Datasheet

OPA695
18
SBOS293G
www.ti.com
FIGURE 8. Dual Output LO Buffer.
SAW FILTER BUFFER
One common requirement in an IF strip is to buffer the output
of a mixer with enough gain to recover the insertion loss of
a narrowband SAW filter. Figure 6 shows one possible
configuration driving a SAW filter. Figure 7 shows the inter-
cept at the 50Ω load. Operating in the inverting mode at a
voltage gain of –8V/V, this circuit provides a 50Ω input match
using the gain set resistor, has the feedback optimized for
maximum bandwidth (700MHz in this case), and drives
through a 50Ω output resistor into the matching network at
the input of the SAW filter. If the SAW filter gives a 12dB
insertion loss, a net gain of 0dB to the 50Ω load at the output
of the SAW (which could be the input impedance of the next
IF amplifier or mixer) will be delivered in the passband of the
SAW filter. Using the OPA695 in this application will isolate
the first mixer from the impedance of the SAW filter and
provide very low two-tone, 3rd-order spurious levels in the
SAW filter bandwidth. Inverting operation will give the broad-
est bandwidth up to a gain of –12V/V (15.6dB). Noninverting
operation will give higher bandwidth at gain settings higher
than this, but will also give a slight reduction in intercept and
Noise Figure performance.
OPA695
R
F
511Ω
R
G
511Ω
50Ω
50Ω
LO
50Ω
LNA
Diversity Receiver
Antenna
Antenna
IF1
IF2
Bandpass
Filter
Bandpass
Filter
LNA
–5V
+5V
DIS
Power supply decoupling not shown.
LO BUFFER AMPLIFIER
The OPA695 may also be used to buffer the Local Oscillator
(LO) from the mixer(s). Operating at a voltage gain of +2, the
OPA695 will provide almost perfect load isolation for the LO
with a net gain of 0dB to the mixer. Applications through
1.4GHz LOs may be considered, but best operation would be
for LOs < 1.0GHz at a gain of +2. Gain could also be easily
provided by the OPA695 to drive higher power levels into the
mixer. One unique option in using the OPA695 as an LO
buffer is shown in Figure 8. Since the OPA695 can drive
multiple output loads, two identical LO signals may be
delivered to the mixers in a diversity receiver simply by
tapping the output off through two series 50Ω output resis-
tors. This circuit is set up for a voltage gain of +2V/V to the
output pin for a gain of +1V/V (0dB) to the mixers, but could
easily be adjusted to deliver higher gains as well.
OPA695
SAW
Filter
+12V
Matching
Network
= 12dB – (SAW Loss)
50Ω
Source
50Ω
50Ω
P
O
P
O
P
I
400Ω
50Ω
P
I
0.1µF1000pF
1000pF
5kΩ
5kΩ
FIGURE 6. IF Amplifier Driving SAW Filter.
FIGURE 7. 2-Tone, 3rd-Order Intermodulation Intercept.
50
40
30
20
10
Center Frequency (MHz)
0 50 150 250200100
Output Intercept (dBm)