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    Lab 2

    Analog Circuits and Filtering


    To amplify a signal from an IR (infrared) receiver, change its DC level and amplitude, filter it to remove unwanted noise, and convert its amplitude to a DC signal (peak detect).    These functions are the core elements of an infrared homing system.



    1. Derive the gain functions for the inverting, non-inverting, and differential amplifier configurations shown below.
    2. Draw a frequency response plot for the passive low pass filter shown in Fig. 1.5
    3. Derive a transfer function for the active bandpass filter in Fig. 1.6
    4. Read data sheets on OP805, TL082.
    5. Learn the difference between DC and AC coupling on your Oscilloscopes.  (this is often a problem when trying to interpret the voltage signals on the scopes)

    Operational Amplifier Circuits

    The Inverting Amplifier.  In this configuration, the input signal is connected to the inverting (negative) terminal, while the noninverting (positive) terminal is connected to ground.

    The Noninverting Amplifier

    To avoid the negative gain present in the inverting amplifier, the noninverting amplifier configuration is used.  Just as the name indicates, the input signal is connected to the noninverting terminal.


    The Differential Amplifier

    This configuration is used in situations where the difference between two signals needs to be amplified.  The basic differential amplifier circuit is shown below.


    High Pass Filters (passive)


    Low Pass Filter (passive)

    Active first order band pass filter

    Peak Detector This circuit is capable of tracking the amplitude of a sinusoid with a response time constant of R1C1, where R1C1 determines the response time of the peak tracking. 


    1.)  Wire up a phototransistor (OP 805) on your protoboard. You can use the test circuit shown in the data sheet. Make sure you do not exceed the maximum voltage ratings for these devices.


    • The phototransistor has a base lead to allow the transistor to be turned on electrically rather than with light. You do not need to wire the base to anything – if you do run current into the base, it will tend to turn on the transistor just like light falling on it would.
    • The outer case of the OP805 is electrically connected to the collector pin as well (look carefully at the pinout diagram).  This means the outer case of the OP805 should not come into electrical contact with any other live element on the circuit.

    2.)  Expose the phototransistor to a flashing IR source.    Measure the output of the waveform on the oscilloscope.   Note the amplitude and frequency response of the phototransistor.

    2. a.) Build your own IR Emitter (optional but recommended).    There are six IR sources in the classroom (the grey emitter boxes) which can produce 1 kHz and 10 kHz sine waves and square wave output in the infrared.  These are the same emitters that will be used on the competition surface.  However, there are only six of these IR sources, and the frequencies are fixed.   It is advantageous for you to make your own IR LED emitter source, and drive it with the function generator.  You can keep this setup at your workbench, and you can vary the frequency continuously and see the frequency response of your system.  See the diagram below for a schematic of the IR LED with resistor, and the desired output from the function generator (question:  what happens if the voltage going to the IR LED goes below ground?)

    3.)  Design and wire up amplification and filtering in your circuit. You may decide to put amplification before filtering, filtering before amplification, or amplification before and after filtering, it’s up to you.  Keep in mind that to make debugging easier, it is usually best to to build and test each stage separately if possible (or at least to allow for each stage to be disconnected and tested separately if it doesn’t work out nicely). 

    3.a.) Design and wire up an amplifier or sets of amplifiers to amplify the output waveform. A few points to keep in mind as you include amplification in your circuit

    • Do not exceed the gain-bandwidth product of the TL082 op-amps, use multiple stages if necessary to achieve your desired gain without losing frequency response.
    • Remember to block the DC component of the signal before amplifying.

    3.b.)  Design and wire up a band pass filter to detect a 10kHz IR sine or square signal and reject noise at other frequencies. Test the amount of rejection you have achieved by exposing your circuit to a 1 kHz IR sine wave.  When designing the circuit, make sure the output of your peak detector swings between 0-5V to match the input range of the TINAH Board.

    4.)  Design and build a peak detector to produce a DC analog signal proportional to the amplitude of the detected IR signal.


    5.)  DO NOT TAKE APART YOUR CIRCUIT AS YOU WILL NEED IT NEXT WEEK. Keep careful notes on all your circuits and the problems / solutions you have encountered. You will need all this for the IR detector on your robot.


    • Show that your circuit produces a DC signal proportional to the amplitude (or distance) of the IR beacon
    • Demonstrate the level of noise rejection due to the filtering stages of your circuit .