Rail-to-rail op amp provides biasing in RF amp

Frank Cox, Linear Technology Corp, Milpitas, CA -- EDN, 1/20/2000

It is often useful to monitor the dc level of an RF signal. However, most RF systems use capacitive coupling; thus, the dc information is lost. The circuit in Figure 1 is an RF amplifier comprising two monolithic microwave integrated circuits (MMICs), IC₁ and IC₂, and a quad rail-to-rail op amp (IC₃, an LT1633). IC_3A restores the dc level at the output. Inductors at both the input and the output of the op amp isolate the amplifier from the RF signal. The isolation is good practice, because frequencies higher than the bandwidth of the op amp can undergo rectification in the amplifier's input stages, thereby introducing offset. MMICs IC₁ and IC₂ are Hewlett- Packard HP MSA-0785 devices, which have an inverting gain of 13 dB; the result is a total gain of approximately 26 dB and a noninverted signal. IC₁ and IC₂ have a 3-dB bandwidth of approximately 2 GHz. The 1.5-nF blocking capacitors set the low-frequency cutoff at 2 MHz.

IC₁ and IC₂ have a 1-dB compression point of 4 dBm, or 1V p-p, into 50?, allowing for an input level as high as 18 mV rms. The maximum output current of IC_3A, typically 40 mA with a single 5V supply, limits the dc level on the output to 2V into 50?. The output saturation (low) voltage of the LT1633, typically 40 mV, sets the minimum pedestal voltage. IC₁ and IC₂ use constant-current bias sources to stabilize their gain with respect to temperature. Two other sections of the quad op amp, IC_3B and IC_3C, form active 22-mA current sources. You can make the voltage dividers on the noninverting inputs of IC_3B and IC_3C adjustable to trim the gain of the RF amplifier. The rail-to-rail inputs of IC₃ allow the circuit to operate to within 110 mV of the positive rail. (DI #2467)