OPA687 Burr-Brown, OPA687 Datasheet - Page 11

• Manufacturer Part Number: OPA687
• Description: Wideband / Ultra-Low Noise / Voltage Feedback OPERATIONAL AMPLIFIER With Power Down
• Manufacturer: Burr-Brown

LOW NOISE FIGURE, HIGH DYNAMIC
RANGE AMPLIFIER
The low input noise voltage of the OPA687 and its very high
2-tone intercept can be used to good advantage as a fixed-
gain IF amplifier. While input noise figures in the 10dB
range (for a matched 50
just the OPA687, Figure 5 shows a technique to reduce the
noise figure even further while providing a broadband, high
gain IF amplifier stage using two stages of the OPA687.
This circuit uses two stages of forward gain with an overall
feedback loop to set the input impedance match. The input
transformer provides both a noiseless voltage gain and a
signal inversion to retain an overall non-inverting signal
path from P
ing to provide the correct feedback polarity through the
6.19k
primary of the 1:2 transformer, the secondary must see a
200
provided by the 200 resistor in series with 10pF. At lower
signal frequencies (f < 80MHz), the input match is set by the
feedback through the 6.19k resistor. The low noise figure
(5dB) for this circuit is achieved by using the transformer,
the low voltage noise OPA687, and the input match set by
feedback. The first stage amplifier provides a gain of +15.
The very high SFDR is provided by operating the output
stage a low signal gain of –2 and using the inverting
compensation to hold it stable. Depending on the load that
is driven, this circuit can give a 2-tone SFDR that exceeds
90dB through 30MHz. Besides offering a very high dynamic
range, this circuit improves on standard IF amplifiers by
offering a precisely controlled gain and a very flexible
output load driving capability.
FIGURE 5. Very High Dynamic Range High Gain Amplifier.
load impedance. At higher frequencies, the match is
resistor. To achieve a 50
I
to P
O
—since the 2nd amplifier stage is invert-
P
Figure
I
Noise
50 Source
5dB
input) are easily achieved with
1:2
input match at the
feedback path
200
10pF
Input match
set by this
30.1
OPA687
420
11
DESIGN-IN TOOLS
DEMONSTRATION BOARDS
Two PC boards are available to assist in the initial evalua-
tion of circuit performance using the OPA687 in its two
package styles. Both of these are available free as an
unpopulated PC board delivered with descriptive documen-
tation. The summary information for these boards is shown
in the table below.
Contact the Burr-Brown applications support line to request
any of these boards.
MACROMODELS AND APPLICATIONS SUPPORT
Computer simulation of circuit performance using SPICE is
often useful when analyzing the performance of analog
circuits and systems. This is particularly true for video and
RF amplifier circuits where parasitic capacitance and induc-
tance can have a major effect on circuit performance. A
SPICE model for the OPA687 is available through either the
Burr-Brown Internet web page (http://www.burr-brown.com)
or as one model on a disk from the Burr-Brown Applica-
tions department (1-800-548-6132). The Applications de-
partment is also available for design assistance at this
number. These models do a good job of predicting small-
signal AC and transient performance under a wide variety of
operating conditions. They do not do as well in predicting
the harmonic distortion characteristics. These models do not
attempt to distinguish between the package types in their
small-signal AC performance.
PRODUCT
OPA687U
OPA687N
6.19k
750
46pF
Overall Gain
6-Lead SOT23-6
PACKAGE
8-Pin SO-8
OPA687
1.5k
1.6pF
P
P
O
I
= 35.6dB
DEM-OPA68xU
DEM-OPA6xxN
OPA687
NUMBER
P
BOARD
O
PART
LITERATURE
REQUEST
NUMBER
MKT-351
MKT-348
®