Chopper amplifier
A chopper amplifier (or chopper amplifier ) is an electronic circuit for amplifying very small DC voltages in measurement technology . The chopper amplifier was developed in 1918 by Joseph W. Milnor .
background
If extremely low voltages in the frequency range from 0 Hz to around 100 Hz have to be measured, some effects can disturb the result:
- the 1 / f noise increases with decreasing frequency
- the heat noise (can be reduced by cooling)
- the zero point drift and offset change depending on the temperature or the aging of the components.
The smaller the voltage to be measured, the more serious the effects of all possible interference. With special differential amplifiers these problems can only be reduced, with chopper amplifiers they can be almost completely eliminated. To this end, some procedures have been devised. Usually the voltage is not amplified immediately, but converted into an alternating voltage, which can be amplified more easily. This conversion can be done either purely electronically or through electromechanical solutions such as the rotary voltmeter and oscillating capacitor amplifier types .
The picture shows using the example of an amplifier with a bandwidth of 100 Hz, which is why this “pink noise” is greatly reduced in the chopper amplifier due to the principle involved.
- If the amplification takes place immediately and without frequency conversion, the left blue-colored “noise volume” is added to the input voltage, which increases sharply with decreasing frequency.
- In the case of a chopper amplifier, for example, the input voltage is "chopped up" with the much higher frequency f = 10 kHz. This corresponds to the amplitude modulation of a 10 kHz carrier with the measurement voltage. The Fourier analysis of the signal shows that the input voltage is contained in the sidebands of the carrier. These extend over the range f - 100 Hz to f + 100 Hz. Only this high range is subsequently amplified, there the 1 / f noise is considerably lower than in the base range 0 to 100 Hz. After the subsequent demodulation, only this low level appears Noise component (the right-hand blue “noise amount”) in the output signal.
Principle of the chopper process
In the early days, the input voltage by a mechanical switch several hundred times per second was (hence the term short- Chopper or Chopper ), which creates a weak AC signal. In current circuits, the periodic polarity reversal is much faster and bounce-free thanks to a group of four JFETs, especially since an OP with two inputs follows. This amplifies the signal about 10,000 times, then it is rectified with a second JFET in the same rhythm . After the resulting pulsating DC voltage has been freed of all interfering AC voltage components by means of an RC low-pass , the amplified input voltage is obtained.
Because the central AC amplifier always is coupled by insulating capacitors chopper amplifier can also for very simple potential separation are used between input and output range: you are then used as potential isolation amplifier called that play a role for security, for example, in medical instrumentation.
The advantages:
- An AC amplifier can be built more simply than a corresponding DC amplifier
- has better long-term stability despite changes (aging) of the components
- the individual compensation of the offset voltage is superfluous
- has practically no 1 / f noise
Chopper amplifiers also have disadvantages:
- The highest frequency to be processed must be well below the chopper frequency in order to avoid intermodulation effects. The usual cut-off frequency is below 100 Hz.
- The price, volume and complexity are significantly higher than that of the operational amplifier .
- They are more difficult to implement in the form of integrated circuits and are usually implemented as discrete electronic circuits.
Variant with changeable capacitors
Instead of the electronic switch, which loads the input voltage due to the periodic short circuit, bridge circuits with capacitance diodes are also used, which are more high-impedance and offer higher input resistances. This circuit is called a varicap amplifier .
An "oscillating capacitor amplifier" contains an electromechanically varied capacitor as a modulator. There was an electrostatically variable capacitor with a Magnoval base (Valvo / Philips XL7900) with an insulation resistance of 100 TΩ.
In earlier ionization measuring chambers, an electromechanically varied capacitor using the old headphone capsule was common. The second capacitor plate was mounted well insulated at a small, adjustable distance from its earthed iron membrane. The receiver capsule was operated with mains frequency, the capacity variation took place over the distance between the plates, so it was very small, but controllable. A further development with increased capacity variation was a coaxial capacitor in the form of a diving trimmer with moving coil drive. A larger stroke and lower frequency for modulation are possible here. The input resistance depends only on the insulation of the second plate and that of the isolating capacitor dielectric and can reach extreme values.
In rotary voltmeter and electric field meter motor-driven variable capacitors are used. With these designs, the voltage source is not loaded at all, only the electrical field generated in the environment is measured. In return, the measurement accuracy is low.
Applications
Applications were in the field of measurement technology for the amplification of direct voltages or low-frequency alternating voltages. For example, with voltage measuring devices in the microvolt range , strain gauges , Hall sensors and thermocouples . In these areas of application, they have largely been replaced by auto-zero / zero-drift amplifiers , which have higher bandwidths, are easier to implement as finished integrated circuits and offer similar limit data.
In contrast to chopper amplifiers, lock-in amplifiers do not measure a direct signal, but an alternating signal of a certain frequency.
literature
- Manfred Seifart: Analog circuits . 3. Edition. VEB Verlag Technik Berlin (GDR), 1989, ISBN 3-341-00740-7 .
- Ulrich Tietze, Christoph Schenk: Semiconductor circuit technology . 1st edition. Springer, 1969 (title no.1565).
Web links
- The chopper (stabilized) amplifier ( Memento from September 20, 2016 in the Internet Archive ) (pdf; 37 kB)
Individual evidence
- ↑ CS3001 / 2/11/12 & CS3003 / 4/13/14 Chopper-stabilized Operational Amplifiers. (PDF; 622 kB) (No longer available online.) Archived from the original on October 29, 2013 ; Retrieved October 27, 2013 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ Patent US1353060 : Voltage regulator for direct and alternating current generators. Filed January 17, 1918 , published September 14, 1920 , applicant: Western Union Telegraph Co., inventor: Joseph W. Milnor.
- ↑ XL7900 Vibrating Capacitor data sheet from Philips
