The diversity of applications that use amplifiers is greater than ever before. They can buffer the inputs of high-speed analogue- to-digital converters, drive multiple video loads, amplify high-speed pulse signals for testinstrumentation applications, etc. Most high-speed (>50MHz) amplifiers on the market today are easy to use and can become very stable oscillators if given the right chance.
A high-performance amplifier may start to oscillate for a number of reasons:
- Driving a capacitive load without buffering the amp’s output
- Added inductance or capacitance caused by board layout
- Improper supply bypassing
- An amplifier’s design rule has been broken
This article discusses these culprits in more detail. Follow the general guidelines described here and your design experience with a high-speed amplifier will be less frustrating.
DRIVING A CAPACITIVE LOAD OR REACTIVE LOAD
Driving a capacitive load directly reduces the phase margin of an amplifier. The capacitive load and the amplifier’s output impedance cause phase lag, which will result in an under-damped pulse response or oscillation. Some amplifiers can directly drive 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 best fits your amplifier. A small series resistance (RS) at the output of the amplifier, illustrated in Figure 1, will improve both stability and settling performance.
Driving a coaxial cable without using a series resistor can also cause frequency peaking or oscillation. Figure 2 illustrates a typical circuit configuration for driving coaxial cable. 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; it compensates for the amplifier’s increasing output impedance.
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, minimise the lead lengths, especially RF and RG to reduce series inductances at the inverting input of the amplifier.
- Use a compact layout and minimise 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 flushmount 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. Most manufacturers recommend the use of 6.8µF tantalum capacitors and 0.1µF 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 6.8µF capacitor within 0.75 inches of the power pin, and Place the 0.1µF capacitor within 0.1 inches of the power pin. It’s important to place the ceramic capacitors within 0.1 inches of the power pins. As the distance increases, the capacitor becomes less effective due to the added trace inductance. Figure 3 illustrates an example for a single-supply amplifier. If a dual-supply amplifier is used, simply include the same bypass capacitors for the other 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:
- Employ the manufacturer’s recommended feedback resistor value for your gain requirement.
- Do not utilise either 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.
SUMMARY
When designing with a highspeed amplifier, follow the basic guidelines listed below:
- Use a series resistance when driving a capacitive load
- Use a ground plane for board layout, but eliminate the ground plane near inputs and outputs
- Eliminate any long lead lengths or use surface-mount components
- Eliminate any parasitic capacitances or inductances near the I/O terminals
- Use supply bypass capacitors on each supply pin
- Place the bypass capacitors as close as possible to the amplifier’s supply pins
- Review the manufacturer’s data sheet
- Ensure that the amplifier’s minimum stable gain has not been violated
- For current-feedback amplifiers, use recommended feedback resistance. And, don’t use either capacitors or diodes in the feedback loop, unless special care is taken to ensure stability.