PIN diode RF switch design circuit
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. Let us understand, PIN diode rf switch design and PIN diode attenuator circuit diagrams.
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 5 volt and VC2 biased to 0 volt 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.
PIN diode attenuator circuit diagram
This circuit utilizes unique properties of a PIN diode to control the attenuation (reduction in signal strength) of an RF or microwave signal. PIN diode is connected in series between RF input and RF output. Its intrinsic layer's resistance is controlled by a varying control voltage. The control voltage is applied to the PIN diode to vary its forward bias. This in turn, adjusts resistance of PIN diode and consecutively amount of attenuation.
Figure-3 depicts PIN diode attenuator circuit which uses PIN diode as variable RF attenuator. When control voltage across the PIN diode is low, it's resistance is high. Hence very little RF signal would pass through to the output.
When control voltage across the PIN diode is increased, it's resistance decreases. This allows more current to flow through the diode. This results in a lower attenuation level and more RF signal passes through to the output.
Conclusion : Due to unique properties, PIN diodes find widespread use in microwave, RF, communication systems, radar and other applications where fast switching and variable resistance are essential. RF switch and attenuator circuits are among these applications.
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