OPA861IDBVT datasheet(16/28 Pages) TI

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OUT +

OUT

INdt

OUT

OUT + 1 S

j+1

OUT

OUT + 1 S

j+1

OUT

OPA861 APPLICATIONS

Control-Loop Amplifier

DC-Restore Circuit

OPA861

SBOS338 – AUGUST 2005

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

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,

OPA861. The circuit of Figure 34 illustrates a series

DC-restore circuit. Figure 35 illustrates this design.

connection of two voltage control current sources that

Depending

the

collector

current

the

have an integral (and at higher frequencies, a pro-

transconductance amplifier (OTA) of the OPA861, a

portional) behavior versus frequency. The control

switching function is realized with the diodes D1 and

loop amplifiers show an integrator behavior from DC

D2.

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

C1 is being charged. When the C-output is sinking

frequency, they operate as an amp with constant

current, D1 is turned off and D2 is turned on, letting

gain. The series connection increases the overall gain

the voltage across C1 be discharged through R2.

to about 110dB and thus minimizes the control loop

deviation. The differential configuration at the inputs

The condition to charge C1 is set by the voltage

enables one to apply the measured output signal and

difference between VREF and VOUT. For the OTA

the reference voltage to two identical high-impedance

C-output to source current, VREF has to be greater

inputs. The output buffer decouples the C-output of

than VOUT. The rate of charge of C1 is set by both R1

the second OTA in order to insure the AC perform-

and C1. The discharge rate is given by R2 and C1.

ance and to drive subsequent output stages.

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