Datasheet
I
OUT
I
1
R
R
1
I
n
R
R
n
I
OUT
+ 1 S
n
j+1
I
j
R
j
R
I
OUT
I
2
I
n
I
1
I
OUT
+ 1 S
n
j+1
I
j
I
OUT
I
IN
C
R
I
OUT
+
1
C R
ŕ
I
IN
dt
I
OUT
I
IN
R
1
R
2
I
OUT
+
R
1
R
2
I
IN
OPA861
SBOS338G –AUGUST 2005–REVISED MAY 2013
www.ti.com
Table 1. Current-Mode Analog Computation Using the OTA Section
FUNCTIONAL ELEMENT TRANSFER FUNCTION IMPLEMENTATION WITH THE OTA SECTION
Current Amplifier
Current Integrator
Current Summer
Weighted Current Summer
OPA861 APPLICATIONS
Control-Loop Amplifier DC-Restore Circuit
A new type of control loop amplifier for fast and The OPA861 can be used advantageously with an
precise control circuits can be designed with the operational amplifier, here the OPA656, as a DC-
OPA861. The circuit of Figure 37 illustrates a series restore circuit. Figure 38 illustrates this design.
connection of two voltage control current sources that Depending on the collector current of the
have an integral (and at higher frequencies, a transconductance amplifier (OTA) of the OPA861, a
proportional) behavior versus frequency. The control switching function is realized with the diodes D
1
and
loop amplifiers show an integrator behavior from DC D
2
.
to the frequency represented by the RC time constant
When the C-output is sourcing current, the capacitor
of the network from the C-output to GND. Above this
C
1
is being charged. When the C-output is sinking
frequency, they operate as an amp with constant
current, D
1
is turned off and D
2
is turned on, letting
gain. The series connection increases the overall gain
the voltage across C
1
be discharged through R
2
.
to about 110dB and thus minimizes the control loop
deviation. The differential configuration at the inputs
The condition to charge C
1
is set by the voltage
enables one to apply the measured output signal and
difference between V
REF
and V
OUT
. For the OTA C-
the reference voltage to two identical high-impedance
output to source current, V
REF
has to be greater than
inputs. The output buffer decouples the C-output of
V
OUT
. The rate of charge of C
1
is set by both R
1
and
the second OTA in order to insure the AC
C
1
. The discharge rate is given by R
2
and C
1
.
performance and to drive subsequent output stages.
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