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參數(shù)資料
| 型號(hào)： | OPA686U |
| 元件分類： | 運(yùn)算放大器 |
| 英文描述： | Wideband, Low Noise, Voltage Feedback OPERATIONAL AMPLIFIER |
| 中文描述： | 寬帶，低噪聲，電壓反饋運(yùn)算放大器 |
| 文件頁(yè)數(shù)： | 10/15頁(yè) |
| 文件大?。?/td> | 158K |
| 代理商： | OPA686U |
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10
OPA686
12
9
6
3
0
–3
–6
–9
–12
–15
–18
Frequency (MHz)
G
1
10
100
500
170MHz
To choose the values for both C
S
and C
F
, two parameters and
only three equations need to be solved. The first parameter
is the target high frequency noise gain NG
2
, which should be
greater than the minimum stable gain for the OPA686. Here,
a target NG
2
of 10.5 will be used. The second parameter is
the desired low frequency signal gain, which also sets the
low frequency noise gain NG
1
. To simplify this discussion,
we will target a maximally flat second-order low pass
Butterworth frequency response (Q = 0.707). The signal
gain of –2 shown in Figure 4 will set the low frequency noise
gain to NG
1
= 1 + R
F
/R
G
(= 3 in this example). Then, using
only these two gains and the GBP for the OPA686
(1600MHz), the key frequency in the compensation can be
determined as:
Physically, this Z
0
(10.6MHz for the values shown above) is
set by 1/(2
π
R
F
(C
F
+ C
S
)) and is the frequency at which the
rising portion of the noise gain would intersect unity gain if
projected back to 0dB gain. The actual zero in the noise gain
occurs at NG
1
Z
0
and the pole in the noise gain occurs at
NG
2
Z
0
. Since GBP is expressed in Hz, multiply Z
0
by 2
π
and use this to get C
F
by solving:
1
2π
R
F
Z
O
NG
2
Finally, since C
S
and C
F
set the high frequency noise gain,
determine C
S
by [Using NG
2
= 10.5]:
C
S
=
NG
2
–1
(
The resulting closed-loop bandwidth will be approximately
equal to:
f
–
3
dB
Z
O
GBP
For the values shown in Figure 4, the f
–3dB
will be approxi-
mately 130MHz. This is less than that predicted by simply
dividing the GBP product by NG
1
. The compensation
network controls the bandwidth to a lower value while
providing the full slew rate at the output and an excep-
tional distortion performance due to increased loop gain at
frequencies below NG
1
Z
0
. The capacitor values shown
in Figure 4 are calculated for NG
1
= 3 and NG
2
= 10.5 with
no adjustment for parasitics.
Figure 5 shows the measured frequency response for the
circuit of Figure 4. This shows the expected gain of
–2 (6dB) with exceptional flatness through 70MHz and a
–3dB bandwidth of 170MHz. Measured distortion into a
100
load shows > 5dB improvement through 20MHz over
the performance shown in the Typical Performance Curves.
Into a 500
load, the 5MHz, 2Vp-p, 2nd harmonic improves
from –90dBc to –96dBc.
Z
O
=
GBP
NG
1
2
1–NG
1
NG
2
– 1– 2NG
1
NG
2
FIGURE 5. G = –2 Frequency Response with External
Compensation.
LOW NOISE FIGURE, HIGH DYNAMIC RANGE “IF”
AMPLIFIER
The low input noise voltage of the OPA686 and its high
two-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
input) are easily achieved with
just OPA686 alone, Figure 6 shows a technique which
reduces the noise figure even further while providing a
broadband, low gain IF amplifier stage using the OPA686.
FIGURE 6. Low Noise Figure IF Amplifier.
Bringing the signal in through a step-up transformer to the
inverting input gain resistor has several advantages for the
OPA686. First, grounding the non-inverting input elimi-
nates the contribution of the non-inverting input current
noise to the output noise. Secondly, the non-inverting
input voltage noise 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 50
source impedance at
C
F
=
)
C
F
(= 2.86pF)
(= 27.2pF)
(= 130MHz)
OPA686
+5V
–5V
R
G
200
R
F
1k
50
V
O
2pF
20pF
50
Source
50
Load
1:2
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相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| OPA686U/2K5 | 制造商:未知廠家 制造商全稱:未知廠家 功能描述:Voltage-Feedback Operational Amplifier |
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| OPA687N | 制造商:BB 制造商全稱:BB 功能描述:Wideband, Ultra-Low Noise, Voltage Feedback OPERATIONAL AMPLIFIER With Power Down |
| OPA687N/250 | 制造商:未知廠家 制造商全稱:未知廠家 功能描述:Voltage-Feedback Operational Amplifier |
| OPA687N/3K | 制造商:未知廠家 制造商全稱:未知廠家 功能描述:Voltage-Feedback Operational Amplifier |
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