refers to the maximum forward average current that a diode is allowed to pass through during long-term continuous operation; The maximum reverse operating current refers to the maximum reverse current allowed for a diode to pass through when reverse biased; The cut-off frequency refers to the frequency at which a diode begins to lose its unidirectional conductivity; Reverse recovery time refers to the time required for a diode to transition from a forward conducting state to a reverse cutoff state. For ordinary series stabilized power supply circuits, the main focus is on whether the maximum rectified current and maximum reverse operating current meet the requirements, and 1N series, 2CZ series, RLR series, etc. can be selected; For the rectification circuit and pulse rectification circuit of switch mode stabilized power supply, it is necessary to choose rectifier diodes with higher operating frequency and shorter reverse recovery time, such as RU series, EU series, V series, and 1SR series, or choose fast recovery diodes to ensure efficient and stable operation of the circuit.
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