Electronic amplifiers that handle very-low-frequency signals are difficult to design because thermal drifts of offset voltages cannot be distinguished from the signals. For this reason a popular technique for designing low-frequency amplifiers is the so called “chopper-stabilized” amplifier in Figure E-20.
A chopper-stabilized amplifier “chops” the excitation signal by switching it on and off periodically. This action is equivalent to a pulse amplitude modulation in which the pulse train being modulated by the excitation is a 50% duty-cycle square wave which alternates between zero and one. Then the “chopped” signal is bandpass filtered to remove any slow thermal drift signals from the first amplifier. Then the amplified signal is “chopped” again at exactly the same rate and in phase with the chopping signal used at the input of the first amplifier. Then this signal may be further amplified. The last step is to lowpass filter the signal out of the last amplifier to recover an amplified version of the original signal. (This is a simplified model but it illustrates the essential features of a chopper-stabilized amplifier.)
Let the following be the parameters of the chopper-stabilized amplifier:
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