參數(shù)資料
型號(hào)： OPA685
英文描述： Ultra-Wideband, Current-Feedback OPERATIONAL AMPLIFIER With Disable TM
中文描述： 超寬帶，電流反饋運(yùn)算放大器商標(biāo)使用禁用
文件頁(yè)數(shù)： 17/28頁(yè)
文件大小： 283K
代理商： OPA685
17
OPA685
for fixed-gain RF amplifiers, but also makes the input match
dependent on 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 their more complex internal
circuits (giving higher input noise voltage and current terms)
and the fact that, 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. This higher Noise
Figure can be acceptable if the OPA685 has enough gain
preceding it in the IF chain.
Op amp Noise Figure equations include at least 6 terms (see
the Noise Performance section of this data sheet) due to the
external resistors. 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
non-inverting configuration. One easy way to improve the
Noise Figure for the non-inverting configuration of the
OPA685 is to include a 1:2 step-up transformer at the input
(Figure 5).
FIGURE 5. IF Amplifier with Improved Noise Figure.
The transformer provides a noiseless voltage gain at the
expense of higher source impedance for the OPA685’s non-
inverting input current noise. The input impedance is still set
to 50
by the 200
resistor on the transformer secondary.
Using a 1:2 step-up will cut the required amplifier gain in
half for any particular desired overall gain.
The following tables summarize the recommended resistor
values and resulting Noise Figures over desired gain setting
for three circuit options for the OPA685 operated as a
precision IF amplifier. In each case, R
F
and R
G
are adjusted
for both best bandwidth and to get the required gain.
Table I.
Non-inverting circuit of Figure 1
Table II. Non-inverting circuit of Figure 5 with a 1:2 input
step up transformer
Table III. Inverting circuit of Figure 2
GAIN TO LOAD
(dB)
R
F
(
)
R
G
(
)
NOISE
FIGURE
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
478
468
458
446
433
419
402
384
363
340
314
284
252
215
174
159
134
113
96
81
68
57
48
40
33
27
21
16
12
9
17.20
16.55
15.95
15.40
14.91
14.47
14.09
13.76
13.23
13.23
13.03
12.86
12.72
12.60
12.51
TABLE I. Non-Inverting Wideband Op Amp (Figure 1).
GAIN TO LOAD
(dB)
R
F
(
)
R
G
(
)
NOISE
FIGURE
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
516
511
506
500
493
486
478
469
458
447
434
419
403
384
364
518
412
334
275
228
190
160
135
114
96
81
69
58
48
40
16.34
15.54
14.78
14.07
13.40
12.78
12.21
11.70
11.25
10.85
10.15
10.21
9.96
9.74
9.57
TABLE II. Non-Inverting with a 1:2 Input Step-Up Trans-
former (Figure 5).
GAIN TO LOAD
(dB)
OPTIMUM
R
F
(
)
R
G
(
)
INPUT
MATCH R
T
NOISE
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).
In all cases, exact computed values for resistors are shown.
Choose standard resistor values which are closest to those in
the tables for implementation.
OPA685
+5V
DIS
–5V
50
Load
Supply decoupling
not shown.
50
R
G
200
V
I
V
O
R
F
50
Source
1:2
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