Most high-speed amplifiers on the market today are very easy to use, but they can also be very stable oscillators if given the chance. The most common reasons a high-speed amplifier oscillates are:

• Driving a capacitive load without buffering the amplifier's output.
• Added inductance or capacitance caused by board layout.
• Improper supply bypassing.
• Amplifier's design rule has been broken.

Driving a Capacitive Load or Reactive Load such as a Coaxial Cable

Driving a capacitive load directly reduces the phase margin of an amplifier. The capacitive load and the amplifier's output impedance cause phase lag, resulting in an under-damped pulse response or oscillation. Some amplifiers are capable of directly driving large capacitive loads, yet others require a series resistance to buffer the output stage. Refer to the amplifier's data sheet to determine which category your amplifier falls into. A small series resistance (Rs) at the output of the amplifier (see Fig. 1) will improve stability and settling performance.

Fig. 1: Typical Topology For Driving A Capacitive Load

Driving a coaxial cable without using a series resistor can also cause frequency peaking, or oscillation. In a typical circuit configuration for driving coaxial cable (Fig. 2) the resistors Rs and RL are equal to the characteristic impedance, Zo, of the cable or transmission line. The amplifier's output impedance increases with increased frequency. The capacitor C can be used to match the cable over a greater frequency range compensating for the amplifier's increasing output impedance.

Fig. 2: Driving Cable Or Transmission Line

General Layout Guidelines

General layout and supply bypassing play major roles in high-frequency performance. The most sensitive pins of a high-speed amplifier are the inverting input and output pins.

Follow these general layout guidelines:

• Use a ground plane on the board to provide components with a low inductive ground connection. However, remove the ground plane under and around the high-speed amplifier, especially near the input and output pins to reduce stray capacitance.
• Use surface-mount components whenever possible because they offer low lead inductances. If leaded components are used, minimize the lead lengths, especially Rf and Rg, to reduce series inductances at the inverting input of the amplifier.
• Use a compact layout and minimize all trace lengths, especially Rf and Rg, to reduce series inductances at the inverting input of the amplifier.
• Do not use sockets. Soldering a surface mount package directly to the printed-circuit board provides the best results. If necessary, use flush-mount socket pins rather than high-profile socket pins.

General Supply Bypassing Considerations

Use bypass capacitors on each supply. Bypass capacitors provide a low-impedance return current path at the power pins, improved power supply noise rejection, and high-frequency filtering on the power supply traces. Refer to the manufacturer's data sheet for recommended capacitor values. Many manufacturers recommend the use of 6.8-mF tantalum capacitors and 0.1-mF ceramic capacitors. In some cases several amplifiers can share the tantalum capacitor. But, for optimum results, use a ceramic capacitor for every amplifier in your system.

To achieve optimum performance, place the capacitors as shown below:

• Place the 6.8-mF capacitor within 0.75 inches of the power pin.
• Place the 0.1 mF capacitor within 0.1 inches of the power pin.

It is important to place the ceramic capacitors within 0.1 inches of the power pins. As the distance increases, the capacitor becomes less effective because of the added trace inductance. An example for a single supply amplifier is shown in Fig. 3. If a dual supply amplifier is used, simply include the same bypass capacitors for the other rail.

Fig. 3: Basic Non-Inverting Amplifier Configuration For A Single Supply

Basic Amplifier Design Rules

Some amplifiers have minimum stable gain requirements. If an amplifier is used at gains lower than the recommended minimum stable gain, it could oscillate.

If using a Current-Feedback Amplifier:

• Use the manufacturers recommended feedback resistor value for your gain requirement.
• Do not use a capacitor or other non-linear element in the amplifier's direct feedback loop.
• Use a feedback resistor for unity-gain configurations; do not use the standard voltage-follower circuit.

For more information regarding the use of a current feedback amplifier, refer to previous articles: An Internal Look at Current and Voltage Feedback Amplifiers and Design Hints for Current Feedback Amplifiers.

Summary

When designing with a high-speed amplifier, follow these basic guidelines:

Capacitive Load:

• Use a series resistance when driving a capacitive load.

Layout:

• Use a ground plane, but eliminate near inputs/outputs.
• Eliminate long lead lengths or use surface-mount components.
• Eliminate any parasitic capacitances or inductances near the I/O terminals.

Supply Bypassing:

• Use Bypass Capacitors on each supply pin.
• Place the bypass capacitors as close as possible to the supply pins.
• A design rule for the specific amplifier has been broken.
• Review the manufacturer's data sheet for additional information.

If you are using a Current Feedback Amplifier:

• The value of Rf may be too high or too low.
• Do not use capacitors or diodes in the feedback loop, unless special care is taken.

Analog Main | Product of the Week | Columns | Editorial | Tech Notes

		Click here to get your listing up.