RF diodes are a key part of RF circuits. These diodes are tasked with handling high-power RF signals in such applications as RF and microwave devices, TV monitors, radio transmitters and stereo amplifiers. A diode is asymmetric in its transfer so that there is a low resistance to current flow in one direction and a high resistance in the opposite direction. There are several types of RF diodes, including:
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PIN diodes that, when forward-biased, are linear and behave like a resistor. They are effectively used as a variable resistor in a variable attenuator and also used as an RF switch. In a forward direction and sufficiently biased, it has a low resistance to the RF that needs to be passed. In comparison, when a reverse bias is applied, it acts as an open circuit, yielding a relatively small level of capacitance. Applications include antenna switching for cordless and cell phones, band switching for tuners, RF attenuation and overvoltage protection. PIN diodes are also used in RF protection circuits to protect a sensitive receiver from the effects of a large transmitter when placed across the receiver input. PIN diodes also have many applications as photodiodes.
Figure 1: An RF diode schematic diagram. (Source: General Microwave)
- Schottky diodes have a metal to semiconductor contact. They are used in high-frequency and fast-switching applications such as voltage clamping or to avoid transistor saturation. Examples include the RF081L2STE25 Schottky diode from Rohm. The diode’s temperature range is -55°C to 150°C. It has a peak reverse current of 10 uA, peak reverse repetitive voltage of 200 V, maximum continuous forward current of 1.1 A, peak reverse recovery time of 25 ns and peak forward voltage of 0.98 V at 1.1 A. It is a 2-pin surface-mount device with a J-Lead shape. Another Schottky example is the CDB7619-000 RF diode by Skyworks Solutions. The surface-mount device is used in such applications as detectors and mixers. The beam-lead and chip Schottky barrier detector diodes are used in applications through 40 GHz in the Ka band. The manufacturing process and materials yield low series resistance and a narrow spread of capacitance values for close impedance control. N-type silicon is also available. Mounted beam-lead diodes are ideal for use in waveguide, coaxial, and stripline applications, while unmounted are suited for use in Microwave Integrated Circuit (MIC) applications.
- Other types include varicap, a.k.a. varactor diodes, used in voltage-controller capacitors so that tuning circuits such as TV receivers lock fast. In this and other uses, they eliminate moving parts.
- Impact ionization avalanche transit-time (IMPATT) diodes operate at very high frequencies and are used as elements in RF/microwave devices, while tunnel and Gunn diodes are used in oscillators and other RF apps.
- Also available is TE Connectivity’s RF2524-000 ESD Suppressor Diode Arrays. A member of the SESD series of ultra-low capacitance diode arrays, it provides signal integrity while preserving unidirectional ESD protection of high-speed serial interfaces. There are several packaging options ranging from the standard 2.5 mm x1.0 mm layout to a 0802 and 1103 DFN that minimizes trace layout complexity and saves PCB space. There is more than 20 kV contact ESD protection (IEC61000-4-2) combined with extremely low leakage and dynamic resistance. Applications include USB 3.1, 3.0 and 2.0, HDMI 2.0, 1.4a, 1.3, DisplayPort, V-by-One, Thunderbolt (Light Peak), LVDS interfaces and applications in consumer, mobile, portable electronics that need high ESD performance but are housed in a small package.