Biomedical Engineering Reference
In-Depth Information
FIGURE 5-6
Op
amp symbol and
common pin
configurations.
Differential-mode noise generated by electrode movement or any other mechanism is
indistinguishable from the electromyogram (EMG) signal and cannot be removed by this
technique. It is therefore output along with the EMG signal and would have to be removed
by the signal processor.
5.3.2 Amplifiers
Most analog signal processing is performed using combinations of operational amplifiers
shown schematically in Figure 5-6. Known as op amps, these versatile integrated circuits
consist of hundreds of transistors, resistors, and capacitors to produce an extremely high-
gain, wide-bandwidth amplifier with a differential input.
The ideal op amp is considered to have infinite gain at DC, infinite input impedance,
and zero output impedance. As a consequence of the infinite input impedance, no current
is drawn by the inputs, and as a result of the infinite gain any difference between the two
inputs results in an infinite output voltage. The zero output impedance results in an output
voltage that is independent of the output current.
These characteristics may seem to be illogical, but they provide a good approximation
of the actual performance of an op amp in the case where negative feedback is used to
reduce the actual gain.
5.3.2.1 Negative Feedback
If the output of an op amp is connected to its inverting input and a voltage signal is applied
to the noninverting input, as shown in Figure 5-7, the output voltage of the op amp closely
follows that input voltage.
As
V
in
increases,
V
out
will increase in accordance with the differential gain. However,
as
V
out
increases that output voltage is fed back to the inverting input, thereby acting
to decrease the voltage difference between inputs. This acts to reduce the output. What
happens for any given voltage input is that the op amp will output a voltage very nearly
equal to
V
in
but just low enough for the voltage difference left between
V
in
and the negative
(
−
) input to be amplified to generate the output voltage.
FIGURE 5-7
Op
amp circuit with
negative feedback.
