Datasheet

OPA164x
IfR >2k orR ||R >2k
R =R ||R
W W
S 1 2
S 1 2
V
IN
V
OUT
R
1
R
1
R
2
OPA1641
OPA1642
OPA1644
www.ti.com SBOS484B –DECEMBER 2009–REVISED AUGUST 2010
SOURCE IMPEDANCE AND DISTORTION PHASE-REVERSAL PROTECTION
For lowest distortion with a source or feedback The OPA1641, OPA1642, and OPA1644 family has
network, the impedance seen by the positive and internal phase-reversal protection. Many FET- and
negative inputs in noninverting applications should be bipolar-input op amps exhibit a phase reversal when
matched. The n-channel JFETs in the FET input the input is driven beyond its linear common-mode
stage exhibit a varying input capacitance with applied range. This condition is most often encountered in
common-mode input voltage. In inverting noninverting circuits when the input is driven beyond
configurations, the input does not vary with input the specified common-mode voltage range, causing
voltage because the inverting input is held at virtual the output to reverse into the opposite rail. The input
ground. However, in noninverting applications, the circuitry of the OPA1641, OPA1642, and OPA1644
inputs do vary, and the gate-to-source voltage is not prevents phase reversal with excessive
constant. This effect produces increased distortion as common-mode voltage; instead, the output limits into
a result of the varying capacitance for unmatched the appropriate rail (see Figure 14).
source impedances.
OUTPUT CURRENT LIMIT
To maintain low distortion, match unbalanced source
impedance with appropriate values in the feedback
The output current of the OPA164x series is limited
network as shown in Figure 34. Of course, the
by internal circuitry to +36mA/–30mA
unbalanced impedance may be from gain-setting
(sourcing/sinking), to protect the device if the output
resistors in the feedback path. If the parallel
is accidentally shorted. This short-circuit current
combination of R
1
and R
2
is greater than 2kΩ, a
depends on temperature, as shown in Figure 28.
matching impedance on the noninverting input should
Although it is uncommon for most modern audio
be used. As always, resistor values should be
applications to require 600Ω load drive capability,
minimized to reduce the effects of thermal noise.
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
Figure 34. Impedance Matching for Maintaining
PROTECTION
Low Distortion in Noninverting Circuits
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
CAPACITIVE LOAD AND STABILITY
single-supply configuration, where the load is
The dynamic characteristics of the OPA164x have
connected to the negative supply voltage, the
been optimized for commonly encountered gains,
minimum load resistance is 2.8kΩ at a supply voltage
loads, and operating conditions. The combination of
of +36V. For lower supply voltages (either
low closed-loop gain and high capacitive loads
single-supply or symmetrical supplies), a lower load
decreases the phase margin of the amplifier and can
resistance may be used, as long as the output current
lead to gain peaking or oscillations. As a result,
does not exceed 13mA; otherwise, the device
heavier capacitive loads must be isolated from the
short-circuit current protection circuit may activate.
output. The simplest way to achieve this isolation is to
Internal power dissipation increases when operating
add a small resistor (R
OUT
equal to 50Ω, for example)
at high supply voltages. Copper leadframe
in series with the output.
construction used in the OPA1641, OPA1642, and
Figure 21 and Figure 22 illustrate graphs of
OPA1644 series devices improves heat dissipation
Small-Signal Overshoot vs Capacitive Load for
compared to conventional materials. PCB layout can
several values of R
OUT
. Also, refer to Applications
also help reduce a possible increase in junction
Bulletin AB-028 (literature number SBOA015,
temperature. Wide copper traces help dissipate the
available for download from the TI web site) for
heat by acting as an additional heatsink. Temperature
details of analysis techniques and application circuits.
rise can be further minimized by soldering the
devices directly to the PCB rather than using a
socket.
Copyright © 2009–2010, Texas Instruments Incorporated 13
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