If the load is too heavy (too low a resistance) for the output

power supply rail the output will ‘‘foldback’’ as shown in

Figure 4 This will distort the output signal This should be

avoided There are many ways to avoid this such as limiting

the input signal lowering the gain of the amplifier or using a

lighter (higher resistance) load

For designs which require low distortion it is recommended

to keep the output of the amplifier more than 300 mV away

from the levels where visible distortion can be seen on an

oscilloscope For designs with wide temperature ranges

which have low distortion requirements additional margin

may be required which should be determined expermental-

ly See

Figure 5 Figure 5 was recorded when visible distor-

tion was just visible

External Compensation Capacitor

An external compensation capacitor of 27 pF is recom-

mended for use with the LM6311 The capacitor should be

connected between pin 5 and ground and should be placed

close to the LM6311 This capacitor increases the phase

margin of the LM6311 allowing it to be used in low gain

A lower value of compensation capacitor (such as 10 pF)

will increase bandwidth at the expense of phase margin

This will result in more peaking and ringing with low gain

A lower value of compensation capacitor can be useful for

Designer should avoid very low values of compensation ca-

pacitors in low gain circuits since this will reduce phase mar-

gin and may cause some circuits to oscillate

Reflections

The output slew rate of the LM6311 is fast enough to pro-

duce reflected signals in many cables and long circuit

traces For best pulse performance it may be necessary

to terminate cables and long circuit traces with their charac-

teristic impedance to reduce reflected signals

Reflections should not be confused with overshoot Reflec-

tions will depend on cable length while overshoot will de-

pend on load and feedback resistance and capacitance

When determining the type of problem often removing or

drastically shortening the cable will reduce or eliminate re-

flections Overshoot can exist without a cable attached to

the op amp output

Other High Speed and Video Amplifiers

National Semiconductor has an extensive line of high speed

amplifiers with a range of operating voltage from 3V single

143 mmabout the size of a grain of rice) and a wide S0-8

for better power dissipation

This op amp line includes

LM6171

3000 V ms slew rate Voltage Feedback design

supplies

LM7131

TinyPaK (SOT23-5) Video amplifier with 70 MHz

supplies

LM7171

200 MHz Voltage Feedback amplifier with 100

mA output current and 4000V ms slew rate Sup-

g

5V

Information on these parts is available from your National

Semiconductor representative

SPICE Macromodel

A SPICE macromodel of the LM6311 and many other Na-

tional Semiconductor op amps is available at no charge

from your National Semiconductor representative

http

www nationalcom

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