LF147, LF347-N
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SNOSBH1D –MAY 1999–REVISED MARCH 2013
APPLICATION HINTS
The LF147 is an op amp with an internally trimmed input offset voltage and JFET input devices (BI-FET II).
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 accommodated without a large
increase in input current. The maximum differential input voltage is independent of the supply voltages. However,
neither of the input voltages should be allowed to exceed the negative 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.
The amplifiers will operate with a common-mode input voltage equal to the positive supply; however, the gain
bandwidth and slew rate may be decreased in this condition. When the negative common-mode voltage swings
to within 3V of the negative supply, an increase in input offset voltage may occur.
Each amplifier is individually biased by a zener reference which allows normal circuit operation on ±4.5V power
supplies. Supply voltages less than these may result in lower gain bandwidth and slew rate.
The LF147 will drive a 2 kΩ load resistance to ±10V over the full temperature range. If the amplifier is forced to
drive heavier load currents, however, an increase in input offset voltage may occur on the negative voltage swing
and finally reach an active current limit on both positive and negative swings.
Precautions should be taken to ensure that the power supply for the integrated circuit never becomes reversed in
polarity or that the unit is not inadvertently installed backwards 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.
As with most amplifiers, care should be taken with lead dress, component placement and supply decoupling in
order to ensure stability. For example, resistors from the output to an input should be placed with the body close
to the input to minimize “pick-up” 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
capacitance from the input of the device (usually the inverting input) 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 6 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.
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