OPA861
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SBOS338–AUGUST 2005
Table 1. Current-Mode Analog Computation Using the OTA Section
FUNCTIONAL ELEMENT
TRANSFER FUNCTION
IMPLEMENTATION WITH THE OTA SECTION
IOUT
IIN
R1
R2
IOUT
+
IIN
R1
Current Amplifier
R2
IOUT
IIN
1
IOUT
+
C
Current Integrator
ŕ
C R IINdt
R
IOUT
n
IOUT + 1 S Ij
j+1
Current Summer
I1
I2
In
IOUT
n
Rj
IOUT + 1 S Ij
j+1
R
Weighted Current Summer
R1
Rn
R
R
I1
In
OPA861 APPLICATIONS
Control-Loop Amplifier
DC-Restore Circuit
The OPA861 can be used advantageously with an
A new type of control loop amplifier for fast and
precise control circuits can be designed with the
OPA861. The circuit of Figure 34 illustrates a series
connection of two voltage control current sources that
have an integral (and at higher frequencies, a pro-
portional) behavior versus frequency. The control
loop amplifiers show an integrator behavior from DC
to the frequency represented by the RC time constant
of the network from the C-output to GND. Above this
frequency, they operate as an amp with constant
gain. The series connection increases the overall gain
to about 110dB and thus minimizes the control loop
deviation. The differential configuration at the inputs
enables one to apply the measured output signal and
the reference voltage to two identical high-impedance
inputs. The output buffer decouples the C-output of
the second OTA in order to insure the AC perform-
ance and to drive subsequent output stages.
operational amplifier, here the OPA656, as
a
DC-restore circuit. Figure 35 illustrates this design.
Depending on the collector current of the
transconductance amplifier (OTA) of the OPA861, a
switching function is realized with the diodes D1 and
D2.
When the C-output is sourcing current, the capacitor
C1 is being charged. When the C-output is sinking
current, D1 is turned off and D2 is turned on, letting
the voltage across C1 be discharged through R2.
The condition to charge C1 is set by the voltage
difference between VREF and VOUT. For the OTA
C-output to source current, VREF has to be greater
than VOUT. The rate of charge of C1 is set by both R1
and C1. The discharge rate is given by R2 and C1.
16
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