Electronics and Communication Engineering - Microwave Communication
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Since reactance does not contribute thermal noise to the circuit, it is a low noise device.
Assertion (A): PIN diode is commonly used for microwave control.
Reason (R): A PIN diode uses heavily doped p and n materials.
A PIN diode has an intrinsic (i) layer between p and n layers. When reverse bias is applied depletion layers are formed at p-i and i-n junctions.
The effective/width of depletion layer increases by the width of i layer. It can be used as a voltage controlled attenuator.
At high frequencies the rectification effect ceases and impedance of diode is effectively that of i layer.
This impedance varies with the applied bias. It is used in high frequency switching circuits, limiters, modulators etc.
At high frequencies transit time is large as compared to the period of microwave signal.
Assertion (A): A line of length 
and short circuited at far end has an input impedance of infinity.
Reason (R): A line of length and short circuited at far end behaves as a parallel resonant circuit.
A parallel tuned circuit has an infinite impedance if R is zero.