Amplifiers

Amplifiers are electronic devices that serve to multiply an input signal by a constant. This amplification factor is called gain and is related to the ratio, Vou/Vin. It is common to express gain in decibels as 20 x log10(Vou/Vin). The dynamic range of an amplifier refers to the voltage range over which the amplifier behaves linearly. The sensitivity control on an EEG machine helps to modify the dynamic range of the amplifier. Sensitivity is expressed as microvolts per millimeter and refers to the size of the deflection on the paper or screen that represents this voltage. Typical sensitivity settings for EEG are 7 ^V/mm. Increasing the amplifier gain requires lowering the sensitivity; they are inversely related. EEG amplifiers have multiple circuit elements that include voltage regulators, filters, and calibration circuits, among other elements.

The heart of the EEG machine is the differential amplifier. The difference between the input voltages from two electrodes relative to a reference electrode (ideally close to the

Shorter T

Shorter T

0.1 60 100 Frequency (Hz)

Fig. 4. Frequency response of filters. This figure shows the percentage of input voltage that is allowed as a function of frequency in relation to applied filters based on analog resistance/capacitance (RC) circuits with various time constants, T. Such filters exhibit a "roll off' in their attenuation of input frequencies. As t shortens, the curves for both the high- and low-pass filters are shifted toward higher frequencies. The cutoff frequency is inversely related to T. For a given filter circuit, it is the frequency above which approx 70% (0.16/t) of the input amplitudes will pass through for analysis. The notch filter is designed to specifically filter out 60-Hz inputs, because these are frequently arti-factual in origin.

0.1 60 100 Frequency (Hz)

Fig. 4. Frequency response of filters. This figure shows the percentage of input voltage that is allowed as a function of frequency in relation to applied filters based on analog resistance/capacitance (RC) circuits with various time constants, T. Such filters exhibit a "roll off' in their attenuation of input frequencies. As t shortens, the curves for both the high- and low-pass filters are shifted toward higher frequencies. The cutoff frequency is inversely related to T. For a given filter circuit, it is the frequency above which approx 70% (0.16/t) of the input amplitudes will pass through for analysis. The notch filter is designed to specifically filter out 60-Hz inputs, because these are frequently arti-factual in origin.

recording leads) is amplified and serves as output. This method serves to subtract common artifactual noise that may be contaminating both input electrodes. One example is ambient 60-Hz noise from the local recording environment. This subtraction of common noise is called common mode rejection. The capacity of an amplifier to perform common mode rejection is described by the common mode rejection ratio, which is equal to the common signal voltage divided by the nonamplified output voltage.

The common mode rejection ratio for many amplifiers in modern EEG devices is 10,000. In a differential amplifier, by convention, if input 1 is negative with respect to input 2, then the pen deflection is upward. If input 1 is positive with respect to input 2, then the deflection is downward.

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