OPERATING
CONSIDERATIONS
PA98 • PA98A
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.
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.
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.
CURRENT LIMIT
For proper operation, the current limit resistor (R
CL
) must be
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
CL
=
I
LIM
- .016
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.
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
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
C
must be rated at 500V working voltage. An NPO
capacitor is recommended. The compensation network C
C
R
C
must be mounted closely to the amplifier pins 7 and 8 to avoid
spurious oscillation.
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