Chopper Stabilized Amplifier Principle

CHOPPER STABILIZED AMPLIFIER PRINCIPLE: 

Chopper amplifiers are dc amplifiers. Some minute amplitude signals needs high gain amplification .But very high dc amplifier design can be complicated because they are very difficult to build up with low offset and I/fnoise. Also stability and bandwidth may not be up to the mark. For the lowest offset and drift performance chopper stabilized amplifiers may be the only solution. Offset voltages less than 5 µV with extremely low offset drifts are available with chopper stabilized amplifiers.

Chopping
So chopper amplifier is used to break up the input signal so that it can be processed as if it were an ac signal and then integrate back to the dc signal at the output. In this way extremely small input signals can be amplified. So DC chopper amplifier provides high accuracy and stability which is a key requirement in biomedical instrumentation field.



So in the case of chopper amplifiers, the signal is sampled to chop (convert) the signal at a frequency that will pass through the ac-amplifier. Many biomedical chopper amplifiers use different chopping frequencies such as 100 Hz .400 Hz, 1000 Hz etc. It should be noted that the input frequency must be less than one half the chopping frequency in order to prevent errors due to aliasing. The main disadvantages of chopper stabilized amplifiers are the clock noise, slow speed and variable AC input impedance. A simple chopper amplifier is shown in figure. The chopper is a SPDT (Single Pole Double Throw) switch. Depending on the excitation voltage the switch grounds the amplifier input and output terminals on periodic intervals. Then the chopped waveform is passed through the ac amplifier is used at the output side to reduce the noise. The input and output waveforms are shown in figure.
A type of chopper amplifier
Chopped waveform

Advantages of Chopper Stabilized Amplifiers:

1. Very low noise operation.
 2. Gives high stability.
3.Highly useful for EEG amplifiers to get high gain.


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