9 OPA134/2134/4134
®
V
OUT
V
IN
R
1
If R
S
> 2kΩ or R
1
II R
2
> 2kΩ
R
S
= R
1
II R
2
R
2
OPA134
configuration alters the feedback factor or noise gain of the
circuit. The closed-loop gain is unchanged, but the feedback
available for error correction is reduced by a factor of 101,
thus extending the resolution by 101. Note that the input
signal and load applied to the op amp are the same as with
conventional feedback without R
3
. The value of R
3
should
be kept small to minimize its effect on the distortion mea-
surements.
Validity of this technique can be verified by duplicating
measurements at high gain and/or high frequency where the
distortion is within the measurement capability of the test
equipment. Measurements for this data sheet were made
with an Audio Precision distortion/noise analyzer which
greatly simplifies such repetitive measurements. The mea-
surement technique can, however, be performed with manual
distortion measurement instruments.
SOURCE IMPEDANCE AND DISTORTION
For lowest distortion with a source or feedback network
which has an impedance greater than 2kΩ, the impedance
seen by the positive and negative inputs in noninverting
applications should be matched. The p-channel JFETs in the
FET input stage exhibit a varying input capacitance with
applied common-mode input voltage. In inverting configu-
rations the input does not vary with input voltage since 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. The effect is increased
distortion due to the varying capacitance for unmatched
source impedances greater than 2kΩ.
To maintain low distortion, match unbalanced source im-
pedance with appropriate values in the feedback network as
shown in Figure 3. Of course, the unbalanced impedance
may be from gain-setting resistors in the feedback path. If
the parallel combination of R
1
and R
2
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.
FIGURE 3. Impedance Matching for Maintaining Low
Distortion in Non-Inverting Circuits.
NOISE PERFORMANCE
Circuit noise is determined by the thermal noise of external
resistors and op amp noise. Op amp noise is described by
two parameters—noise voltage and noise current. The total
noise is quantified by the equation:
With low source impedance, the current noise term is
insignificant and voltage noise dominates the noise perfor-
mance. At high source impedance, the current noise term
becomes the dominant contributor.
Low noise bipolar op amps such as the OPA27 and OPA37
provide very low voltage noise at the expense of a higher
current noise. However, OPA134 series op amps are unique
in providing very low voltage noise and very low current
noise. This provides optimum noise performance over a
wide range of sources, including reactive source imped-
ances, refer to the typical curve, “Voltage Noise vs Source
Resistance.” Above 2kΩ source resistance, the op amp
contributes little additional noise—the voltage and current
terms in the total noise equation become insignificant and
the source resistance term dominates. Below 2kΩ, op amp
voltage noise dominates over the resistor noise, but com-
pares favorably with other audio op amps such as OP176.
PHASE REVERSAL PROTECTION
OPA134 series op amps are free from output phase-reversal
problems. Many audio op amps, such as OP176, exhibit
phase-reversal of the output when the input common-mode
voltage range is exceeded. This can occur in voltage-fol-
lower circuits, causing serious problems in control loop
applications. OPA134 series op amps are free from this
undesirable behavior even with inputs of 10V beyond the
input common-mode range.
POWER DISSIPATION
OPA134 series op amps are capable of driving 600Ω loads
with power supply voltage up to ±18V. Internal power
dissipation is increased when operating at high supply
voltages. Copper leadframe construction used in OPA134
series op amps improves heat dissipation compared to con-
ventional materials. Circuit board layout can also help
minimize junction temperature rise. Wide copper traces help
dissipate the heat by acting as an additional heat sink.
Temperature rise can be further minimized by soldering the
devices to the circuit board rather than using a socket.
OUTPUT CURRENT LIMIT
Output current is limited by internal circuitry to approxi-
mately ±40mA at 25°C. The limit current decreases with
increasing temperature as shown in the typical performance
curve “Short-Circuit Current vs Temperature.”
V total i R e kTR
nnSns
()( )=++
2
2
4
