PIN diode application note | PIN diode RF switch,attenuator
This PIN diode application note covers basic description on PIN diode rf switch and PIN diode attenuator. The circuit diagram is mentioned.
PIN diode is widely used in microwave and RF circuits. At these higher frequencies this diode acts as a device whose impedance is controlled by DC excitation. Due to this fact PIN diode is used to control large amounts of radio frequency power.
Figure-1 depicts PIN diode cross section consisting of intrinsic I region(having high resistivity) sandwitched between P-type & N-type regions. It acts as a current controlled resistor at higher frequencies. It also depicts PIN diode equivalent circuit when it is in forward biased mode and in reverse biased mode.
When it is forward biased, holes as well as electrons get injected into I region. These charges do not extinguish each other but stay alive for some time known as carrier lifetime. Hence average charge gets stored. This lowers effective resistance of I region to value designated as "Rs".
At lower frequencies, PIN diode acts like a varactor. Here, value of RP is proportional to voltage and inversely proportional to frequency.
When it is reverse biased, there will be no stored charge in I-region. Hence PIN diode behaves as capacitor CT shunted by parallel resistance RP.
The variation in width of I region and diode area is exploited for different characteristics. Thicker I region provides high breakdown voltage and good distortion properties. Thinner I region of PIN diode will provide fast switching speed.
Figure-2 depicts PIN diode used as RF switch in series and shunt configurations. In this application, PIN diode can be biased to either a low or high impedance state. These device states depend on level of stored charge in I-region.
The series connected PIN diode switch is typically used in the situation when minimum insertion loss is needed over broad frequency range. Since no through-holes are required in the circuit, it is simpler to manufacture. If VC1 biased to 5Volt and VC2 biased to 0Volt then PIN diode will become forward biased. Hence it behaves as low impedance for the RF signal. When VC1 and VC2 polarities are reversed, PIN diode appears as open circuit. Hence it introduces large resistance along with reverse bias capacitance.
As shown in the figure-2, shunt mounted diode produces higher isolation values across wider frequency range. PIN diodes are widely used in the design of SPST and SPDT switches.
Figure-3 depicts PIN diode used as variable RF attenuator.
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