RF Design with Operational Amplifiers: Part I

Originally, operational amplifiers (op amps) were not thought of as candidates for RF design, but the advent of newer, faster devices has now made them an attractive option for some applications. The RF designer who wishes to use op amps as gain stages can benefit from many advantages such as simplicity. However, they will also have to familiarize themselves with new terminology.

Traditional RF design techniques, using discrete transistors, have been practiced successfully for decades. RF designers whose systems are cost-sensitive may ask, "Why replace a transistor, which costs a few cents, with a component that may cost several dollars?" Alternatively, RF designers of high-end systems may have been following the development of op amps keenly, wanting to use them in their designs, but have been hesitant due to the learning curve involved.

Advantages
When discrete transistors are used, the bias and operating point of the transistor interacts with the gain and tuning of the stage. When op amps are used, all the designer needs to do is connect the appropriate power source to the op amp power pins. With careful design, this can be a single supply. Gain of the stage is set with two resistors and does not affect the tuning of the stage.

Unlike transistors, when op amps are used, thermal drift effects are all but eliminated.

Disadvantages
If the op amp is to be powered from a single supply, the designer must learn how to set the op amp's operating point. (The process is considerably easier than biasing a transistor stage.)

The RF designer is accustomed to describing RF performance in certain ways. Op amp AC performance is described in terms of AC performance. The RF designer must learn how to translate op amp AC performance parameters into an RF context.

One difference in terminology will be confusing if not addressed from the onset. RF designers typically discuss gain in terms of power dB, which is matched to an impedance (usually 50 Ohms). When discussing power dB, a gain of 10 dB is a gain of ten, 20 dB is a gain of 100. Op amp designers are more familiar with voltage dB, which is independent of impedance, and is different from power dB by a factor of two. For voltage dB, 20 dB is a gain of 10, 40 dB is a gain of 100.

Voltage Feedback or Current Feedback?
The RF designer must decide " are voltage feedback amplifiers or current feedback amplifiers better for the design? The bandwidth specification given in op amp data sheets only refers to the point where the unity gain bandwidth of the device has been reduced by 3 dB (voltage) by internal compensation and/or parasitics. This is not very useful for determining the actual operating frequency range of the device.

Internally compensated voltage feedback amplifier bandwidth is dominated by an internal "dominant pole" compensation capacitor. This gives them a constant gain/bandwidth limitation. In contrast, current feedback amplifiers have no dominant pole capacitor. Therefore, they can operate much closer to their maximum frequency at higher gain. Stated another way, the gain/bandwidth dependence has been broken.

To illustrate this, a voltage feedback and current feedback op amp are compared:

• THS4001, a voltage feedback amplifier with a 270 MHz (-3 dB voltage) open loop bandwidth, is only usable to about 10 MHz at a gain of 10 (20 dB voltage).
• THS3001, a current feedback amplifier with a 420 MHz (-3 dB voltage) open loop bandwidth, is usable to about 150 MHz at a gain of 10 (20 dB voltage).

The RF designer must be aware of some things with current feedback amplifiers:

• Conventional circuit topologies are unchanged for current feedback amplifiers.
• Current feedback amplifiers recommend values for R_f, the feedback resistor. These recommendations should be taken seriously. Gain adjustment should be made with R_g. Keep capacitors out of the feedback loop.

Other than these restrictions, no additional care is needed with the current feedback amplifier above and beyond the normal care in layout and bypassing of high-speed RF circuitry.

For both voltage and current feedback amplifiers, limit capacitance on the inverting op amp input. This is a major cause of instability. It is very easy to accumulate stray capacitance on a sloppy PCB layout. To reduce this stray capacitance, Texas Instruments recommends a hole in the ground and power planes under the inverting input of an op amp on a multi-layer board.

Op Amps as a 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 shows a non-inverting RF amplifier. The input impedance of the non-inverting input is high, so the input is terminated with a 50 Ω resistor. Voltage gain is set by the ratio of R_f and R_g:

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