When recording biopotentials noise and drift are the two problems encountered. Noise is due to the recording device and by the patient when they move. Drift is a shift in baseline created due to various thermal effects. A DC amplifier has a shift or sudden peak in the output when the input is zero. Therefore, a chopper amplifier solves the problems of drift in DC amplifiers. The name Chop means to sample the data. The amplifier circuit samples the analog signal. So it is known as chopper amplifier.
The general diagram of a chopper amplifier is as shown below. The first block chopper accepts the DC input signal and converts them to an AC signal. The AC amplifier block amplifies the chopped AC signal.
Next, in the demodulator rectifier block, an amplified chopped AC signal is converted to amplified DC signal.
Chopper amplifier is classified into two types. Mechanical and non-mechanical choppers. The chopper converts DC or low-frequency signal to high-frequency signal. An AC amplifier amplifies the modulated high-frequency signal. The amplified signal is demodulated and filtered to obtain the low frequency or DC signal.
Mechanical Chopper Amplifier
From the figure, chopper S1 acts as electromagnetically operated switch or relay. ‘A’ is the AC amplifier that has an input terminal and a ground terminal. ‘Q’ acts as reference term. Chopper acts a switch, so it connects the amplifier input terminal alternatively to reference term Q. Consider a condition in which chopper S1 is closed. At this position, the amplifier input terminal connects to Q1. The entire circuit is short-circuited, so input voltage is zero. Now, let us consider the reverse operation when chopper S1 is open. The AC amplifier starts receiving the signal from P terminal. Finally, the amplifier input has an alternating voltage that varies between zero and input voltage. At this stage, conversion of DC signal to square wave pulse occurs with amplification. Diode ‘D’ rectifies the chopped signal.
After rectification, the rectified signal is filtered and amplified. At the output terminal M and N, the amplified DC output signal occurs. Chopping or sampling rate determines the chopper response time.
Non-Mechanical Chopper Amplifier
In comparison with mechanical type, a non-mechanical chopper uses photodiodes or photoconductors for modulation (convert DC signals to AC signals) and demodulation (convert AC signals to DC signals). When light is not incident on the photodiode, no current flows through the circuit. However, when light falls on the photosensor, the resistance becomes low. So, the current flows through the sensor. This system is similar to a switching operation.
From the figure, an oscillator has two neon bulbs, which operates on half cycles of oscillation. PC1, PC2, PC3, and PC4 are photodiodes. Neon lamp 1 flashes light on PC1 and PC2. Neon lamp 2 flashes light on PC3 and PC4. When light falls on PC1, its resistance value reduces making the capacitor to charge. Light falls on PC3 making the input to flow through it when there is no light on PC1. Therefore, the light incidence on PC1 and PC3 takes place alternatively to generate a square wave pulse across the output capacitor. The generated square wave pulse is the input for the AC amplifier. The amplifier output is an amplified square wave pulse. The other two photodiodes PC2 and PC4 are in the output circuit. It recovers DC signal and makes the capacitor fully charged to the peak value of output voltage. At the final stage, a low pass filter removes the unwanted noise and ripples. The output is an amplified DC signal.
Differential Chopper Amplifier
A type of chopper used for EEG measurement is a differential chopper. It has a transformer. A chopper vibrator connects the input of the transformer. The center tap of the transformer acts as one of the terminals for the input connector. The chopper switch acts as another terminal. AC coupled amplifier provides the gain. The output from this amplifier goes to filter and demodulator block. Finally, an amplified DC output signal is obtained.