Authors: Denard Lynch Date: Oct 24, 2012 Revised: Oct 21, 2013, D. Lynch Description: This laboratory explores the characteristics of operational amplifiers in a simple voltage gain configuration as well as a threshold comparator. The threshold detector is then used in a simple temperature alarm, exploiting the reverse-biased leakage current characteristic of a silicon diode. The student will also be introduced to the use of a FET transistor as a solid-state switch to control an LED. Learning Objectives: In this laboratory, the student will: Use a FET as a switch to control a static load (LED) Calculate component values for an operational amplifier circuit with a specified AC voltage gain. Construct and verify operation of an op-amp circuit as a threshold detector Utilize the threshold detector in a simple temperature alarm Safety Considerations: In addition to general electrical safety considerations, the student should also be aware of the following considerations specific to this laboratory exercise: Resistors and solid-state components carrying current will generate heat energy and can be overheated in AC circuits. Power is based on RMS voltages and currents. In all other respects, the same considerations as in DC circuits apply. Diodes and filter capacitors used to rectify AC power are polarized and must be connected so that they are appropriately forward biased (positive to positive). Reverse biasing these components (i.e. positive to negative), can result in excess currents, and heating, in these and other circuit components. Please verify the correct polarity of components before energizing circuits. Measurement of AC circuit parameters requires suitable test gear or the selection of the appropriate scale and range. Use of DC instruments or incorrect scales can result in equipment damage or safety risks. Background and Preparation: In addition to the oscilloscope, waveform generator and power supply functions of your Digilent ADM, this lab will provide an opportunity to use the static digital input/output features. Denard Lynch Page 1 of 12 Oct 21, 2013
Additional background information can be found in Appendix A at the end of this document. Procedure: The procedure will involve constructing several different circuits: A half-wave and full-wave rectifier circuit A FET switch circuit to control an LED An inverting Op-Amp circuit with a specified voltage gain An Op-Amp set up as a threshold detector Modeling- This will involve using theory of operation of various solid-state devices and integrated circuits to predict their behaviour and performance in a circuit situation. The required parameters are described in the detailed procedure below. Please read the entire procedure over carefully before the lab and calculate circuit component values where possible. Measurements- Use your solderless breadboard and set up the circuits, in turn, for each procedure. You may be able to construct more than one circuit on your breadboard at the same time. In some cases, it may be advantageous to maintain a circuit for later parts of the laboratory. A good first step is to examine the circuit, make a list of the parts you will need, obtain the necessary parts and construct the circuit. You may also need to measure the actual value of your passive components (e.g. using the RLC Meter in 2C80 or equivalent) versus their nominal value. For example: Component Nominal Value Measured Value 1/4W resistor 1000Ω 986Ω Capacitor 100WVDC 0.1µF 0.092µF Etc. You will verify parameters of active, solid-state devices in-circuit if necessary. Start by assuming the values given in datasheets for the components. Your lab instructor will indicate where to obtain the necessary parts if they are not already in your parts kit, and how to measure their actual value. You can use your Analog Discovery Module (ADM) as the AC source or the DC source for applicable parts of these procedures. (Remember to allow for the current drive limitations of the ADM.) Terms: LED Light Emitting Diode. A specially constructed diode that Denard Lynch Page 2 of 12 Oct 21, 2013
FET Op Amp Threshold detector emits light when it conducts. Because of the specific materials used to obtain light emission at different wavelengths, the forward voltage, V f, can vary considerably from colour to colour and from that of other diodes. Field Effect Transistor: a device that controls the flow of current from its Drain to its source by applying a voltage to its Gate. These are voltage-controlled devices, and require almost no current at all from the controlling source. They come in N-channel and P-channel varieties for different circuit applications. Abbreviation for operational amplifier. These are integrated circuit differential amplifiers with close to ideal characteristics. They can be easily configured as a noninverting, inverting and various other configurations. A circuit that will provide a significant change in output level based on a very slight change around a specific input level. Can be used to detect conditions in instrumentation applications. I. FET Solid-state Switch The object in this part of the lab is to verify the proper switching function of the FET transistor. Also verify a sufficient turn on voltage (V GS ) by consulting the datasheet or experimenting with different levels of drive voltage. (Note: leave this part of the circuit set up for use later in the laboratory.) Use your solderless breadboard and set up a FET switch circuit as shown in Figure 1. Use the +5V DC source in your ADM and a ground (black) wire to provide a 5V DC source for the circuit. Use the red LED in your parts kit as a load (you will need the 1kΩ resistor (or some other suitable value) to limit the current through the LED to about 5mA). R 2, the pull-down resistor, can be any reasonably large value from ~ 2kΩ to 100kΩ. It is only required to drain any accumulated charge from the gate to ensure the FET turns off when not driven on. (If you want to verify the reason why this is good design practice, leave R 2 off, power the circuit and then use a jumper to temporarily connect the control input to the +5V source. When you remove it, the LED may stay on, depending on the actual FET you have (some have enough internal leakage to drain the gate themselves). Grounding the control input will turn the switch off. We want to ensure that, should the control input go open, the FET switch will not inadvertently stay on, possibly causing other damage!).. R G is usually added to safeguard the source driving the FET (good G D S Denard Lynch Page 3 of 12 Oct 21, 2013
design practice). In this circuit, and with the ADM source, it is not really needed at all in this circumstance. +5VDC 1 LED1 (red) R1 1000 Control Input 2 RG 10 Q1 IRL2703 G D S GND 3 R2 2.2k Figure 1: FET Switch circuit There are several ways to provide a control input that will turn the switch on. Experiment with all three if time permits; use i) as a minimum. iii) is necessary if you want to dim the LED (a worthwhile exercise!). i) Manually: simply use a jumper and connect the gate, through the gate resistor, to the +5V source. This will provide the required forward bias between the gate and source to turn the FET on. (You can also do this by connecting a mechanical switch between terminals 2 and 1 in the figure.) In this mode, measure V DS while the switch is on (i.e. the saturation voltage). You can also use one of your oscilloscope inputs to monitor the gate voltage, V GS. ii) Use the Static I/O on your ADM. Click the Static I/O button on the main window. The window shown in Figure 2 will appear. By default, the digital I/O pins come up as an input LED indicator, which will show as a red light when that input is high (e.g. >3.3V). A right click on any of the numbered I/O pins will allow you to set them up as outputs, either a switch or a pushbutton. Right click on Pin 0 area and select Push-pull Switch. When you slide it on with your mouse, it will then output 3.3V DV (with respect to ground) on D0, the first digital pin (pink wire). If you check the spec s for the IRL2703 FET, you will note that a gate bias of 3.3 volts will allow ~ 7A of current flow in the Drain more than enough to drive the LED. (There is also a Config pull-down menu to the left of each bank of 8 digital i/o pins allowing other options. These may be of use in subsequent laboratories.) There is a little monitor light in the upper right-hand corner of the switch, but you could also connect an input LED indicator to your circuit (e.g. D1, a Denard Lynch Page 4 of 12 Oct 21, 2013
green wire) at the control input or perhaps the Drain, so you can see when it is low. Figure 2: Static I/O Configuration Window iii) Use the (first or) second WaveGen to repeatedly turn the FET on and off to observe the result. To open the second Arbitrary Waveform Generator by clicking Channel 2 on the Select Channels pull-down menu. This will open up a second, completely independent WaveGen below Channel 1 which will be output on the W2 (Ye/Wh) wire (again, with respect to ground). Set this channel up for a 0 5V, square-wave output (2.5V, 2.5V offset, 50% symmetry). Set the frequency to 1Hz or less and observe the LED. To test the required drive voltage experimentally, try lowering the drive voltage (from the WaveGen) until the switch no longer operates as expected. If you set the frequency to ~200Hz or higher and vary the symmetry (duty cycle) between 0 and 100%, you will see how the brightness of LEDs are typically controlled. (This technique will also be utilized in a future laboratory to control the power to different type of load.) Hint: You may want to leave this circuit in tact on one part of your board for later use in the temperature alarm. Procedure Summary and information to note: 1. What is the minimum Gate voltage required to turn the LED on fully? Denard Lynch Page 5 of 12 Oct 21, 2013
II. Op Amps The objective of this part of the lab is to verify the gain of an inverting op amp configuration, and the limits of the output voltage swing given a fixed supply voltage. In addition, utilize the high gain and high input impedance of the TL082 in a threshold detection application. Fixed-gain Amplifier: In this part, you will require both a positive and negative power supply (with respect to the ground/black wires). On your ADM, the red wire is the +5V supply (w.r.t. ground), and the white wire is -5V supply (w.r.t. ground). Remember, these have to be enabled and turned on from the Voltage screen on the Digilent Control window A note about schematic representation: Simple circuits are usually drawn without any wires crossing so connections are obvious. As circuits get more complex, it becomes too difficult, and more confusing to follow, without some wires crossing. The convention for crossing wires on most schematics is that if they simply cross, they are not connected. An intended connection between the crossed wires is indicated by a dot. (See below.) Crossing, not connected: connected: There is a TL082 Op Amp Integrated Circuit (IC) in your parts package. Obtain any additional resistors needed from the laboratory staff or instructors. The TL082 Op Amp IC has two identical Op Amps in one package. In the schematic in Figure 3, they are both shown, although you will only need one for this circuit. Note the inputs of the unused amp are shown grounded (ground, not the -5V supply). This is good design practice, just to make sure they do not cause any unanticipated output or oscillation from the unused op amp. Denard Lynch Page 6 of 12 Oct 21, 2013
CH1+ R2 5.1k +5V (pin 8) CH1- WaveGen+ H1 WaveGen Gnd H2 R1 1k 3 2 5 6 IC1A 1 TL082P IC1B 7 TL082P R3 10k CH2+ Output + CH2- Output - -5V (pin 4) Figure 3: Inverting Op Amp Circuit You can connect the ±5V supplies to pins 4 and 8 directly rather than use the (+) and ( ) bus strips on your breadboard. You can then use the (-) bus strip as a signal ground (black wires) which will provide the reference required for both input and output for measuring voltages. ICs are usually marked with a dot or notch to help you identify the end where pin 1 is located. Use this to orient and wire the op amp on your breadboard. Also note, to connect to pins on both sides of the IC, you must insert the IC straddling the centre gap so there is a unique connection strip for each pin. Recall from the theory that the voltage gain of an inverting amplifier is: A V = R Feedback Rinput = R 2 R1 = 5.1kΩ 1kΩ = 5.1 in this case. (You can use other values for the feedback and input resistors and just adjust your expected gain accordingly, but use at least 1kΩ as an input resistor to avoid overloading the WaveGen output). Start your WaveGen and set it to output a sine wave at 1kHz. Start with an input amplitude of approximately 0.5V 0-P. (Note: In this case we are not dealing with power and impedance as much as ratios for voltage gain and input and output levels. As the instruments you are using show peak values directly, it is convenient to use these instead of RMS values for these calculations and measurements, but be sure to note that they are 0-P.) After verifying the expected gain by measuring the output / input voltage ratio, increase the input until you notice clipping of the output waveform. In theory, the amplifier can only output a magnitude that is no more than the supply voltage, but practically, it can only come within a certain range of the supply because of internal circuit overhead. What is the output limit for your TL082 given the ±5V supplies? Next, set the input level back to 0.5 V and increase the frequency of the input until the gain starts to fall below the value you measured at 1kHz. If you can increase the frequency enough (2 3MHz), you will reach the point when the voltage gain reduces to 1. (Recall, this is how the gain-bandwidth product is defined. The frequency at which the gain reduces to 1 is the maximum useful bandwidth of the amp. This is a Denard Lynch Page 7 of 12 Oct 21, 2013
characteristic of the specific type of op amp you are using. You can check the datasheet for the unity-gain bandwidth of a TL082: ~3MHz) Also observe that there appears to be a phase shift that increases as the frequency increases. This is due to a delay (latency) for the signal passing through the op amp. If you can increase the frequency enough (the voltage gain, A V, will drop well below 1), you will approach a frequency where this apparent phase shift reaches ~180 0. Procedure Summary and information to note: 1. How much error is there between your calculated gain and your measured gain? 2. What is the maximum output voltage swing when using a ±5V supplies? 3. At what frequency, approximately, does the designed gain start to reduce? 4. At what frequency, if measurable, does the gain reduce to unity (1)? Threshold Detector: Modify your circuit as shown in Figure 4 using either of the amps in the TL082 package. You can eliminate the capacitor altogether for this part, and use only the 1kΩ resistor as an input (no need for the 100kΩ). (You could set up this second circuit separately, leaving the inverting amplifier in tact for now.) This is a threshold detector; the output will jump from the negative limit to the positive limit as soon as the non-inverting (+) input level goes above the inverting (-) input level (threshold). For this example, the threshold is fixed a 0V by tying the inverting (-) input to ground, half way between the positive and negative supplies, but a potentiometer (variable resistor) or a simple voltage divider will set the threshold at any value within the output operating range of the op amp. Set one of the WaveGens for a sinewave output with a range -1V to +1V and about 50Hz. When this is connected to the input through the 100kΩ resistor, the output will change abruptly from its positive limit to its negative limit each time the waveform goes through 0V. You will observe that when the input sinewave is vertically symmetrical (i.e. 0 offest) the output is a square wave. You can vary the duty cycle (the time the output is high compared to the total period) by adjusting the offset of the input. How much offset is required to provide a 25% duty cycle (where the output is +ve 25% of the time)? (Note: you can easily try this with a triangular wave as well.) Denard Lynch Page 8 of 12 Oct 21, 2013
CH1+ +5V (pin 8) CH2+ Output + CH2- Output - CH1- WaveGen+ OR WaveGen Gnd BK R1 100k R2 1k C1 3 2 5 6 IC1A 1 TL082P IC1B 7 TL082P R3 10k -5V (pin 4) Figure 4: Op Amp Threshold Detector Procedure Summary and information to note: 1. How much time does it take for the output to change from the negative to the positive limit (or vice versa)? 2. How much input offset is required to achieve a 25% duty cycle output? Does this correspond to what you could calculate? Denard Lynch Page 9 of 12 Oct 21, 2013
III. Temperature Alarm The objective of this part is to integrate the components used in previous parts of this lab in a practical application demonstrating the use of an op amp as a threshold detector ( or comparator) and a FET solid-state switch to control power to a simple load (an LED). A diode s reverse leakage current is a reasonably linear function of temperature. This characteristic can be exploited to make a simple temperature alarm using the parts in your lab kit. You can combine two of the circuits you already assembled to make such an alarm, as shown in Figure 5. +5V (pin 8) 1N4007 1k IC1A 1 TL082P IC1B 7 TL082P 10 Q2 IRL2703 R5 R3 10k LED1 (red) 3M R1 3 2 5 6 R2 R4 1000-5V (pin 4) Figure 5: Temperature Alarm Use three separate 1 MΩ resistors in series to form a voltage divider with a reverse biased 1N4007 (used in Part 1 of this lab). Use two jumpers to extend your probe away from the board to facilitate heating or cooling. Alternately, you may ask one of the lab technicians, if they are able, to solder some 5 8 cm wire leads onto your 1N2007 in lieu of using jumpers. The input impedance of your ADM is very high, so you can use the oscilloscope probes to measure the voltage at the input to your op amp without loading the voltage divider noticeably. A 3 MΩ resistor and a typical 1N4007 will provide an approximately 50% voltage divider at room temperature. You can adjust the actual resistance used to obtain a voltage just slight higher than ground potential (~ +.5V) if required. As the diode temperature rises, the leakage current increases causing the voltage at the divider point to drop ( increased IR drop across the 3MΩ resistor). With the threshold detector configured as inverting, when the voltage drops Denard Lynch Page 10 of 12 Oct 21, 2013
below ground potential (< 0V), the output of the op amp will go high (~ +4V), turning on the FET switch and illuminating the LED. An increase from room temperature to ~body temperature should be enough to trigger your alarm. The circuit is sensitive enough so that your body resistance or capacitance are enough to interfere with the circuit, so avoid touching any bare leads. (You may be able to use your breath to raise the temperature of your probe. Test it by monitoring the voltage with your oscilloscope.). What simple modification would change the alarm so that it would trigger if the temperature fell below a set limit? How could you make the alarm adjustable? Procedure Summary and information to note: 1. At what voltage input (to pin 2) does the alarm trigger? 2. How could you make the trigger temperature adjustable? Denard Lynch Page 11 of 12 Oct 21, 2013
APPENDIX A: Background Theory Solid-state switches, like the example in Part I of this lab, offer many advantages over mechanical devices, especially when it is desirable to control the switch with electronic controllers. Determining the required current and voltage specifications for the device is usually a simple matter of circuit analysis to determine the required average and maximum current handling capability, as well as the voltage that will be imposed across various terminals of the device. Another consideration is the V GS I D characteristic. If there are drive restriction (e.g. only 5VDC available), a deice will be sought which will provide the requisite current at that drive voltage. Once a candidate device is selected based on preliminary analysis, the power dissipation capability can be re-analyzed using actual device data. With FET switches, the input impedance is typically extremely high and the input buffer resistances are chosen to i) protect the driving source, and ii) to ensure the transistor turns off when required. The device suggested for use in this laboratory is considerably more capable than required for the actual circuit spec s, but is chosen for its availability and to provide an adequate safety margin in an experimental environment. The critical characteristics of LEDs used as indicators, as in this laboratory, are the current requirement for proper illumination and the foreword bias voltage required (V F ). A datasheet will contain this information. In the absence of a data sheet, V F can be experimentally determined by using the LED in a simple rectifier circuit as in Part I of this laboratory (use a suitably large series ballast resistor). Also, a good general starting point for current requirement for a medium-sized LED such as the ones in your parts kit is 5mA. If this provides insufficient illumination, slowly increment the level until adequate illumination is provided. Determining any required buffer resistance is a simple application of Ohm s Law. Operational amplifiers, like the TL082 you will be using in this laboratory, operate close to ideally in many common configurations and applications. Recall that the output will automatically change in a certain direction in response to a difference in voltage between the inverting (-) and non-inverting (+) inputs. It is up to the circuit designer to arrange a circuit so that the desired outcome is achieved when the voltages at the input change. For the inverting amplifier used in this experiment, the gain is determined by providing a feedback resistor between the output and the inverting input (-). The voltage gain is simply: A V = R Feedback RInput. The negative sign simply indicates that the output is inverted, or shifted 180 0 from the input. If R Feedback is very high (e.g. open = ), the gain also approaches infinity (or the upper limit of the amp). This configuration can be used as a threshold detector, with the threshold being controlled by a simple voltage divider connected to one of the inputs. References: EE204 Course Notes various datasheets (1N4007, IRL2703, TL082) Denard Lynch Page 12 of 12 Oct 21, 2013