IC Substrate - Design with diodes: protect sensitive components

In radar or radio receivers, sensitive low-noise amplifiers (LNAs) are bound to be damaged when subjected to large input signals. So, what is the solution?

We can use receiver protection device limiter (RPL) circuits to protect sensitive components. The "heart" of the RPL circuit is usually composed of PIN diodes, which can protect the components from large input signals without adversely affecting small signal operation.

The operation of the RPL circuit does not require external control signals. This type of circuit includes at least one PIN diode connected in parallel with the signal path, and one or more passive components, such as RF choke inductors and DC isolation capacitors. The following is a simple (but possibly complete) RPL circuit.

When there is no RF input signal or only a small RF signal, the impedance characteristic of the limiter PIN diode will reach the maximum value, usually several hundred ohms or more. Therefore, the diode produces a very small impedance mismatch, and accordingly, can bring low insertion loss.

When a large input signal appears, the RF voltage forces charge carriers (holes in the P layer and electrons in the N layer) into the I layer of the PIN diode. After entering the I layer, free charge carriers will reduce its RF resistance. From the perspective of the RF port of the RPL circuit, this creates an impedance mismatch.

This mismatch will cause the energy from the input signal to be reflected to the corresponding signal source. The reflected signal cooperates with the incident signal to produce a standing wave with the smallest voltage in the PIN diode, because the reflected signal temporarily presents the lowest impedance on the transmission line. Each minimum voltage on the transmission line has a corresponding maximum current. The maximum current flowing through the PIN diode results in an increase in the amount of free charge carriers in the diode I layer, resulting in lower series resistance, greater impedance mismatch, and a "smaller" minimum voltage. In the end, the resistance of the diode will reach its minimum value-this value depends on the design of the PIN diode and the amplitude of the RF signal. When the amplitude of the RF signal increases, the diode is forced to reach a fully conductive state, thereby further reducing the resistance of the diode until the diode saturates and produces the smallest resistance possible. The comparison curve of output power and input power is thus obtained, as shown below.

When the large RF signal no longer appears, if the amount of free charge carriers in the I layer is large, the resistance of the diode will remain at a low level (the insertion loss is still large at this time). After a large RF signal is interrupted, two mechanisms can be used to reduce the amount of free charge carriers: (1) Charge conduction outside the I layer (2) Charge recombination inside the I layer.

The magnitude of charge conduction is mainly determined by the DC resistance in the current path outside the diode.

The rate of charge recombination is determined by many factors, including the density of free charge carriers in the I layer, the concentration of doped atoms in the I layer, and other charge trapping points, and so on. Considering the necessary parameters of the diode, the larger the RF signal that a PIN diode can safely handle, the longer it takes to recover to low insertion loss.

Therefore, the characteristics of the I layer of the PIN diode determine the performance of the RPL circuit. The thickness of the I layer (sometimes called the width) determines the input power when the diode reaches its limit: the thicker the I layer, the higher the input reference 1dB compression level (also called the threshold level). The thickness of the I layer, the area of the diode junction and the material of the diode determine the resistance, capacitance and thermal resistance of the diode.

Only a PIN diode, an RF choke inductor and a pair of DC isolation capacitors are needed to realize the simplest PIN RPL circuit. The RF choke inductor is very important to the performance of the RPL circuit, and its main function is to complete the DC current path of the PIN diode. When a large signal forces charge carriers into the I layer of the diode, a direct current is generated in the diode. If a complete path is not provided for the DC current, the resistance of the diode cannot be reduced, and the diode will not reach its limit. The direct current will flow in the direction of the rectified current, but this is not caused by rectification.

Installing choke inductors in RPL circuits is a very challenging task, because inductors are the most undesirable components in RPL circuits. Based on the inductance value and the capacitance between the parasitic windings, all inductances have series and parallel resonances. Therefore, great care must be taken to ensure that no series resonance occurs in the operating frequency band. In addition, the DC resistance of the choke must be minimized to shorten the recovery time of the RPL circuit.

Note: The DC isolation capacitor is optional. Only when there is a DC voltage or current that may bias the PIN diode on the input or output transmission line, a DC isolation capacitor is required.

Instance

Assuming that the maximum input power that the low noise amplifier (LNA) can withstand is 15 dBm, the I layer thickness of the PIN diode in the RPL circuit is required to be about 2 microns. The designer can determine the acceptable capacitance of the PIN diode based on the RF signal frequency and the acceptable maximum value of the small signal insertion loss. If the designer assumes that the RPL circuit works in the X-band and the acceptable maximum insertion loss is 0.5dB, then the maximum capacitance of the diode can be calculated.

The insertion loss (IL) (in decibels) of the shunt capacitor can be obtained according to the following formula:

We can solve the C value according to the formula:

When f = 12GHz, IL = 0.5dB and Z0 = 50Ω, C = 0.185pF.

The capacitance value obtained together with the thickness of the I layer determines the area of the diode junction.

If the I-layer is thinner and the junction area is small, the diode will have a relatively high thermal resistance. As a result, the junction temperature has to be forced to exceed its maximum rating of 175°C in order to dissipate more energy. Generally speaking, a 2 micron diode with a capacitance of 0.185pF can safely handle a large CW input signal of about 30-33dBm. Since Joule heat is generated when current flows through the diode resistance, a large signal may damage or burn the diode instantly.

The PIN diode RPL circuit can provide reliable protection for sensitive components such as the LNA in the radar or radio receiver, and protect it from the influence of larger incident signals. When RPL applications require extremely low steady-state leakage output power and higher input power handling capabilities, additional diode stages and other circuit enhancement components can be added to the input side of the RPL circuit.

If you are choosing the most suitable diode and circuit topology for RPL applications, the MACOM application engineering team is here to provide you with help and advice at any time.