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Designing RF with op amps (Part 2)

Posted: 03 Apr 2008 ?? ?Print Version ?Bookmark and Share

Keywords:RF design? operational amplifier? voltage feedback?

By Bruce Carter
Texas Instruments Inc.

The Part 1 of this article introduced RF design using operational amplifiers. In Part 2, we address practical circuit examples of RF and intermediate frequency (IF) amplifiers.

While the introduction gave the "How's and Why's" of RF design with op amps, the information is valueless unless working circuits can be constructed and measurements taken to prove the concept. This article describes wideband RF and narrow band IF amps. Except where noted, this article speaks of voltage gain of the stage.

Wideband RF amps
As discussed in Part 1, it is best to use current feedback amplifiers for RF design. We chose to use the THS3202 2GHz low-distortion, current-feedback amplifiers as an example due to their wide bandwidth and fast slew rate. For example, the circuit of Figure 1 produces an amplifier voltage gain of 20, and a stage voltage gain of 10.

Wideband RF amplifier
Figure 1

This circuit in Figure 1 produces the amplitude curve illustrated in Figure 2.

Wideband response for circuit of Figure 1
Figure 2

The voltage gain of the op amp stage itself is 20, but this is cut in half by the action of the back termination resistor in combination with the load. The -3dB (voltage) point of the RF amplifier is about 390 MHz. If a flat gain over frequency is required, this circuit is only usable to about 200 MHz. Input and output voltage standing wave ratio (VSWR) values are better than 1.01:1 for most of the bandwidth, only degrading to about 1.1:1 near 200MHz. S12 is -75dB over most of the bandwidth, only degrading to -50dB near the bandwidth limit.

More gain requires cascading multiple stages of THS3202 op amps. Fortunately for the designer, the THS3202 is a dual device: making a two stage RF amplifier easy to implement at very little additional cost. The circuit in Figure 3 shows a cascaded RF amplifier, used to create a voltage gain of 100, (40dB voltage, 20dB power).

40dB wideband RF amplifier
Figure 3

The circuit comprises two identical gain stages. Isolation is accomplished by using inter-stage termination resistors. The inter-stage 39 pF capacitor provides peaking to compensate for some high frequency roll-off, but better IP3 performance can be achieved by removing it and living with the roll-off. Overall circuit response is shown in Figure 4.

40dB RF amplifier response
Figure 4

There is significant peaking above 100MHz. An amplifier of this gain should be shielded. It is also a good idea to include some low pass filtering to reduce spurs.

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