Designing With Diodes: Protecting Sensitive Components

Sensitive low noise amplifiers (LNAs) in radar or radio receivers cannot tolerate large input signals without sustaining damage. What’s the solution? Receiver-protector limiter (RPL) circuits, the “heart” of which typically comprises PIN diodes, can be utilized to protect sensitive components from large input signals without adversely affecting small-signal operation.

RPL circuits do not require external control signals. These circuits comprise at least one PIN diode connected in shunt with the signal path, along with one or more passive components, such as RF choke inductors and DC-blocking capacitors. A simple (but possibly complete) RPL circuit is shown below.

When there is no RF input signal or when only a small RF input signal is present, the impedance of the limiter PIN diode is at its maximum value, the magnitude of which is typically in the few-hundreds of ohms or greater. Consequently, the diode produces a very small impedance mismatch and correspondingly low insertion loss.