Chopper And Chopper-Stabilised Amplifiers, What Are They All About Then?

One of my first jobs as a freshly minted graduate engineer involved the maintenance of a set of analogue chart recorders. They were museum pieces by the early 1990s: a motorized roll of graph paper across which a pen would traverse in proportion to the voltage on the input terminals. Inside was a simple servo, with a differential amplifier comparing the feedback via a potentiometer from the mechanism with the amplified input.

The recorders dated from the early 1960s, and internally their electronics were from the germanium transistor era: many Mullard OC-series devices, black-painted glass tubes with a red dot, and, unexpectedly, a large electromagnet connected to the 50 Hz AC supply with a reed switch through its middle, something completely new to an overconfident youngster who thought she knew everything.

What I’d stumbled upon was a chopper amplifier, a slightly ungainly and long superseded solution to the problem of DC amplification from the days before ubiquitous integrated circuit op-amps. We have become so used to DC amplifiers that just work, that we have forgotten that there was a time when such devices were an impossibility. The close matching of properties between devices on the same wafer allowed integrated circuit op-amps to achieve stable DC amplification in a way that the best attempts at the same circuits with discrete transistors had failed, but before they happened some desperate measures were called for.

The classic chopper amplifier. From Analog Devices, MT-055 tutorial.
The classic chopper amplifier. From Analog Devices, MT-055 tutorial.

The chopper amplifier relied on the simple premise that, using 1950s technology, it was difficult to make a good DC amplifier but easy to make a good AC amplifier. Thus a DC input signal was modulated onto an AC source, fed through an AC amplifier, then demodulated and passed through a low-pass filter to recover the amplified DC signal. In my chart recorders the modulating and demodulating was performed by the 50 Hz reed switch, and the germanium transistor AC amplifier was able to do a good job of working with the resulting 50 Hz square wave.

Chopper amplifiers were commonly found in instrumentation and other applications through the period until the advent of affordable integrated circuit op-amps through the 1960s.  When these components arrived, they provided a single component that was a differential amplifier rather than the chopper’s single-ended amplifier, with better gain and a hugely higher bandwidth at a fraction of the price and with none of the complexity. You might imagine that the chopper would be consigned to history then by this development, but that’s not so! An integrated circuit op-amp is an astonishingly versatile component which can be had in many different variants to suite a huge array of applications, but it has one Achilies’ heel. It is difficult to make an op-amp with a low noise level at fractions of a Hz, i.e. at near-DC frequencies.

A chopper-stabilised amplifier, from the Texas Instruments App note: Auto-zero amplifiers ease the design of high-precision circuits
A chopper-stabilised amplifier, from the Texas Instruments app note: Auto-zero amplifiers ease the design of high-precision circuits.

The solution to the problem of poor op-amp DC noise performance came from a return to the chopper, but only as an adjunct to the op-amp rather than its replacement. The chopper-stabilised op-amp is a standard op-amp configured as an AC amplifier with a traditional chopper amplifier (labelled “Stabilizing amplifier” in our diagram) in the path to its non-inverting input. This becomes a two-path device, with the high-frequency components being amplified by the op-amp with its superior bandwidth, but the DC component being amplified by the chopper amplifier for which the op-amp becomes simply a buffer. The whole circuit is still a single-ended one, but it has become a high-bandwidth amplifier with an ultra-low-noise DC performance.

A basic auto-zero amplifier, from Maxim Integrated app note 4179.
A basic auto-zero amplifier, from Maxim Integrated app note 4179.

There is a final refinement of the chopper amplifier, and it is one which can be bought off the shelf from multiple manufacturers. The auto-zero amplifier uses a chopper amplifier and an op-amp just as the chopper-stabilised amplifier does, except the chopper does not contribute directly to the output. Instead it forms a pair of sample-and-hold amplifiers, alternately trimming its own offset as a differential amplifier and then trimming the offset of the other op-amp which does the amplification. This has the effect of reducing the op-amp’s DC noise by the same extent as the simple chopper amplifier, but while preserving the differential inputs of the op-amp.

It is unlikely that any of you reading this piece will deal with vintage chart recorders for a living, though the Hackaday readership never cease to surprise us. So if you encounter a chopper amplifier it’s likely to be of the final variety, an auto-zero amplifier chip that will probably appear to you to be as just another op-amp that you’re using for a strain gauge or similar. But even if the joys of maintaining a 1950s vibrating reed switch will never bring a wrinkle to your brow, it’s still worth knowing something about how they work. For further information I’d suggest your next port of call should be the multiple application notes and data sheets on the subject that can be found on the web or the few linked from this page. You can never have too many pieces of circuitry stored in your brain!

Image: a mechanical vibrating switch used in a vintage chopper amplifier. Binarysequence [CC BY-SA 4.0].

18 thoughts on “Chopper And Chopper-Stabilised Amplifiers, What Are They All About Then?

  1. Actually, every one of us has a chopper-stabilized opamp right on our bench, that we use every day. DMMs use them to automatically guarantee a zero reading with 0 volts input. DMM chips as far back as the Intersil 7106 from the early 1980’s have had them.

    1. A lot of modern multimeters do not use auto zero ADCs. Quite a few use Delta Sigma, a very versatile sort of device that really needs an article of its own. There are also low offset opamps that do not use chopper techniques, for applications like audio where the chance of adding noise is unacceptable.

  2. Chopping is still a subject of active research, and the vocabulary has evolved a bit. The circuit in the Analog app note (labeled ‘classic chopper amplifier’ above) now falls in the category of ‘offset cancellation’.

    These days chopping is considered a modulation technique similar to radio: you combine the low-frequency signal of interest with a high-frequency carrier, run it through an AC-coupled amplifier with high gain, feed the output of that into a voltage divider that drops the gain back to 1, then remove the carrier. The AC coupling blocks the amp’s DC input offset voltage, and the amplify-then-divide step reduces the amplifier’s drift to 1/gain in the output.

    The defining feature is that chopping produces continuous output. No matter when you look at the chopper’s output, it’s a low-offset, low-drift copy of the input.

    Offset cancellation is considered different because it uses sampling instead of modulation: the circuit spends 50% of its time capturing its own offset-and-drift error, and 50% of its time subtracting the captured error from the amplified input. The output is a square wave that’s only related to the input half the time.

    The big advantage of offset cancellation is that the amplifier can run at full speed during the half where the output is connected to the input. Chopping, like all modulation processes, runs into problems when the input is more than half the modulation frequency.

    There are three main ways to deal with the square-wave output:

    1) You can filter it, but that loses the advantage of being able to run the amp at full speed.

    2) You can use a track-and-hold circuit to capture the parts of the output related to the signal, and skip over the parts that drop to 0V. That’s better, but the output is still only related to the input half the time.

    3) You can use two offset-cancelled amps running out of phase with each other. When one is capturing its own error, the other is producing valid output. That’s called a ‘ping-pong amplifier’, and provides the best mix of offset cancellation and full-speed operation. It’s also the most complex though.. you need two complete amplifiers and a switching network for the outputs.

    1. I think SDR detectors do this. A brilliant bit of work in the early 1990’s showed how to replace a mixer with a (quadrature) chopper. It eliminates the need to do a product of high freq signals. Turning the signal off and on is the same as multiplying by a square wave. And since a square wave is the sum of all the odd frequencies in a Fourier Series, well you can see where this is going. And there is no distortion as in semiconductor mixers that use the non-linear regions to get products.

  3. I still have an old Grass-Telefactor setup we used for EEG but was originally I think used for polygraphs, that had choppers in it. And I’ll just leave this here, a site made by old friend Joe Sousa on Philbrick. I have a ton of this stuff too, and in some of the physics work I do, it’s still valuable because it’s one heck of a lot harder to hurt a twin triode than any combo of semiconductors, even MOV and protection diodes, with very fast HV pulses in arcs.
    http://www.philbrickarchive.org/
    If you pay attention to the names in the philbrick archive, you’ll note that many of the people who wrote the app notes we learned electronics from – worked there before National, Motorola, Analog Devices, TI, and so forth. That’s really where quite a bit of this started.
    (Good one, Jenny)

  4. The HP/Agilent/Keysight RF power sensor heads (good for measuments down in the -70dbm at 25 GHz range) use chopper amplifiers too. They take either a diode pair or thermistor exposed to rf and feed it to a chopper amp that passes the signal back to the meter itself. Take a look at an HP8484A schematic sometime. Pretty impressive bit of engineering.
    KI2L

  5. 1. Similar techniques (diode rings used as (de)modulators + transformer coupled valve amplifiers) were used in the direction control system of the V2 rocket during WW2. The schematics are available online … somewhere.

    2. There’s an important caveat regarding any sort of opamp auto-zero system that works by nulling the differential input voltage: it relies on the external feedback circuit keeping the inputs at roughly the same voltage. If the opamp output hits the rails, the input voltage isn’t controlled any more, and the auto-zero will start to wind up (in its futile attempt to null the now large differential input voltage). This can lead to very long overdrive recovery times.
    The only real way to deal with this is to avoid it, either by limiting the input voltage before the opamp, or by using some sort of non-linear device (e.g. inverse series connected zeners) in the feedback path.

    1. These techniques also appear in the RADAR circuits which are described in the Rad Lab series of books. I’m just now reading “Waveforms” (volume 19 of 28), and it mentions modulation and demodulation as a way of avoiding the need for DC amplifiers. It also mentions the Syncho as a type of mechanical modulator.

  6. “…with better gain and a hugely higher bandwidth at a fraction of the price…”

    Considering at the module-level, this component is a DC amplifier(DC in, DC out), what value does an increased bandwidth have in this application? DC, by definition, has no band to be… “widthed”…

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