RFPIN Diode RF Switch and Attenuator Design
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PIN diodes are 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 diodes are used to control large amounts of radio frequency power.
Let’s understand PIN diode RF switch design and PIN diode attenuator circuit diagrams.
PIN Diode Characteristics
Figure 1 depicts a PIN diode cross-section consisting of an intrinsic I region (having high resistivity) sandwiched between P-type & N-type regions. It acts as a current-controlled resistor at higher frequencies.
It also depicts the 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 the 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 the effective resistance of the I region to a value designated as “Rs”. At lower frequencies, the PIN diode acts like a varactor. Here, the value of RP is proportional to voltage and inversely proportional to frequency.
When it is reverse biased, there will be no stored charge in the I-region. Hence, the PIN diode behaves as a capacitor CT shunted by a parallel resistance RP. The variation in the width of the I region and diode area is exploited for different characteristics. A thicker I region provides high breakdown voltage and good distortion properties. A thinner I region of the PIN diode will provide fast switching speed.
PIN Diode RF Switch
Figure 2 depicts the PIN diode used as an RF switch in series and shunt configurations. In this application, the PIN diode can be biased to either a low or high impedance state. These device states depend on the level of stored charge in the I-region.
The series-connected PIN diode switch is typically used in situations where minimum insertion loss is needed over a broad frequency range. Since no through-holes are required in the circuit, it is simpler to manufacture. If VC1 is biased to 5 volts and VC2 is biased to 0 volts, then the PIN diode will become forward biased. Hence, it behaves as low impedance for the RF signal. When the VC1 and VC2 polarities are reversed, the PIN diode appears as an open circuit. Hence, it introduces large resistance along with reverse bias capacitance.
As shown in Figure 2, a shunt-mounted diode produces higher isolation values across a wider frequency range. PIN diodes are widely used in the design of SPST and SPDT switches.
PIN Diode Attenuator Circuit
This circuit utilizes the unique properties of a PIN diode to control the attenuation (reduction in signal strength) of an RF or microwave signal. The PIN diode is connected in series between the 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 the resistance of the PIN diode and, consequently, the amount of attenuation.
Figure 3 depicts a PIN diode attenuator circuit which uses the PIN diode as a variable RF attenuator. When the control voltage across the PIN diode is low, its resistance is high. Hence, very little RF signal would pass through to the output. When the control voltage across the PIN diode is increased, its 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 their 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.
