Chopper (electronics)

Schematic of an inverter using a vibrator as a chopper.

A chopper circuit is used to refer to numerous types of electronic switching devices and circuits. The term has become somewhat ill-defined, and as a result is much less used nowadays than it was perhaps 30 or more years ago.

Essentially, a chopper is an electronic switch that is used to interrupt one signal under the control of another. Most modern uses also use alternative nomenclature which helps to clarify which particular type of circuit is being discussed. These include:

• switched mode power supplies, including DC to DC converters.
• Speed controllers for DC motors
• Class D Electronic amplifiers
• Switched capacitor filters
• Variable Frequency Drive

Chopper amplifiers

One classic use for a chopper circuit and where the term is still in use is in chopper amplifiers. These are DC amplifiers. Some types of signal that need amplifying can be so small that an incredibly high gain is required, but very high gain DC amplifiers are much harder to build with low offset and 1/f noise, and reasonable stability and bandwidth. It's much easier to build an AC amplifier instead. A chopper circuit is used to break up the input signal so that it can be processed as if it were an AC signal, then integrated back to a DC signal at the output. In this way, extremely small DC signals can be amplified. This approach is often used in electronic instrumentation where stability and accuracy are essential; for example, it is possible using these techniques to construct pico-voltmeters and Hall sensors.

The input offset voltage of amplifiers becomes important when trying to amplify small signals with very high gain. Because this technique creates a very low input offset voltage amplifier, and because this input offset voltage does not change much with time and temperature, these techniques are also called "Zero-Drift" amplifiers (because there is no drift in input offset voltage with time and temperature). Related techniques that also give these Zero-drift advantages are Auto-zero and Chopper-stabilized Amplifiers.

Auto-zero amplifiers use a secondary auxiliary amplifier to correct the input offset voltage of a main amplifier. Chopper-stabilized amplifiers use a combination of auto-zero and chopper techniques to give some excellent DC precision specifications. ^[1]

Some example chopper and auto-zero amplifiers are LTC2050,MAX4238/MAX4239 and OPA333.

1. ^ US Patent 7132883 - Chopper chopper-stabilized instrumentation and operational amplifiers

References

• C. Enz, G. Temes, Circuit techniques for reducing the Effect of Op-Amp Imperfections: Autozeroing, Correlated Double Sampling and Chopper Stabilization - Proceedings of the IEEE, vol. 84 No. 11, Nov. 1996
• A. Bilotti, G. Monreal, Chopper-Stabilized Amplifiers with a Track-and-hold Signal Demodulator - Allegro Technical Paper STP 99-1
• A. Bakker, K. Thiele, J. Huijsing, A CMOS Nested-Chopper Instrumention Amplifier with 100-nV Offset - IEEE J. Solid-State Circuits, vol. 35 No. 12, Dec 2000