Amplifiers are used in different applications. They can buffer theinputs of high-speed ADCs, drive multiple video loads, and amplifyhigh-speed pulse signals for test instrumentation applications.
Most high-speed (> 50MHz) amplifiers in the market today are veryeasy to use, but can become very stable oscillators if given thechance. A high-speed amplifier oscillates due to:
1) Driving a capacitiveload without buffering the amplifier's output;
2) Added inductance orcapacitance caused by board layout;
3) Improper supplybypassing; and,
4) Broken amplifier's designrule.
This article discusses in detail these culprits as well as generalguidelines in designing a high speed amplifier.
Driving load, cable
Driving a capacitive load directly reduces the phase margin of anamplifier. The capacitive load and the amplifier's output impedancecause phase lag, which results in an under-damped pulse response oroscillation.
Some amplifiers can directly drive large capacitive loads, whileothers require a series of resistance to buffer the output stage. Referto the amplifier's data sheet to determine which category youramplifier falls into.
A small series resistance (R s ) at the output of the amplifierimproves stability and settling performance (Figure 1, below ).
|Figure1: A small series resistance at the output of the amplifier improvesstability and settling performance.|
Driving a coaxial cable without using a series resistor can alsocause frequency peaking or oscillation. Figure 2 below shows a typical circuitconfiguration for driving a coaxial cable. The resistors R s and R L are equal to the characteristicimpedance (Z o ) of the cable or transmission line.The amplifier's output impedance increases with increased frequency.
The capacitor C can beused to match the cable over a greater frequency range – it compensatesfor the amplifier's increasing output impedance.
|Figure2: Driving a coaxial cable without using a series resistor can alsocause frequency peaking or oscillation.|
General layout and supply bypassing play major roles in high-frequencyperformance. The most sensitive pins of a high-speed amplifier are theinverting input and output pins. Follow these general layoutguidelines:
1) Use a ground plane onthe board to provide components with a low inductive ground connection.However, remove the ground plane under and around the high-speed ampli-fier, especially near the input and output pins, to reduce straycapacitance.
2) Eliminate any parasiticcapacitances or inductances near the I/O terminals.
3) Use surface-mountcomponents whenever possible because they offer low lead inductances.If leaded components are used, minimize the lead lengths, especially R f and R g to reduce series inductances at the inverting input of the amplifier.
4) Use a compact layout, andminimize all trace lengths (especially R f and R g )to reduce series inductances at the inverting input of the amplifier.
5) Do not use sockets.Soldering a surface-mount package directly to the PCB provides the bestresults. If necessary, use flush-mount socket pins rather thanhigh-profile socket pins.
6) Review the manufacturer'sdata sheet to ensure that the amplifier's minimum stable gain is notviolated.
Use bypass capacitors on each supply. Bypass capacitors provide a lowimpedance return current path at the power pins, improved power supplynoise rejection and high-frequency filtering on the power supplytraces. Refer to the manufacturer's data sheet for recommendedcapacitor values.
Most manufacturers recommend the use of 6.8 microFarad tantalumcapacitors and 0.1 microFarad ceramic capacitors. In some cases,several amplifiers can share the tantalum capacitor.
For optimum results, however, use a ceramic capacitor for everyamplifier in your system. To achieve optimum performance, place the 6.8microFarad capacitor within 0.75 inches of the power pin, and the 0.1microFarad capacitor within 0.1 inch of the power pin.
It is important to place the ceramic capacitors within 0.1inch ofthe power pins. As the distance increases, the capacitor becomes lesseffective because of the added trace inductance.
|Figure3: It is important to place the ceramic capacitors within 0.1 inch ofthe power pins. As the distance increases, the capacitor becomes lesseffective because of the added trace inductance.|
Figure 3 above shows anexample for a single supply amplifier. If a dual-supply amplifier isused, simply include the same bypass capacitors for the other supply.
Some amplifiers have minimum stable gain requirements. If anamplifier is used at gains lower than the recommended minimum stablegain, 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 nonlinear element in the amplifier'sdirect feedback loop. Use a feedback resistor for unity gainconfigurations; do not use the standard voltage follower circuit.
DebbieBrandenburg is Marketing Engineer for Signal Conditioning Products atFairchild Semiconductor
To read a PDF version of this story, go to Techniquesin designing high-performance amplifiers.