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Designing RF with op amps (Part 1)

Posted: 27 Mar 2008 ?? ?Print Version ?Bookmark and Share

Keywords:RF design? operational amplifier? voltage feedback?

Gain stage
By themselves, op amps are differential input, open loop devices. They are intended to be operated in a closed loop topology (different from a receiver's AGC loop). The feedback loop for each op amp must be closed locally, within the individual RF stage.

There are two ways of accomplishing this. The op amp designer refers to them as "inverting" and "non-inverting." These terms refer to whether or not the output of the op amp circuit is inverted from the input. From the standpoint of RF design, this is seldom of any concern. For all practical purposes, either configuration will work and give equivalent results. For that reason, the non-inverting configuration will be given priority here, because it is the simplest to use.

Figure 1: RF op amp gain stage

Figure 1 shows a non-inverting RF amplifier. The input impedance of the non-inverting input is high, so the input is terminated with a 50? resistor. Voltage gain is set by the ratio of Rf and Rg:

The gain of this stage as shown should never be below half (-6dB voltage), because most op amps are unity gain stable.

The output of the stage is converted to 50? by placing a 50? resistor in series with the output. This combined with a 50? load, means that the gain is divided by two (-6dB voltage) in a voltage divider. So, a unity gain (0dB) gain stage becomes a gain of half, or -6dB voltage.

Driving the amplifier into two 50? resistors is not an ideal situation for some op amps. Many of them are designed to drive 600? loads. It is important to look at the load recommendation for the op amp.

The RF designer may notice that the power supply requirements have been complicated by the adding a second negative supply. The stage can be modified easily for single supply operation as shown in Figure 2.

Figure 2: Single supply RF amplifier

A virtual ground is generated on the non-inverting input after the coupling capacitor. This raises the operating point of the op amp to a virtual ground halfway between the supply voltage and ground. Because the op amp portion of the stage is now referenced to a voltage that is half the power supply, it must be DC isolated. Coupling capacitors are needed to isolate preceding and succeeding stages, and the gain resistor Rg from the virtual ground DC potential. These capacitors should be selected to have low impedance at the operating frequency.

Scattering parameters
Forward Transmission S21
The forward transmission S21 is specified over the operating frequency range of interest. S21 is never specified on an op amp data sheet because it is a function of the gain, which is set by the input and feedback resistors Rf and Rg. The forward transmission of a non-inverting op amp stage is:

Op amp data sheets show open loop gain and phase. It is the designer's responsibility to know the closed loop gain and phase. Fortunately, this is not hard to do. Many times the data sheets include excellent graphs of open loop bandwidth, and sometimes include phase. Closing the loop produces a straight line across the graph at the desired gain, curving to meet the limit. The open loop bandwidth plot should be used as an absolute maximum. The designer who approaches the limit does so at the expense of extensive compensation and complex PCB layout techniques.

To illustrate this point, two hypothetical 1GHz op amps are shown in Figure 3: one voltage feedback; the other current feedback. If a voltage gain of 20 dB is required, the voltage feedback amplifier is limited to just over 10.7MHzbarely adequate for an FM IF amplifier. If a voltage gain of 40dB is required, the voltage feedback amplifier is limited to just over 1MHzbarely adequate for medium wave/AM amplification.

The current feedback amplifier, on the other hand, can be used to about 50MHz in both cases. The designer is cautioned to take extra care to avoid oscillations at high gains. Remember the RF designer's rule: all oscillators begin as amplifiers, and all amplifiers begin as oscillators.

Figure 3: Gain bandwidth comparison of voltage and current feedback op amps

Reverse Transmission S12
Op amp topologies, in particular current feedback amplifiers, assume both inputs are connected to low impedances and, therefore, have excellent performance. Another reason why op amps have such good reverse isolation is that, instead of the gain element being a single transistor with its associated leakage, the reverse signal has to go through the leakage of dozens or hundreds of transistors fabricated on the silicon of the op amp.

Figure 4: Reverse transmission path through op amps

Reverse isolation is somewhat better in non-inverting current feedback amplifier configurations. This is because the output signal must also leak through the circuitry connecting the non-inverting and inverting inputs to get to the source.

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