OPA4830 Datasheet, PDF (32/45 Pages) National Semiconductor (TI) – Quad, Low-Power, Single-Supply, Wideband Operational Amplifier

OPA4830
SBOS350A – DECEMBER 2006 – REVISED MAY 2008.................................................................................................................................................... www.ti.com
BANDWIDTH VS GAIN:
NONINVERTING OPERATION
Voltage-feedback op amps exhibit decreasing
closed-loop bandwidth as the signal gain is
increased. In theory, this relationship is described by
the gain bandwidth product (GBP) shown in the
Electrical Characteristics. Ideally, dividing GBP by the
noninverting signal gain (also called the noise gain, or
NG) predicts the closed-loop bandwidth. In practice,
this calculation only holds true when the phase
margin approaches 90°, as it does in high-gain
configurations. At low gains (increased feedback
factors), most amplifiers exhibit a more complex
response with lower phase margin. The OPA4830 is
compensated to give a slightly peaked response in a
noninverting gain of 2V/V (see Figure 74). This
compensation results in a typical gain of +2V/V
bandwidth of 110MHz, far exceeding that predicted
by dividing the 110MHz GBP by 2V/V. Increasing the
gain causes the phase margin to approach 90° and
the bandwidth to more closely approach the predicted
value of (GBP/NG). At a gain of +10V/V, the 11MHz
bandwidth illustrated in the Electrical Characteristics
agrees with that predicted using the simple formula
and the typical GBP of 110MHz.
Frequency response in a gain of +2V/V may be
modified to achieve exceptional flatness simply by
increasing the noise gain to 3V/V. One way to do this,
without affecting the +2V/V signal gain, is to add a
2.55kΩ resistor across the two inputs (see Figure 78).
A similar technique may be used to reduce peaking in
unity-gain (voltage follower) applications. For
example, by using a 750Ω feedback resistor along
with a 750Ω resistor across the two op amp inputs,
the voltage follower response is similar to the gain of
+2V/V response of Figure 73. Further reducing the
value of the resistor across the op amp inputs further
dampens the frequency response because of
increased noise gain. The OPA4830 exhibits minimal
bandwidth reduction going to single-supply (+5V)
operation as compared with ±5V. This minimal
reduction is because the internal bias control circuitry
retains nearly constant quiescent current as the total
supply voltage between the supply pins changes.
INVERTING AMPLIFIER OPERATION
All of the familiar op amp application circuits are
available with the OPA4830 to the designer. See
Figure 88 for a typical inverting configuration where
the I/O impedances and signal gain from Figure 72
are retained in an inverting circuit configuration.
Inverting operation is one of the more common
requirements and offers several performance
benefits. It also allows the input to be biased at V_S/2
without any headroom issues. The output voltage can
be independently moved to be within the output
voltage range with coupling capacitors, or bias
adjustment resistors.
+5V
0.1mF
2R_T
1.5kW
2R_T
1.5kW
+
0.1mF
6.8mF
1/4
OPA4830
150W
+V_S
2
50W Source
R_G
0.1mF _374W
R_M
57.6W
R_F
750W
Figure 88. AC-Coupled, G = –2V/V Example Circuit
In the inverting configuration, three key design
considerations must be noted. The first consideration
is that the gain resistor (R_G) becomes part of the
signal channel input impedance. If input impedance
matching is desired (which is beneficial whenever the
signal is coupled through a cable, twisted pair, long
PCB trace, or other transmission line conductor), R_G
may be set equal to the required termination value
and R_F adjusted to give the desired gain. This
approach is the simplest and results in optimum
bandwidth and noise performance.
However, at low inverting gains, the resulting
feedback resistor value can present a significant load
to the amplifier output. For an inverting gain of 2,
setting R_G to 50Ω for input matching eliminates the
need for R_M but requires a 100Ω feedback resistor.
This configuration has the interesting advantage of
the noise gain becoming equal to 2 for a 50Ω source
impedance—the same as the noninverting circuits
considered above. The amplifier output now sees the
100Ω feedback resistor in parallel with the external
load. In general, the feedback resistor should be
limited to the 200Ω to 1.5kΩ range. In this case, it is
preferable to increase both the R_F and R_G values, as
shown in Figure 88, and then achieve the input
matching impedance with a third resistor (R_M) to
ground. The total input impedance becomes the
parallel combination of R_G and R_M.
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