OPA1641_15 Datasheet, PDF (13/29 Pages) Texas Instruments – High-Performance, JFET-Input AUDIO OPERATIONAL AMPLIFIERS

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OPA1641
OPA1642
OPA1644
SBOS484B – DECEMBER 2009 – REVISED AUGUST 2010
SOURCE IMPEDANCE AND DISTORTION
PHASE-REVERSAL PROTECTION
For lowest distortion with a source or feedback
network, the impedance seen by the positive and
negative inputs in noninverting applications should be
matched. The n-channel JFETs in the FET input
stage exhibit a varying input capacitance with applied
common-mode input voltage. In inverting
configurations, the input does not vary with input
voltage because the inverting input is held at virtual
ground. However, in noninverting applications, the
inputs do vary, and the gate-to-source voltage is not
constant. This effect produces increased distortion as
a result of the varying capacitance for unmatched
source impedances.
To maintain low distortion, match unbalanced source
impedance with appropriate values in the feedback
network as shown in Figure 34. Of course, the
unbalanced impedance may be from gain-setting
resistors in the feedback path. If the parallel
combination of R₁ and R₂ is greater than 2kΩ, a
matching impedance on the noninverting input should
be used. As always, resistor values should be
minimized to reduce the effects of thermal noise.
R₁
R₂
OPA164x
V_IN
V_OUT
If R_S > 2kW or R₁ || R₂ > 2kW
R_S = R₁ || R₂
Figure 34. Impedance Matching for Maintaining
Low Distortion in Noninverting Circuits
CAPACITIVE LOAD AND STABILITY
The dynamic characteristics of the OPA164x have
been optimized for commonly encountered gains,
loads, and operating conditions. The combination of
low closed-loop gain and high capacitive loads
decreases the phase margin of the amplifier and can
lead to gain peaking or oscillations. As a result,
heavier capacitive loads must be isolated from the
output. The simplest way to achieve this isolation is to
add a small resistor (R_OUT equal to 50Ω, for example)
in series with the output.
Figure 21 and Figure 22 illustrate graphs of
Small-Signal Overshoot vs Capacitive Load for
several values of R_OUT. Also, refer to Applications
Bulletin AB-028 (literature number SBOA015,
available for download from the TI web site) for
details of analysis techniques and application circuits.
The OPA1641, OPA1642, and OPA1644 family has
internal phase-reversal protection. Many FET- and
bipolar-input op amps exhibit a phase reversal when
the input is driven beyond its linear common-mode
range. This condition is most often encountered in
noninverting circuits when the input is driven beyond
the specified common-mode voltage range, causing
the output to reverse into the opposite rail. The input
circuitry of the OPA1641, OPA1642, and OPA1644
prevents phase reversal with excessive
common-mode voltage; instead, the output limits into
the appropriate rail (see Figure 14).
OUTPUT CURRENT LIMIT
The output current of the OPA164x series is limited
by internal circuitry to +36mA/–30mA
(sourcing/sinking), to protect the device if the output
is accidentally shorted. This short-circuit current
depends on temperature, as shown in Figure 28.
Although it is uncommon for most modern audio
applications to require 600Ω load drive capability,
many audio op amp applications continue to specify
the total harmonic distortion (THD+N) at 600Ω load
for comparative purposes. Figure 7 and Figure 9
provide typical THD+N measurement curves for the
OPA164x series, where the output drives a 3V_RMS
signal into a 600Ω load. However, it should be noted
that correct device operation cannot be ensured when
driving 600Ω loads at full supply. Depending on
supply voltage and temperature, it may well trigger
the output current limit circuitry of the device.
POWER DISSIPATION AND THERMAL
PROTECTION
The OPA164x series of op amps are capable of
driving 2kΩ loads with power-supply voltages of up to
±18V over the specified temperature range. In a
single-supply configuration, where the load is
connected to the negative supply voltage, the
minimum load resistance is 2.8kΩ at a supply voltage
of +36V. For lower supply voltages (either
single-supply or symmetrical supplies), a lower load
resistance may be used, as long as the output current
does not exceed 13mA; otherwise, the device
short-circuit current protection circuit may activate.
Internal power dissipation increases when operating
at high supply voltages. Copper leadframe
construction used in the OPA1641, OPA1642, and
OPA1644 series devices improves heat dissipation
compared to conventional materials. PCB layout can
also help reduce a possible increase in junction
temperature. Wide copper traces help dissipate the
heat by acting as an additional heatsink. Temperature
rise can be further minimized by soldering the
devices directly to the PCB rather than using a
socket.
Copyright © 2009–2010, Texas Instruments Incorporated
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