Chopper-stabilized amplifier

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patent was for tube amplifiers, transistor-based Chopper Stabilized Amplifiers soon emerged and were eventually superseded by monolithic OPAMP circuits. These modern OPAMPs maintain microvolt levels of input offset voltage and exhibit high temperature stability.

Where it Used

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 sensor types. 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 (US2684999) 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 addresses the common challenge in DC amplifiers regarding the drift of zero output voltage, which typically required manual adjustment over time due to the changing characteristics of electronic tubes. Goldberg and Lehmann's design automates 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 a direct current to stabilize the amplifier's zero output effectively.

This innovation not only enhanced the reliability and maintenance of DC amplifiers but also expanded their utility in various electronic applications by improving 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. The Chopper Stabilized Amplifier hence represents a significant evolution in amplifier technology, enabling more complex and reliable electronic systems.

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 light 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 "black boxes," containing all the necessary circuitry to ensure outstanding zero offset and zero drift characteristics. These integrated circuits are highly optimized to provide excellent stability and accuracy, making them ideal for precision applications where minimal voltage offset and drift are crucial.