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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.

You will also connect the final output from the circuit into analog input of the TINAH board.




  • Datasheets for OP805, QSD124 phototransitor, and the TL082 op-amp.   Download
    • You will be using the QSD124 as the phototransitor for ENPH 253.   They have better performance than the older OP805’s, and are far cheaper (almost 10x cheaper!)


  1. Draw a frequency response plot for the passive low pass filter shown in Fig. 1.5
  2. Derive a transfer function for the active bandpass filter in Fig. 1.6
  3. Review the data sheets on OP805   QSD124 and TL082.
  4. 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 scope)
  5. Read the notes on the TINAH Analog Input pins, and be sure to note the following
    1. The labeling on the Analog Input pins is incorrect, the pin labeled “7” is actually “0”, and vice-versa.
    2. Knob 6 and Knob 7 are connected to Analog Inputs 6 and 7 by default.  To gain access to the analog inputs on lines 6 and 7 and bypass either of the knobs, shift the position of the two jumpers next to the knobs.
  6. The TINAH Board’s A/D resolution is 10 bits. Determine the resolution to which you will be able to measure the angular position of the potentiometer.



Operational Amplifier Circuits


The Inverting AmplifierIn this configuration, the input signal is connected to the inverting (negative) terminal, while the noninverting (positive) terminal is connected to ground.
The Noninverting AmplifierTo 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 AmplifierThis configuration is used in situations where the difference between two signals needs to be amplified.  
High Pass Filter (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.









0.   Read through all lab steps, and draw initial schematics for each stage in your logbook.  


1.  Wire up a phototransistor (OP 805) QSD124  n 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.


You will be using the QSD124 as the phototransitor for ENPH 253.   They have better performance than the older OP805’s, and are far cheaper (almost 10x cheaper!)


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 highly  recommended).

There are four IR Emitter Box 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.  If you use the IR Emitters, you may want to carry the battery charger with them as well, as they will only last ~1hr without charging.    These are the same IR Emitters that will be used on the competition surface.

However, there are only four 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 1 kHz IR sine wave signal and reject noise at other frequencies. Test the amount of rejection you have achieved by exposing your circuit to a 10 kHz IR sine wave and compare against the desired 1 kHz signal.  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.

As you put this together, think about what RC time constant you should aim for in your peak detector circuit.


5.   Examine the input and output voltage limits of the amplifier.

Note what happens to the output voltage as the input voltage gets close to the power rails values (close to -15V or to +15V).  You can see this effect when you have a stage with  high gain, and increase the input to “saturate” the circuit.   You should examine your output stage by stage, and see the effects of saturating your signal and how it cascades through each stage of your circuit.

***  NB:   Go through Step 5 slowly, as there are often subtle issues with amplification, saturation, and exceeding voltage levels which can be traced back to input or output levels exceeding the maximum ranges.



6.  Connect the IR circuit output to an Analog Input on the TINAH board.

Hook up the output of your IR detector circuit to an analog input.  Write a program to display IR amplitude readings on the LCD display.    You will also want to protect the output using Overvoltage protection, as described below:

Overvoltage protection. Use a zener diode to make sure the signal is in the range 0 to 5V before plugging it into the TINAH Board.    The zener diodes available in the lab are BZX79C5V, rated for 0.5 Watt of power.  Work out an appropriate resistor value for placing in line with the output value if the input of the circuit.  See how to use zener diodes as voltage clamps for overvoltage protection [The Circuit Designer’s Companion, pg. 120, Tim Williams]





Show the following to a TA/instructor.

    • A DC signal proportional to the amplitude (or distance) of the IR beacon to the TINAH board on the oscilloscope.
    • The DC signal above acquired by the TINAH board.
    • Demonstrate the the level of noise rejection due to the filtering stage of your circuit.


    • The schematics for your circuit (so we can check your component selections and values) 


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