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參數(shù)資料
| 型號： | OPA643PB |
| 英文描述： | Wideband Low Distortion, High Gain OPERATIONAL AMPLIFIER |
| 中文描述： | 寬帶低失真，高增益運算放大器 |
| 文件頁數(shù)： | 11/17頁 |
| 文件大小： | 262K |
| 代理商： | OPA643PB |
OPA643
11
This will increase the Q for the closed-loop poles, peaking
up the frequency response and extending the bandwidth. A
peaked frequency response will show overshoot and ringing
in the pulse response as well as a higher integrated output
noise. Operating at a noise gain less than +3 runs the risk of
sustained oscillation (loop instability). However, operation
at low gains would be desirable to take advantage of the
much higher slew rate and lower input noise voltage available
in the OPA643, as compared to performance offered by
unity gain stable op amps. Numerous external compensation
techniques have been suggested for operating a high gain op
amp at low gains. Most of these give zero/pole pairs in the
closed-loop response that cause long term settling tails in the
pulse response and/or phase non-linearity in the frequency
response. Figure 5 shows an external compensation method
for the non-inverting configuration that does not suffer from
these drawbacks.
gain for the op amp and the noise gain pole, set by 1/R
F
C
F
,
is placed correctly, a very well controlled second-order low
pass frequency response will result.
R
F
402
R
I
133
R
G
402
R
T
50
50
OPA643
+5V
–5V
V
O
50
Source
FIGURE 5. Broadband Low Gain Non-Inverting External
Compensation.
The R
I
resistor across the two inputs will increase the noise
gain (i.e. decrease the loop gain) without changing the signal
gain. This approach will retain the full slew rate to the output
but will give up some of the low noise benefit of the
OPA643. Assuming a low source impedance, set R
I
so that
1+R
F
/(R
G
||
R
I
) is
≥
+3.
Where a low gain is desired, and inverting operation is
acceptable, a new external compensation technique may be
used to retain the full slew rate and noise benefits of the
OPA643 while maintaining the increased loop gain and the
associated improvement in distortion offered by the
decompensated architecture. This technique shapes the loop
gain for good stability while giving an easily controlled
second-order low pass frequency response. Figure 6 shows
this circuit (the same amplifier circuit as shown on the front
page). Considering only the noise gain for the circuit of
Figure 6, the low frequency noise gain, (NG
1
) will be set by
the resistor ratios while the high frequency noise gain (NG
2
)
will be set by the capacitor ratios. The capacitor values set
both the transition frequencies and the high frequency noise
gain. If this noise gain, determined by NG
2
= 1+ C
S
/C
F
, is set
to a value greater than the recommended minimum stable
R
F
806
C
12.6pF
0.1μF
OPA643
+5V
–5V
V
O
V
I
C
1.9pF
R
G
402
R
T
280
FIGURE 6. Broadband Low Gain Inverting External
Compensation.
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 OPA643. Here,
a target NG
2
of 7.5 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 6 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 Gain Bandwidth Product (GBP)
for the OPA643 (800MHz), the key frequency in the
compensation can be determined as:
Physically, this Z
0
(13.6MHz for the values shown in Figure
6) 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:
Finally, since C
S
and C
F
set the high frequency noise gain,
determine C
S
by:
C
S
=
NG
2
–1
(
The resulting closed-loop bandwidth will be approximately
equal to:
F
–3dB
Z
O
GBP
Z
O
=
GBP
NG
1
2
1–NG
1
NG
2
– 1– 2NG
1
NG
2
C
F
=
1
2
π
R
F
Z
O
NG
2
)
C
F
相關PDF資料 |
PDF描述 |
|---|---|
| OPA643U | Wideband Low Distortion, High Gain OPERATIONAL AMPLIFIER |
| OPA643NB | Wideband Low Distortion, High Gain OPERATIONAL AMPLIFIER |
| OPA643UB | Wideband Low Distortion, High Gain OPERATIONAL AMPLIFIER |
| OPA646 | Low Power, Wide Bandwidth OPERATIONAL AMPLIFIER |
| OPA646U | Low Power, Wide Bandwidth OPERATIONAL AMPLIFIER |
相關代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| OPA643U | 制造商:Rochester Electronics LLC 功能描述:HIGH SPEED OP AMP - Bulk |
| OPA643UB | 制造商:Rochester Electronics LLC 功能描述:HIGH SPEED OP AMP - Bulk |
| OPA644 | 制造商:未知廠家 制造商全稱:未知廠家 功能描述:OPA644 - DISCONTINUED PRODUCT. No longer recommended for new design. |
| OPA644U | 制造商:Rochester Electronics LLC 功能描述:- Bulk |
| OPA644UB | 制造商:BB 制造商全稱:BB 功能描述:Low Distortion Current Feedback OPERATIONAL AMPLIFIER |
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