双运放组成的恒流源-TI方案
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2024-02-08
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As Published on EN-Genius.net
A High-Voltage Bidirectional Current Source
by John Caldwell, Analog Applications Engineer, Texas Instruments
Integrated circuit (IC) difference amplifiers are a popular approach to building
bidirectional current sources using the Howland current pump topology. Many engineers
often are surprised to find that these circuits are limited, not by their output current, but
by the output voltage they can produce across the load. Because the vast majority of IC
difference amplifiers are designed to operate on 36 V power supplies, these circuits only
produce the expected output current into low impedances. By combining an IC difference
amplifier featuring an extremely wide input common-mode range with a bootstrapped
power supply scheme, a modified Howland current pump topology is presented that
allows for very wide power supply voltages. These elevated power supply voltages
enable linear circuit operation over a broader range of load impedances. To demonstrate
the performance of this circuit a bidirectional current source capable of linearly sourcing
and sinking 10 mA into a 10 kΩ load was built using low-cost and widely available
components. The demonstration circuit exhibited exceptional linearity and transient
response inherent to the Howland topology without the use of specialized high-voltage
integrated circuits.
The Howland current pump is a commonly used circuit employing an op amp and several
resistors to perform voltage-to-current conversion. Monolithic difference amplifiers offer
a convenient way to create a Howland current pump with excellent performance as is
shown in Fig. 1. If the four on-chip resistors are of equal value, the relationship of the
output current to the input voltage is:
L
OUT
IN
OUTLIN
I
R
V
RIV =!= )(
(Eq. 1)
The on-chip resistances in these difference amplifiers are closely matched allowing this
topology to achieve extremely high output impedances
[1]
. Unfortunately, this circuit has
several practical limitations.
Fig. 1: Bidirectional Current Source Employing Monolithic Difference Amplifier
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