Draft:Chopper-stabilized amplifier

Chopper-stabilized amplifiers are precision operational amplifiers designed to minimize input offset voltage and drift, particularly in direct current (DC) applications. They achieve this by periodically sampling and correcting offset voltage through a modulation technique called chopping. These amplifiers are widely used in digital multimeters, analog sensor circuits, and high-precision instrumentation, where temperature stability and long-term accuracy are critical. First introduced in vacuum tube circuits and later adapted for solid-state designs, modern chopper-stabilized amplifiers are now available as monolithic integrated circuits with microvolt-level precision.

Introduction

Chopper-stabilized amplifiers are essential in scenarios demanding high DC voltage gain, where the voltage offset from standard amplifiers can cause significant disruptions. Additionally, the offset voltage of regular operational amplifiers is temperature-sensitive and can drift with changes in temperature. Previously, instruments designed without chopper-stabilized amplifiers required manual zero offset drift adjustments using a knob connected to a variable resistor, a method that was impractical and necessitated regular tuning. The development of zero offset compensation through chopper-stabilized amplifiers addressed this problem by incorporating an automatic feedback loop to adjust the circuit's zero offset. This innovation not only enhanced the zero offset correction but also improved the overall quality and temperature stability of the final circuit.

While the original patent was for tube amplifiers, transistor-based chopper-stabilized amplifiers soon emerged and were eventually superseded by monolithic op amp circuits. These modern op amps maintain microvolt levels of input offset voltage and exhibit high temperature stability.

Applications

Chopper-stabilized amplifiers are extensively used in precision electronic applications, such as DC amplifiers for voltmeters, digital multimeters (DMMs), and null detectors. They are also integral to precision analog sensor circuits for various measurements, including those involving strain gauges and other low-level signal sources. The critical advantage of using chopper-stabilized amplifiers in these contexts is their exceptional temperature stability and minimal zero offset, which are essential for ensuring accurate and reliable measurement outcomes in sensitive and precision-dependent environments.

History

The patent (US 2,684,999) for the chopper-stabilized amplifier, filed in 1949 by Edwin A. Goldberg and Jules Lehmann and granted in 1954, marks a pivotal advancement in the design and functionality of direct current amplifiers. This invention addressed the common challenge in DC amplifiers of drift in zero output voltage, which previously required manual adjustment over time due to changing characteristics of electronic tubes. Goldberg and Lehmann's design automated the stabilization of zero, drift, and gain in DC amplifiers through a novel use of a chopper mechanism—a contactor-type modulator that converts the error voltage into an alternating current, allowing it to be amplified and then rectified back to direct current to stabilize the amplifier's zero output effectively.

This innovation improved the reliability and maintenance of DC amplifiers and expanded their utility in various electronic applications by enhancing accuracy and reducing the need for frequent recalibrations. The introduction of this technology allowed DC amplifiers to maintain a stable zero output voltage automatically, which was particularly beneficial in precision electronic measurements and systems requiring sustained accuracy over time.

Principle of Operation

A chopper-stabilized amplifier employs a chopping generator that controls switches to compensate for the input offset voltage of the amplifier, correcting both the initial offset and any temperature-induced drift. The frequency of the chopping generator typically ranges from tens of hertz to tens of kilohertz. Initially, reed relays were used as the switching mechanism. During the transistor era, pairs of neon lights coupled to photoresistors were popular. In contemporary integrated chopper amplifier ICs, MOSFET switches are utilized, enhancing the efficiency and reliability of these systems.

Current implementations of chopper-stabilized operational amplifiers are often designed as integrated circuits, containing all the necessary circuitry to ensure outstanding zero offset and zero drift characteristics. These ICs are highly optimized to provide excellent stability and accuracy, making them ideal for precision applications where minimal voltage offset and drift are crucial.

References

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