參數(shù)資料
型號(hào)： VCA810AIDR
英文描述： High Gain Adjust Range, Wideband, Voltage-Controlled Amplifier
中文描述： 高增益調(diào)整范圍，寬帶，電壓控制放大器
文件頁(yè)數(shù)： 14/25頁(yè)
文件大?。?/td> 417K
代理商： VCA810AIDR
"#$%
SBOS275C JUNE 2003 REVISED OCTOBER 2004
www.ti.com
14
LOW DRIFT WIDEBAND LOG AMP
The VCA810 can be used to provide a 2.5MHz (–3dB) log
amp with low offset voltage and low gain drift. The
exponential gain-control characteristic of the VCA810
permits simple generation of a temperature-compensated
logarithmic response. Enclosing the exponential function
in an op-amp feedback path inverts this function,
producing the log response. Figure 7 shows the practical
implementation of this technique. A DC reference voltage,
V
R
, sets the VCA810 inverting input voltage. This
configuration makes the amplifier output voltage
V
OA
= GV
R
, where
G
10
2(VC
1)
.
R
1
470
VCA810
R
2
330
V
OL
V
10mV
OPA820
V
IN
V
OA
=
GV
R
C
50pF
R
3
100
V
OL
=
1 +
1 + 0.5 Log(
V
IN
/V
R
)
R
1
R
2
(
)
V
C
Figure 7. Temperature Compensated Log
Response
A second input voltage also influences V
OA
through
control of gain G. The feedback op amp forces V
OA
to
equal the input voltage V
IN
connected at the op amp
inverting input. Any difference between these two signals
drops across R
3
, producing a feedback current that
charges C
C
. The resulting change in V
OL
adjusts the gain
of the VCA810 to change V
OA
.
At equilibrium:
V
OA
V
IN
V
R
10
2(VC
1)
The op amp forces this equality by supplying the gain
R
1
V
OL
R
1
control voltage,
V
C
R
2
.
Combining the last two expressions and solving for V
OL
yields the circuit’s logarithmic response:
V
OL
1
R
2
R
1
1
0.5
log
V
IN
V
R
An examination of this result illustrates several circuit
characteristics. First, the argument of the log term,
V
IN
/V
R
, reveals an option and a constraint. In Figure 7,
V
R
represents a DC reference voltage. Optionally, making
this voltage a second signal produces log-ratio operation.
Either way, the log term’s argument constrains the
polarities of V
R
and V
IN
. These two voltages must be of
opposite polarities to ensure a positive argument. This
polarity combination results when V
R
connects to the
inverting input of the VCA810. Alternately, switching V
R
to
the amplifier non-inverting input removes the minus sign of
the log term argument. Then, both voltages must be of the
same polarity in order to produce a positive argument. In
either case, the positive polarity requirement of the
argument restricts V
IN
to a unipolar range. Figure 8
illustrates these constraints.
The above V
OL
expression reflects a circuit gain
introduced by the presence of R
1
and R
2
. This feature
adds a convenient scaling control to the circuit. However,
a practical matter sets a minimum level for this gain. The
voltage divider formed by R
1
and R
2
attenuates the voltage
supplied to the V
C
terminal by the op amp. This attenuation
must be great enough to prevent any possibility of an
overload voltage at the V
C
terminal. Such an overload
saturates the VCA810 gain-control circuitry, reducing the
amplifier’s gain. For the feedback connection of Figure 7,
this overload condition permits a circuit latch. To prevent
this, choose R
1
and R
2
to ensure that the op amp cannot
possibly deliver a more negative input than 2.5V to the V
C
terminal.
Figure 8 exhibits three zones of operation described
below:
Zone I:
V
C
> 0V. The VCA810 is operating in full
attenuation (80dB). The non-inverting input of the
OPA820 will see
～
0V. V
OL
is going to be the integration of
the input signal.
Zone II:
2V < V
C
< 0V. The VCA810 is in its normal
operating mode, creating the log relationship in Equation (2).
Zone III:
V
C
< 2V. The VCA810 control pin is out of range,
and some measure should be taken so that it does not
exceed –2.5V. A limiting action could be achieved by using
a voltage limiting amplifier.
(1)
(2)
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參數(shù)描述
VCA810AIDRG4 功能描述:特殊用途放大器 High-Gain Adj Range Wideband Vltg-Cntrl RoHS:否 制造商:Texas Instruments 通道數(shù)量:Single 共模抑制比（最小值）: 輸入補(bǔ)償電壓: 工作電源電壓:3 V to 5.5 V 電源電流:5 mA 最大功率耗散: 最大工作溫度:+ 70 C 最小工作溫度:- 40 C 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:QFN-20 封裝:Reel
VCA810ID 功能描述:特殊用途放大器 High-Gain Adj Range Wideband Vltg-Cntrl RoHS:否 制造商:Texas Instruments 通道數(shù)量:Single 共模抑制比（最小值）: 輸入補(bǔ)償電壓: 工作電源電壓:3 V to 5.5 V 電源電流:5 mA 最大功率耗散: 最大工作溫度:+ 70 C 最小工作溫度:- 40 C 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:QFN-20 封裝:Reel
VCA810IDG4 功能描述:特殊用途放大器 High-Gain Adj Range Wideband Vltg-Cntrl RoHS:否 制造商:Texas Instruments 通道數(shù)量:Single 共模抑制比（最小值）: 輸入補(bǔ)償電壓: 工作電源電壓:3 V to 5.5 V 電源電流:5 mA 最大功率耗散: 最大工作溫度:+ 70 C 最小工作溫度:- 40 C 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:QFN-20 封裝:Reel
VCA810IDR 功能描述:特殊用途放大器 High-Gain Adj Range Wideband Vltg-Cntrl RoHS:否 制造商:Texas Instruments 通道數(shù)量:Single 共模抑制比（最小值）: 輸入補(bǔ)償電壓: 工作電源電壓:3 V to 5.5 V 電源電流:5 mA 最大功率耗散: 最大工作溫度:+ 70 C 最小工作溫度:- 40 C 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:QFN-20 封裝:Reel
VCA810IDRG4 功能描述:特殊用途放大器 High-Gain Adj Range Wideband Vltg-Cntrl RoHS:否 制造商:Texas Instruments 通道數(shù)量:Single 共模抑制比（最小值）: 輸入補(bǔ)償電壓: 工作電源電壓:3 V to 5.5 V 電源電流:5 mA 最大功率耗散: 最大工作溫度:+ 70 C 最小工作溫度:- 40 C 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:QFN-20 封裝:Reel
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