Datasheet

OPA695
17
SBOS293G
www.ti.com
FIGURE 5. IF Amplifier with Improved Noise Figure.
GAIN TO LOAD R
F
R
G
NOISE
(dB) ()() FIGURE
6 478 159 17.20
7 468 134 16.55
8 458 113 15.95
9 446 96 15.40
10 433 81 14.91
11 419 68 14.47
12 402 57 14.09
13 384 48 13.76
14 363 40 13.23
15 340 33 13.23
16 314 27 13.03
17 284 21 12.86
18 252 16 12.72
19 215 12 12.60
20 174 9 12.51
TABLE I. Noninverting Wideband Op Amp (Figure 1).
GAIN TO LOAD R
F
R
G
NOISE
(dB) ()() FIGURE
6 516 518 16.34
7 511 412 15.54
8 506 334 14.78
9 500 275 14.07
10 493 228 13.40
11 486 190 12.78
12 478 160 12.21
13 469 135 11.70
14 458 114 11.25
15 447 96 10.85
16 434 81 10.15
17 419 69 10.21
18 403 58 9.96
19 384 48 9.74
20 364 40 9.57
TABLE II. Noninverting with a 1:2 Input Step-Up Trans-
former (Figure 5).
GAIN TO LOAD
OPTIMUM R
G
INPUT NOISE
(dB) R
F
()() MATCH R
T
FIGURE
6 463.27 116 87 16.94
7 454.61 101 98 16.06
8 444.91 88 114 15.16
9 434.07 77 142 14.23
10 421.95 66 199 13.24
11 408.42 57 380 12.16
12 398.11 50 Infinite 11.03
13 446.68 50 Infinite 10.92
14 501.19 50 Infinite 10.83
15 562.34 50 Infinite 10.75
16 630.96 50 Infinite 10.67
17 707.95 50 Infinite 10.61
18 794.33 50 Infinite 10.55
19 891.25 50 Infinite 10.49
20 1000.00 50 Infinite 10.45
TABLE III. Inverting Wideband RF Amplifier (Figure 2).
NOISE FIGURE
All fixed-gain RF amplifiers show a very good noise figure
(typically < 5dB). For broadband amplifiers, this is achieved
by a low-noise input transistor and an input match set by
feedback. This feedback greatly reduces the noise figure for
fixed-gain RF amplifiers, but also makes the input match
dependent on the load and the output match dependent on
the source impedance at the input.
The noise figure for an op amp is always higher than for
fixed-gain RF amplifiers due to the more complex internal
circuits of an op amp (giving higher input noise voltage and
current terms). Also, for simple circuits, the input match is set
resistively. What is gained is an almost perfect I/O imped-
ance match, much better load isolation, and very high 3rd-
order intercepts versus quiescent power. These higher noise
figures can be acceptable if the OPA695 has enough gain
preceding it in the IF chain.
Op amp noise figure equations include at least six terms (see
the Noise Performance section), due to the external resis-
tors. As a point of reference, the circuit of Figure 1 has an
input noise figure of 14dB while the inverting configuration of
Figure 2 has an input noise figure of 11dB. At higher gains,
it is typical for the inverting noise figure to be slightly better
than for an equivalent gain, noninverting configuration. One
easy way to improve the noise figure for the noninverting
configuration of the OPA695 is to include a step-up, 1:2 turns
ratio transformer at the input. This configuration is shown in
Figure 5.
OPA695
+5V
DIS
5V
50 Load
Supply decoupling
not shown.
50
R
G
200
V
I
V
O
R
F
50 Source
1:2
In all cases, exact computed values for resistors are shown
in application, pick standard resistor values that are closest
to those in the tables.
The transformer provides a noiseless voltage gain at the
expense of higher source impedance for the OPA695
noninverting input current noise. The input impedance is still
set to 50 by the 200 resistor on the transformer second-
ary. A 1:2 turns ratio transformer will reflect the 200 to the
input side as a 50 impedance over the bandwidth of the
transformer. Using a 1:2 step-up transformer will also reduce
the required amplifier gain by 1/2 for any particular desired
overall gain.
Tables I - III summarize the recommended resistor values
and resulting noise figures over the desired gain setting for
three circuit options for the OPA695 operated as a precision
IF amplifier. In each case, R
F
and R
G
are adjusted for both
best bandwidth and to achieve the required gain.