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BIOMEDICAL INSTRUMENTATION LABORATORY

Spring 2004.

TAs:
Jason Brooke mbrooke@bme.jhu.edu

Kartik Murari kmurari@bme.jhu.edu

Abhishek Rege arege@bme.jhu.edu

Lab Exercise 1: Introduction to the op-amp:
Inverting, Non-Inverting and Differential Amplifiers.

The op-amp or Operational
Amplifier is a IC ( Integrated Circuit ), with some extremely useful
features. It forms the basis of a large number of circuits used in the
electronics industry. The purpose of the first part of this exercise
is to introduce the op-amp and demonstrate some of the circuitry possible.

In order to study electrophysiology, we need
to be able to record various biopotentials (i.e. ECG, EM, EOG, EEG,
etc.).  The basic biopotential amplifier requires an appropriate
amplitude amplification range as well as frequency range, and noise
reduction.  The basic building blocks of biopotential amplifiers
are differential and instrumentation amplifiers.  In the second
part of this lab you will characterize a single op-amp differential
amplifier.

LAB EXPERIMENT:

An INVERTING
AMPLIFIER with a gain of 50 has been constructed for the experiment.
To test the circuit, input a sinusoidal wave of frequency 1.0 kHz and
amplitude 10mv peak-peak from the signal generator. Look at the output
and the input waveforms on the oscilloscope. What do you notice about
the peaks and troughs of the input and the output? Record the output
waveform amplitude. Increase the amplitude of the input in steps to
500mV. What happens to the shape of the output for higher amplitudes
of the input signal? Record the output amplitudes. Plot a graph of the
output amplitude vs. the input amplitude.
DIFFERENTIAL
AMPLIFIER - Common-mode rejection: Apply a sinusoidal signal of frequency
10 Hz and amplitude 1V pk-pk to both the inverting and non-inverting
inputs of the amplifier. Record the output voltage amplitude. Tune the
potentiometer so that the output signal is minimized. Record the output
voltage amplitude. Record the output voltage amplitudes and calculate
the common-mode gain.
DIFFERENTIAL
AMPLIFIER Differential gain: Apply a sinusoidal signal of frequency
10 Hz and amplitude 25 mV pk-pk BETWEEN the non-inverting and inverting
inputs. Record the output voltage amplitude. Repeat the experiment for
the same frequencies as in part 1. Calculate the differential gain and
plot it against frequency. Now calculate the Common-mode rejection ratio
(CMRR).

Lab Exercise 2 : Differentiators , Integrators
and Comparators

The op-amp is often used in circuits for analog
computations ( adders, subtractors, multipliers, logarithmic amplifiers
, integrators and differentiators among others ) . The purpose of this
lab is to demonstrate some simple applications of the op-amp in such
circuits. Another circuit with immense application is the comparator,
a digital circuit (with only two values of the output ) which compares
an input voltage with a reference and outputs a signal based on whether
the input is more than or less than the reference.

LAB EXPERIMENT:

Use the
INTEGRATOR for this part of the experiment. For the input use a square
wave of frequency 1Hz and amplitude of 2V peak-peak, from the signal
generator. What is the shape of the output ? Why is it so ? Repeat the
experiment for different types of input signals ( DC, Sinusoidal, Triangular).
Record the output waveforms for the various input signals.
Use the
COMPARATOR for this part of the experiment. Apply a input sinusoidal
signal of frequency 1.0 kHz and a peak-peak amplitude of 2V. What is
the shape of the output ? What are the amplitudes of the output voltages?
Repeat the experiment with a sinusoidal signal of frequency 100 Hz at
the same voltage amplitude and also with a sinusoidal signal of frequency
10 kHz. How does the output change ? Why ?

Lab Exercise 3: Active Filters.

Often, real world
signals of interest are mixed with undesirable noise signals (power
line interference in ECG signals etc.). Circuits such as filters are
used to attenuate the amplitudes of the signals which are not desirable.
Depending on the frequencies which are desirable, filters can be low-pass,
high-pass or band-pass. The principle of action of ideal filters are
shown below:

V

V

V

0

Freq.

0

Freq.

0

Freq.

Band-Pass

High-Pass

Low-Pass

  

Circuits made with real-world
components cannot achieve the sharp cut-off characteristics of the ideal
filters shown above, but with some degree of approximation we can get
fairly close. Filters are essential in circuits such as ECG monitors
where a considerable degree of interference is picked up because of
the surrounding electrical equipment as well as the movements of the
patient. Another area in which filters could be used is in hearing aids.

LAB
EXPERIMENTS:

Use the LOW-PASS Filter for this part of the experiment. Apply a sinusoidal
signal of frequency 0.5 Hz and 500mv pk-pk amplitude to the input of
the circuit. Record the peak to peak amplitude of the output. Increase
, in steps, the frequency of the input signal keeping the voltage constant
, to 25 kHz. Record the output amplitudes versus frequency. Calculate
the decibel gain ( 20 log<sub>10
[Gain] ) and plot it as a function of frequency on a logarithmic scale.
Repeat the above experiment with the HIGH-PASS filter. Plot the gain
as a function of frequency on a logarithmic scale. 

(Note: you dont have to draw every frequency. Just find the flat
gain region, then find the cut-off frequency where the gain reduces
by a factor of 0.707, and then go beyond that frequency to see reduction
in gain. You should get by with measurements done at 5-6 frequencies.

Lab Exercise # 4: Hearing Aid

Speech is the most
important form of communication. Some people are unlucky to lose their
sense of hearing, which greatly impairs their ability to communicate.
There is a lot of research being done to invent devices that improve
the quality of life of hearing-impaired patients. One of such devices
is a hearing aid that interfaces to the cochlear nerve bypassing dysfunctional
or destroyed inner hair cells of the inner ear.

This lab is designed
to review basic operational amplifier (op-amp) and filter circuits that
can be used to build a simple hearing aid.

Lab Procedure

Our first goal is to build
a simple device that will amplify sound to compensate for hearing loss.
The next stage is to make the device capable of stimulating the cochlear
nerve if the inner hairs cells (they are responsible for conversion
of mechanical energy of the sound wave into electrical input to the
brain via the cochlear nerve) are destroyed.

Connect a speaker to the
output of your signal source (you can either use a function generator
or your walkman) and reduce the volume until it is barely audible. This
is your input signal. Record the peak-to-peak amplitude of this signal.
Note: Speakers