Amplification of an evoked potential signal is carried out utilizing a
high pass filter implemented as an integrator in a feedback loop which
drives the DC offset voltage to zero. As a result, the feed-forward
amplifier circuit has almost zero volts at its output since the only
voltage remaining is the offset voltage of the operational amplifier,
which is selected so as to maintain this parameter as low as possible.
Because the voltage impressed across the feed-forward amplifier section
is close to zero, the gain of this section can be set to zero during the
time that the electrical stimulus pulse is present without introducing
any additional artifacts and subsequent amplifier stages are not driven
into saturation. When the electrical stimulus potential is no longer
present or is significantly reduced in amplitude and before the time of
receipt of the response signal, the feed-forward amplifier is brought
back into the circuit to provide the high gain required to amplify the
response signal, which can be measured without interference from
saturation of any of the amplifier stages as they recover to baseline.