Design Idea:
|
by Frank CoxIt is often useful to monitor the DC level of an RF signal; however, in most RF systems signals are capacitively coupled, and thus the DC information is lost. The circuit in Figure 1 is an RF amplifier made with two cascaded monolithic microwave integrated circuit amplifiers (MMICs), with the addition of one section of an LT1633 quad rail-to-rail op amp, which restores the DC level at the output. Inductors at both the input and output of the op amp isolate it from the RF signal. This is good practice because frequencies higher than the bandwidth of the op amp can be rectified in the input stages and induce offset. The MMIC, a Hewlett Packard MSA-0785, has gain of 13dB and inverts the signal, so the total gain is about 26dB and the output is not inverted. The MMIC has a 3dB bandwidth of about 2GHz. The low frequency response is set at 2MHz by the 1500pF DC blocking capacitors. |
Figure 1. RF amplifier with DC-level restore |
| The MMICs have
a 1dB compression point of 4dBm or 1VP-P
into 50W,
allowing for an input level of up to 18mVRMS.
The maximum DC level on the output is limited to 2V into 50 by the maximum
current of the LT1633, which is typically 40mA on a single 5V supply. The
minimum pedestal voltage will be limited by the output saturation voltage
(output low) of the LT1633, which is typically 40mV. This gives a reasonable
range on a 5V supply.
The MMICs are biased with constant
current sources to stabilize their gain with respect to temperature. The
voltage dividers on the noninverting inputs of U3B and U3C can be made
adjustable to trim the gain of the RF amplifier. Two other sections of
the LT1633 are used to make active 22mA current sources. The rail-to-rail
inputs of the LT1633 allow the circuit to operate down to 110mV below the
positive rail. |
