Choosing the Right Diode For Your AGC Detector
Raymond W. Waugh · 1999
Automatic Gain Control (AGC) loops are used to control the gain or output power of amplifiers in a wide variety of applications. A typical circuit is shown in Figure 1. In this circuit, a coupler (distributed transmission line as shown, or lumped element) couples off a small amount of power from the amplifier and feeds it to a Schottky diode detector.. The detector produces a DC voltage proportional to the output power, which is then fed back to the amplifier’s gain control circuit. Diode detectors of this type can be externally biased or self biased [1]. Let us first examine the self-biased detector. The typical self-biased detector (sometimes referred to as “zero-biased”) is shown in Figure 2. A diode is combined with a capacitor of sufficient size on the DC side to present a low impedance (compared to that of the diode), and a shunt 68 ohm resistor is placed on the RF side. This resistor serves two functions — it provides a good impedance match at the input to the detector circuit and furnishes a return path for the DC current generated in the diode. The circuit is completed with a DC load resistor of 1 kohm to 10 kohms. In the lower half of Figure 2, the diode is replaced with its linear equivalent circuit, which can provide insight into the performance of the detector diode. Lp and Cp are package parasitics — little can be done to change their values. The diode chip itself can be represented by a threeelement equivalent circuit, including Rs (parasitic series resistance), Cj (parasitic junction capacitance) and Rj (the junction resistance of the diode, where RF energy is converted to DC voltage). As frequency or junction capacitance increases, the junction resistance of the diode will be shorted out and RF energy will be diverted to Rs where it is converted into heat. When this occurs, output voltage will fail.