APPLICATIONS INFORMATION
WIDEBAND, NONINVERTING OPERATION
The OPA846 provides a unique combination of features. Low
input voltage noise, along with a very low distortion output
stage, gives one of the highest dynamic range op amps
available. The very high Gain Bandwidth Product (GBP) can
be used either to deliver high signal bandwidths at high gain,
or to deliver very low distortion signals at moderate frequen-
cies and lower gains. To achieve the full performance of the
OPA846, careful attention to PC board layout and compo-
nent selection is required, as discussed in the following
sections of this data sheet.
Figure 1 shows the noninverting gain of a 10V/V circuit used
as the basis of the Electrical Characteristics and most of the
Typical Characteristic curves. Most of the curves are charac-
terized using signal sources with a 50Ω driving impedance,
and with a 50Ω load impedance presented by the measure-
ment equipment. In Figure 1, the 50Ω resistor at the V
IN
terminal matches the source impedance of the test genera-
tor, while the 50Ω series resistor at the V
O
terminal provides
a matching resistor for the measurement equipment load.
Generally, the data sheet voltage swing specifications are at
the output pin (V
O
in Figure 1), while the output power (dBm)
specifications are at the matched 50Ω load. The total 100Ω
load at the output, combined with the 503Ω total feedback
network load, presents the OPA846 with an effective output
load of 83Ω for the circuit of Figure 1.
+5V
+V
CC
+
guideline ensures that the noise added at the output due to
the Johnson noise of the resistors does not significantly
increase the total noise over that due to the 1.2nV/√Hz input
voltage noise for the op amp. Higher resistor values can
certainly be used where the application requires it, but can
start to add significantly to the output noise power as de-
scribed in the Setting Resistor Values to Minimize Noise
section.
WIDEBAND INVERTING GAIN OPERATION
Operating the OPA846 as an inverting amplifier has several
benefits and is particularly appropriate when a matched input
impedance is required. Figure 2 shows the inverting gain
circuit used as the basis of the inverting mode of the Typical
Characteristic curves.
+5V
V
CC
+
0.1µF
6.8µF
R
S
50Ω Load
50Ω
0.1µF
V
O
91Ω
OPA846
R
F
1kΩ
50Ω Source
V
I
R
G
50Ω
0.1µF
+
6.8µF
0.1µF
50Ω Source
V
I
V
O
6.8µF
V
EE
–5V
50Ω
R
S
50Ω Load
50Ω
FIGURE 2. DC-Coupled, G = –20V/V, Bipolar Supply, Speci-
fication and Test Circuit.
Driving this circuit from a 50Ω source, and constraining the
gain resistor (R
G
) to equal 50Ω, gives both a signal band-
width and noise advantage. R
G
acts as both the input
termination resistor and the gain setting resistor for the
circuit. Although the signal gain (V
O
/V
I
) for the circuit of
Figure 2 is double that for Figure 1, the noise gains are in fact
equal when the 50Ω source resistor is included. This has the
interesting effect of doubling the equivalent GBP of the
amplifier. This can be seen by observing the 200MHz band-
width for the inverting gain of –20. This implies a GBP of
4GHz, when in fact this extended bandwidth is given by the
reduced noise gain when the matched source resistor is
included. If the signal source is actually the low impedance
output of another amplifier, R
G
is increased to the minimum
load resistance value allowed for that amplifier and R
F
is then
adjusted to achieve the desired gain. For stable operation of
the OPA846, it is critical that this driving amplifier show very
low output impedance at frequencies beyond the expected
closed-loop bandwidth for the OPA846.
OPA846
R
F
453Ω
R
G
50Ω
V
EE
–5V
0.1µF
+
6.8µF
FIGURE 1. DC-Coupled, G = +10V/V, Bipolar Supply, Speci-
fication and Test Circuit.
Voltage-feedback op amps (unlike current-feedback designs)
can use a wide range of resistor values to set the gain,
although these resistors usually have low values to maintain
a low total output noise. The circuit of Figure 1, and the
specifications at other gains, use the constraint that R
G
be
set to 50Ω and R
F
adjusted to get the desired gain. Using this
10
OPA846
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