Datasheet

OPA691
14
SBOS226D
www.ti.com
Each channel includes a bandpass filter. Each bandpass
filter is set for a different frequency band. This allows the
channelizing part of this circuit. The role of the channelizers
OPA691s is to provide impedance isolation. This is done
through the use of four matching resistances (59Ω in this
case). These matching resistors ensure that the signals will
combine during the transition between channels. They have
been used to get a gain of +1 at the load.
This circuit may be used with a different number of channels.
Its limitation comes from the drive requirement for each
channel as well as the minimum acceptable return loss.
The output resistor value (R
O
) to keep a gain of +1 at the load
depends on the number of channels. For the OPA691 with a
gain of 2 using R
F
= 402Ω and R
G
= 402Ω, Equation 1 is:
(1)
R
n
n
O
=
•
(
)
+
[]
•+
•
(
)
+
[]








75 1 804
2
1
241200
75 1 804
1
ΩΩ
Ω
ΩΩ
–
–
–
SINGLE-SUPPLY “IF” AMPLIFIER
The high bandwidth provided by the OPA691 while operating
on a single +5V supply lends itself well to IF amplifier
applications. One of the advantages of using an op amp like
the OPA691 as an IF amplifier is that precise signal gain is
achieved along with much lower 3rd-order intermodulation
versus quiescent power dissipation. In addition, the OPA691
in the SOT23-6 package offers a very small package with a
power shutdown feature for portable applications. One con-
cern with using op amps for an IF amplifier is their relatively
high noise figures. It is sometimes suggested that an opti-
mum source resistance can be used to minimize op amp
noise figures. Adding a resistor to reach this optimum value
may improve the noise figure, but will actually decrease the
signal-to-noise ratio. A more effective way to move towards
an optimum source impedance is to bring the signal in
through an input transformer. Figure 6 shows an example
that is particularly useful for the OPA691.
Bringing the signal in through a step-up transformer to the
inverting input gain resistor has several advantages for the
OPA691. First, the decoupling capacitor on the noninverting
input eliminates the contribution of the noninverting input current
noise to the output noise. Secondly, the noninverting input noise
voltage of the op amp is actually attenuated if reflected to the
input side of R
G
. Using the 1:2 (turns ratio) step-up transformer
reflects the 50Ω source impedance at the primary through to the
secondary as a 200Ω source impedance (and the 200Ω R
G
resistor is reflected through to the transformer primary as a 50Ω
input matching impedance). The noise gain to the amplifier
output is then 1 + 600/400 = 2.5V/V. Taking the op amp’s
2.2nV/
√Hz input voltage noise times this noise gain to the
output, then reflecting this noise term to the input side of the R
G
resistor, divides it by 3. This gives a net gain of 0.833 for the
noninverting input voltage noise when reflected to the input
point for the op amp circuit. This is further reduced when
referred back to the transformer primary.
The relatively low-gain IF amplifier circuit of Figure 6 gives a
12dB noise figure at the input of the transformer. Increasing
the R
F
resistor to 600Ω (once R
G
is set to 200Ω for input
impedance matching) will slightly reduce the bandwidth.
Measured results show 150MHz small-signal bandwidth for
the circuit of Figure 6 with exceptional flatness through
30MHz. Although the OPA691 does not show an intercept
characteristic for the 2-tone, 3rd-order intermodulation distor-
tion, it does hold a very high Spurious-Free Dynamic Range
(SFDR) through high output powers and frequencies. The
maximum single-tone power at the matched load for the
single-supply circuit of Figure 6 is 1dBm (this requires a
2.8V
PP
swing at the output pin of the OPA691 for the 2-tone
envelope). Measured 2-tone SFDR at this maximum load
power for the circuit of Figure 6 exceeds 55dBc for frequen-
cies to 20MHz.
DESIGN-IN TOOLS
DEMONSTRATION FIXTURES
Two printed circuit boards (PCBs) are available to assist in
the initial evaluation of circuit performance using the OPA691
in its two package options. Both of these are offered free of
charge as unpopulated PCBs, delivered with a user's guide.
The summary information for these fixtures is shown in
the table below.
ORDERING LITERATURE
PRODUCT PACKAGE NUMBER NUMBER
OPA691ID SO-8 DEM-OPA-SO-1A SBOU009
OPA691IDBV SOT23-6 DEM-OPA-SOT-1A SBOU010
R
F
600Ω
V
O
= 3V/V (9.54dB)
V
I
R
G
200Ω
OPA691
+5V
DIS
Power-supply
decoupling not shown.
V
I
V
O
50Ω Load
50Ω
50Ω Source
1:2
5kΩ
5kΩ1µF
0.1µF
FIGURE 6. Low-Noise, Single-Supply IF Amplifier.
The demonstration fixtures can be requested at the Texas
Instruments web site (www.ti.com) through the OPA691
product folder.
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