LF355AH National Semiconductor, LF355AH Datasheet - Page 9

• Manufacturer Part Number: LF355AH
• Description: Series Monolithic JFET Input Operational Amplifiers
• Manufacturer: National Semiconductor

Application Hints
The LF155 6 7 series are op amps with JFET input de-
vices These JFETs have large reverse breakdown voltages
from gate to source and drain eliminating the need for
clamps across the inputs Therefore large differential input
voltages can easily be accomodated without a large in-
crease in input current The maximum differential input volt-
age is independent of the supply voltages However neither
of the input voltages should be allowed to exceed the nega-
tive supply as this will cause large currents to flow which
can result in a destroyed unit
Exceeding the negative common-mode limit on either input
will force the output to a high state potentially causing a
reversal of phase to the output Exceeding the negative
common-mode limit on both inputs will force the amplifier
output to a high state In neither case does a latch occur
since raising the input back within the common-mode range
again puts the input stage and thus the amplifier in a normal
operating mode
Exceeding the positive common-mode limit on a single input
will not change the phase of the output however if both
inputs exceed the limit the output of the amplifier will be
forced to a high state
These amplifiers will operate with the common-mode input
voltage equal to the positive supply In fact the common-
mode voltage can exceed the positive supply by approxi-
mately 100 mV independent of supply voltage and over the
full operating temperature range The positive supply can
therefore be used as a reference on an input as for exam-
ple in a supply current monitor and or limiter
Typical Circuit Connections
V
ometer
The potentiometer wiper is con-
nected to V
For potentiometers with tempera-
ture coefficient of 100 ppm C or
less the additional drift with adjust
is
Typical overall drift 5 V C
OS
V C mV of adj )
is adjusted with a 25k potenti-
V
0 5 V C mV of adjustment
OS
Adjustment
a
g
(0 5
LF155 6 R
LF157
Due to a unique output stage design these am-
plifiers have the ability to drive large capacitive
loads and still maintain stability C
Overshoot
Settling time (t
F
Driving Capacitive Loads
R
e
e
s
5k
1 25k
20%
s
) j 5 s
9
Precautions should be taken to ensure that the power sup-
ply for the integrated circuit never becomes reversed in
polarity or that the unit is not inadvertently installed back-
wards in a socket as an unlimited current surge through the
resulting forward diode within the IC could cause fusing of
the internal conductors and result in a destroyed unit
All of the bias currents in these amplifiers are set by FET
current sources The drain currents for the amplifiers are
therefore essentially independent of supply voltage
As with most amplifiers care should be taken with lead
dress component placement and supply decoupling in or-
der to ensure stability For example resistors from the out-
put to an input should be placed with the body close to the
input to minimize ‘‘pickup’’ and maximize the frequency of
the feedback pole by minimizing the capacitance from the
input to ground
A feedback pole is created when the feedback around any
amplifier is resistive The parallel resistance and capaci-
tance from the input of the device (usually the inverting in-
put) to ac ground set the frequency of the pole In many
instances the frequency of this pole is much greater than
the expected 3 dB frequency of the closed loop gain and
consequently there is negligible effect on stability margin
However if the feedback pole is less than approximately six
times the expected 3 dB frequency a lead capacitor should
be placed from the output to the input of the op amp The
value of the added capacitor should be such that the RC
time constant of this capacitor and the resistance it parallels
is greater than or equal to the original feedback pole time
constant
L(MAX)
j 0 01
For distortion
power bandwidth is 500 kHz
LF157 A Large Power BW Amplifier
s
1% and a 20 Vp-p V
TL H 5646 – 15
OUT
swing