Tips for Designing High-performance Amplifier Subsystems

By Debbie Brandenburg

October 30, 2006

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Amplifiers are used in different applications. They can buffer the inputs of high-speed ADCs, drive multiple video loads, and amplify high-speed pulse signals for test instrumentation applications.

Most high-speed (> 50MHz) amplifiers in the market today are very easy to use, but can become very stable oscillators if given the chance. A high-speed amplifier oscillates due to:

1. Driving a capacitive load without buffering the amplifier's output;
2. Added inductance or capacitance caused by board layout;
3. Improper supply bypassing; and,
4. Broken amplifier's design rule.

This article discusses in detail these culprits as well as general guidelines in designing a high-speed amplifier.

Driving Load, 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, which results in an under-damped pulse response or oscillation.

Some amplifiers can directly drive large capacitive loads, while others require a series of 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 improves stability and settling performance (Figure 1).

Figure 1: A small series resistance at the output of the amplifier improves stability and settling performance.

Driving a coaxial cable without using a series resistor can also cause frequency peaking or oscillation. Figure 2 shows a typical circuit configuration for driving a coaxial cable. The resistors R_s and R_L are equal to the characteristic impedance (Z_o) of the cable or transmission line.

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