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4

OPERATING

CONSIDERATIONS

PA98 • PA98A

GENERAL

Please read Application Note 1 "General Operating Consider-

ations" which covers stability, supplies, heat sinking, mounting,

current limit, SOA interpretation, and specification interpreta-

tion. Visit www.apexmicrotech.com for design tools that help

automate tasks such as calculations for stability, internal power

dissipation, current limit; heat sink selection; Apex’s complete

Application Notes library; Technical Seminar Workbook; and

Evaluation Kits.

CURRENT LIMIT

For proper operation, the current limit resistor (R

connected as shown in the external connection diagram. The

minimum value is 1.4 ohm, however for optimum reliability the

resistor value should be set as high as possible. The value

is calculated as follows; with the maximum practical value of

30 ohms.

.7

R

I

SAFE OPERATING AREA (SOA)

The MOSFET output stage of this power operational ampli-

fier has two distinct limitations:

1. The current handling capability of the MOSFET geometry

and the wire bonds.

2. The junction temperature of the output MOSFETs.

NOTE: The output stage is protected against transient flyback.

However, for protection against sustained, high energy flyback,

external fast-recovery diodes should be used.

SAFE OPERATING CURVES

The safe operating area curves define the maximum ad-

ditional internal power dissipation the amplifier can tolerate

when it produces the necessary output to drive an external

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load. This is not the same as the absolute maximum internal

power dissipation listed elsewhere in the specification since

the quiescent power dissipation is significant compared to

the total.

INPUT PROTECTION

Although the PA98 can withstand differential voltages up to

±25V, additional external protection is recommended. Since

the PA98 is a high speed amplifier, low leakage, low capaci-

tance JFETs connected as diodes are recommended (e.g.

2N4416, Q1-Q4 in Figure 1). The differential input voltage will

be clamped to ±1.4V. This is sufficient overdrive to produce

maximum power bandwidth.

POWER SUPPLY PROTECTION

Unidirectional zener diode transient suppressors are recom-

mended as protection on the supply pins. The zeners clamp

transients to voltages within the power supply rating and also

clamp power supply reversals to ground. Whether the zeners

are used or not, the system power supply should be evaluated

for transient performance including power-on overshoot and

power-off polarity reversals as well as line regulation.

Conditions which can cause open circuits or polarity reversals

on either power supply rail should be avoided or protected

against. Reversals or opens on the negative supply rail is

known to induce input stage failure. Unidirectional transzorbs

prevent this, and it is desirable that they be both electrically

and physically as close to the amplifier as possible.

STABILITY

The PA98 is externally compensated and performance can

be tailored to the application. Use the graphs of small signal

response and power response as a guide. The compensation

capacitor C

capacitor is recommended. The compensation network C

must be mounted closely to the amplifier pins 7 and 8 to avoid

spurious oscillation.

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