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| 型號： | 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 |
| 文件頁數(shù)： | 15/28頁 |
| 文件大小： | 283K |
| 代理商： | OPA685 |
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15
OPA685
RF SPECIFICATIONS AND
APPLICATIONS
The ultra-high full power bandwidth and 3rd-order intercept
of the OPA685 may be used to good advantage in IF
amplifier applications. Additional benefits in using a
wideband op amp such as the OPA685 include extremely
good (and independent) I/O impedance matching as well as
very high reverse isolation. A designer accustomed to using
fixed-gain RF amplifiers will get almost perfect gain accu-
racy, much higher I/O return loss, and 3rd-order intercept
points exceeding 40dBm (up to 50MHz) using only 12mA
supply current for the OPA685. Using the considerable
design freedom given by adjusting the external resistors, the
OPA685 can replace a wide range of fixed-gain RF ampli-
fiers with a single part. To understand in RF amplifier terms
how to take advantage of this, first consider the four ‘S’
parameters (this will be done using the example circuits of
Figures 1 and 2 on
±
5V supplies. However, similar results
can be obtained on a single +5V supply).
INPUT RETURN LOSS (S
11
)
This is a measure of how closely (over frequency) the input
impedance matches the source impedance. This is relatively
independent of gain setting for both the non-inverting and
inverting configurations. The Typical Performance Curves
show the magnitude of S
11
through 1GHz for the circuits of
Figures 1 and 2 (non-inverting gain of +8 and inverting gain
of –8 operation, respectively). Non-inverting operation of-
fers better matching to higher frequencies with the only
deviation due to the parasitic input capacitance of the non-
inverting input. The non-inverting input match is set simply
by the resistor to ground on the non-inverting input since the
amplifier itself shows a very high input impedance. Invert-
ing operation is also very good, but S
11
rises more quickly
due to loop gain roll-off effects appearing at the inverting
node. The inverting mode input match is set by the parallel
combination of R
G
and R
T
in Figure 2 since the inverting
amplifier node may be considered a virtual ground. A good
fixed-gain RF amplifier would have an input Voltage Stand-
ing Wave Ratio (VSWR) < 1.2:1. This corresponds to an S
11
of –21dB. The OPA685 exceeds this performance through
100MHz for the inverting mode of operation and through
250MHz for the non-inverting.
OUTPUT RETURN LOSS (S
22
)
This is a measure of how closely (over frequency) the output
impedance matches the load impedance. This is relatively
independent of gain for both non-inverting and inverting
operation. To first-order, the output matching impedance is
simply set by adding a series resistor to the low impedance
output of the op amp. Since the op amp itself shows a very
low output impedance which increases with frequency, an
improvement in the output match can be obtained by adding
a small equalizing capacitor across this output resistor. The
Typical Performance Curves show the measured S
22
with
and without this 3.3pF capacitor across the 50
output
resistor. Again, a very good match for a fixed-gain RF
amplifier would be a VSWR of 1.2:1. Looking at the Typical
Performance Curves for S
22
and where it rises above –21dB,
the OPA685 exceeds this level of performance through
100MHz without the equalizing capacitor and through
250MHz with it.
FORWARD GAIN (S
21
)
In all high-speed amplifier data sheets, this is referred to as
the small-signal gain which is plotted over frequency. The
difference between non-inverting and inverting operation is
that the phase of S
21
starts out at 0
°
for the non-inverting and
–180
°
for the inverting. This initial phase shift for inverting
mode is inconsequential to most IF strip applications. The
phase of OPA685 is shown in the Typical Performance
Curves as a part of the gain flatness curve. It is very linear
with frequency and may be accurately modeled as a constant
time delay through the amplifier.
The Typical Performance Curves for the OPA685 show S
21
over a range of signal gains where the external resistors have
been adjusted to re-optimize flatness at each gain setting.
Since this is a current-feedback op amp, the signal bandwidth
can be held relatively constant as the desired gain setting is
changed. The “Non-Inverting Small-Signal Frequency Re-
sponse” curve shows some change in bandwidth versus gain
(due to parasitic capacitive effects on the inverting node)
with very little variation for inverting operation.
Signal gains are most often referred to as V/V in op amp data
sheets. This is the voltage gain from input to output and is
set by external resistor ratios. Since the output impedance is
set by a physical series resistor, the voltage gain to the
matched load is cut in half by this resistor divider (Figures
1 and 2). The log gain to the matched load for the non-
inverting circuit of Figure 1 is:
(1)
The log gain to the matched load for the inverting circuit of
Figure 2 is:
(2)
The specific resistor values used in Figures 1 and 2 give both
a maximally flat bandwidth and a log gain to the matched
load of 12dB. The design tables at the end of this section
summarize the required resistor values over a range of
desired gains for the circuits of Figures 1 and 2.
As the desired signal gain increases, the achievable band-
width will decrease. In the non-inverting case, it decreases
relatively quickly, as shown in the Typical Performance
Curves. The inverting configuration holds almost constant
bandwidth (with correctly selected external resistor values)
until R
G
reduces to 50
and remains at that value to satisfy
the input impedance matching requirement. Further increases
in gain are achieved by increasing R
F
, shown in Figure 2.
The bandwidth then decreases rapidly as shown by the gain
of –16V/V plot in the Typical Performance Curves.
G
R
R
dB
F
G
+
=
+
20
1
2
1
log
G
R
R
dB
F
G
–
log
=
20
1
2
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相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| OPA685N | 制造商:BB 制造商全稱:BB 功能描述:Ultra-Wideband, Current-Feedback OPERATIONAL AMPLIFIER With Disable TM |
| OPA685N/250 | 制造商:未知廠家 制造商全稱:未知廠家 功能描述:Current-Feedback Operational Amplifier |
| OPA685N/3K | 制造商:未知廠家 制造商全稱:未知廠家 功能描述:Current-Feedback Operational Amplifier |
| OPA685U | 功能描述:高速運(yùn)算放大器 Ultra-Wideband RoHS:否 制造商:Texas Instruments 通道數(shù)量:1 電壓增益 dB:116 dB 輸入補(bǔ)償電壓:0.5 mV 轉(zhuǎn)換速度:55 V/us 工作電源電壓:36 V 電源電流:7.5 mA 最大工作溫度:+ 85 C 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:SOIC-8 封裝:Tube |
| OPA685U/2K5 | 制造商:未知廠家 制造商全稱:未知廠家 功能描述:Current-Feedback Operational Amplifier |
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