Datasheet
Current-Feedback Amplifier
V
OUT
V
IN
+
a
ǒ
1 )
R
F
R
G
Ǔ
1 )
ǒ
1 )
R
F
R
G
Ǔ
1
g
m
R
1
[
1 ) s
(
R
1
C
1
) R
1
C
2
) R
2
C
2
)
) s
2
R
1
C
1
C
2
]
Control-Loop Amplifier
9
6
3
0
−
3
−
6
−
9
−
12
Frequency (Hz)
Gain (dB)
1M 10M 100M 1G
G = +2V/V
R
L
= 500
Ω
V
O
= 2V
PP
R
1
100
Ω
V
I
V
O
3 B
2
E
C
87
65
R
5
133
Ω
R
3
301
Ω
R
IN
50
Ω
G = +2V/V
I
Q
= 20mA
OTA
R
4
453
Ω
R
2
80.6
Ω
Network
Analyzer
+1
+5V
1 4
−
5V
50
Ω
R
Q
250
Ω
50
Ω
Source
OPA860
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....................................................................................................................................................... SBOS331C – JUNE 2005 – REVISED AUGUST 2008
loop amplifiers show an integrator behavior from DC
to the frequency, represented by the RC time
Building a current-feedback amplifier with the
constant of the network from the C-output to GND.
OPA860 is extremely simple. One advantage of
Above this frequency, they operate as an amp with
building a current-feedback amplifier with the
constant gain. The series connection increases the
OPA860 instead of getting an off-the-shelf
overall gain to about 110dB and thus minimizes the
current-feedback amplifier is the control gained on the
control loop deviation. The differential configuration at
bandwidth though the use of external capacitors.
the inputs enables one to apply the measured output
Figure 54 shows a typical circuit for the OPA860 in a
signal and the reference voltage to two identical
noninverting current-feedback amplifier configuration.
high-impedance inputs. The output buffer decouples
Input and output parasitic capacitances are shown.
the C-output of the second OTA in order to insure the
R
1
is the output impedance of the C-output of the
AC performance and to drive subsequent output
OTA section. C
1
is the output parasitic capacitance
stages.
on the C-output pin of the OTA-section. C
2
is the
input parasitic capacitance for the input of the Buffer
section. As shown in Equation 6 , the poles formed by
R
1
, C
1
, R
2
, and C
2
control the frequency response.
The frequency response in this configuration is shown
in Figure 52 . Setting an external capacitor on the
C-output to ground allows adjusting the bandwidth.
(6)
Note that both peaking and bandwidth can be
adjusted by changing the feedback resistance, R
F
.
A new type of control loop amplifier for fast and
precise control circuits can be designed with the
OPA860. The circuit of Figure 55 shows a series
Figure 52. Current-Feedback Architecture
connection of two voltage control current sources that
Frequency Response
have an integral (and at higher frequencies, a
proportional) behavior versus frequency. The control
Figure 53. Direct Feedback Amplifier Specification and Test Circuit
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