Modern op amps have much higher operating bandwidths than their predecessors. These new devices are excellent building blocks that can greatly simplify the design and construction of high-bandwidth video and RF systems. However, these high-bandwidth, fast-settling op amps can easily become unstable if designers don't observe some special precautions. So, designers must understand common instability issues and how to avoid them.
Effective grounding, bypassing, and decoupling are all essential to preserving high-frequency circuit stability. All three have closely related applications. "Grounding" effectively creates a common signal "sink" by providing a low-inductance signal-return path. Also, using a "ground plane" can help isolate the different sections of an RF circuit from each other. Sometimes an entire side of a pc board will be metal to provide a ground plane for the best conduction of RF signals. Or, it may be a series of thick traces that run around different portions of the pc board, providing a common low-impedance ground reference.
Power-supply bypassing transfers most of the RF energy present on the power-supply lines to ground. This minimizes signal transfers between amplifier stages via the common power-supply line. Finally, power-supply decoupling, normally an RC low-pass filter in the power-supply line, is even more effective in preventing RF energy from flowing between amplifier stages that share a common supply line.
Grounding basics: "Ground" or "earth ground" is a common engineering term that has been used for over a hundred years. Yet the generality of this term and its widespread use has led to much confusion.
The IEEE Dictionary defines an earth ground as: "A conducting connection, whether intentional or accidental, by which an electrical circuit or equipment is connected to the Earth, or to some conducting body of relatively large extent that serves in place of the Earth."
Generally, the purpose of "grounding" a piece of equipment, or an individual component, is to provide a low-impedance return path to the power supply. There are many good reasons to do this, including hum and noise reduction, preserving circuit stability (by preventing high-level signals from feeding back into low-level circuitry), and avoiding multiple dc return paths that can lead to offset errors.
Problems arise when designers fail to pay attention to some important details that may, at first, seem trivial. A very typical problem occurs when the same grounding point is used for both high- and low-level signals. An extreme example of this is where a low-level signal component is tied to the same grounding point as a power-supply filter capacitor. The high currents flowing in the capacitor modulate the low-level signal and introduce power-supply hum into the signal path. Similar problems occur when high- and low-level signals share a common ground-return line, especially a thin (high-Z) run on a pc board. The high-level signal mixes with and modulates the weaker signal, often causing crosstalk or oscillation.
Another common problem occurs when high-level digital circuitry shares a common ground-return line with analog circuitry. The digital signal, with its large switching currents, modulates the analog signal and introduces digital noise and interference.
With any high-frequency circuit, it's always good practice to keep all connections as short as possible, and to directly ground all components to the pc-board ground using separate, very short ground wires, or a common ground plane. Avoid daisy-chain grounds, where a ground wire connects to one component and then directly off to another, then another, forming a "chain" of grounds. Because these components are all grounded at different points along the wire, each component's "grounding point" will be at a slightly different potential. This can introduce some very strange effects, including "motor boating" (low-frequency audio oscillations) and other forms of instability. It can also cause dc circuit errors. Even though these may only be a few millivolts, if they occur at the input of a high-gain amplifier, they can add up to a large dc error at the output.
Power-supply bypassing: Adequate power-supply bypassing can also be critical when optimizing the performance of a high-frequency circuit. Although the modern op amp has excellent power-supply rejection in the audio range, this drops off rapidly at video and RF frequencies. Driving high-frequency signal currents, or transient switching currents, into low-impedance loads often creates very high signal levels on the power-supply line.
Usually, an op amp's power-supply bypassing consists of one or more capacitors connected between each power-supply pin and ground. This ensures a low-impedance ac path to ground over a wide frequency range—typically much wider than the amplifier's 3-dB bandwidth.