CHEM 411L Instrumental Analysis Laboratory Revision 2.0. Amplifiers

Transcription

1 CHEM 411L Instrumental Analysis Laboratory Revision 2.0 Amplifiers In this laboratory exercise we will examine the signal Amplification of two circuits employing Operational Amplifiers. In particular, we will construct a Voltage Feedback Amplifier and an Operational Feedback Amplifier. In addition to each circuit's Aplification we will be interested in examining other behavioral characteristics of these circuits. This will provide us with a taste for how Op Amps work. And a taste is all we will have time for. Those with a hunger for a more complete meal should consult Sprott's text on "Modern Electronics". As noted previously, electronic analytical instrumentation is capable of detecting minute amounts of even the most difficult analytes. This is because of their ability to significantly amplify the analyte's Response Signal to the instrument's Stimulus. So, construction of amplifier circuits is a critical component of most analytical instrumentation. As per Wikipedia, an electronic Amplifier is; ".. an electronic device that increases the power of a signal. It does this by taking energy from a power supply and controlling the output to match the input signal shape but with a larger amplitude." In our case we are interested in amplifiers which exhibit a voltage gain; the amplifier's gain being measured in terms of the ratio of the output signal's voltage to that of the input signal's voltage. Historically, the Vacuum Tube provided for an enormous leap forward in electronics, in part because of its ability to provide for signal amplification. However, these devices are fragile, expensive, and tend to overheat. Solid state replacements for these devices were developed beginning in the late 1920's, culminating in the invention of the Transistor during the 1940's; the workhorse of all modern electronics. A Common Emitter Amplifier, one configuration of the Bipolar Junction Transistor, is useful as a voltage amplifier. De Forrest Triode of Vacuum_tube#History_and_development Darlington Transistor Darlington_transistor_MJ1000.jpg

2 P a g e 2 Common Emitter Amplifier (Note the Symbol for the Transistor) Both the vacuum tube and the transistor are "active" devices. They behave as though they contain an internal source; more on this in a moment. Cascaded together they can be used to construct Operational Amplifiers. A high-gain, mulitstage, dc amplifier containing many individual transistors and resistors can be considered as a single, active, circuit component called an operational amplifier (op amp for short). Such a circuit is usually miniaturized and fabricated on a single chip in what is called an integrated circuit (IC)... The op amp is such a useful device that it has a become the basic building block of analog electronics and has revolutionized the way in which complicated electronic circuits are designed and constructed. An Introduction to Modern Electronics J.C. Sprott The circuit diagram (from Sprott) for a typical Op Amp is given below: A couple of features are noteworthy. The device has two inputs V + and V - ; a non-inverting input and an inverting input, respectively. It has two points of connection to its power source; +V CC

3 P a g e 3 and -V CC. Typical Op Amps will be powered by sources of ±5V, ±10V, etc. Finally, note the device's output point, V out. Instead of working through how these devices operate at the level of their constituent components, we will consider them to be blackboxes with a certain number of terminals that have specified characteristics. The Op Amp is a four-terminal device (V +, V -, V out and GND) with the following properties: Large Open-Loop Gain (A o ) [see below] High Input Impedance Low Output Impedance The symbol for a general Op Amp is: (If V out is referenced to the midpoint of +V CC and -V CC, then the GND terminal can be omitted.) The Op Amp whose circuit is diagramed above, has an open-loop gain of 10 4 ; the gain being the ability of the device to amplify the input signal by converting energy from its power source to its input signal. Typical values for A o range from 10 2 to An Ideal Op Amp acts a Voltage Source with: This is diagramed as: V out = A o (V + - V - ) (Eq. 1) If Ideal, then the Op Amp will act as though A o is infinite.

4 P a g e 4 We will construct two Amplifiers using Op Amps, each circuit employing what is called Negative Feedback. In this case, the device's output V out is "fed back" to its inverting input V - via some passive circuit element such as a resistor. (f is the fraction of V out fed-back to V -.) This means: V out = A o (V in - f V out ) (Eq. 2) If the Op Amp is ideal, we can take A o ~. This means, since V out ~ finite, that V + ~ V -, such that V + - V - ~ 0. This allows us to calculate the amplification A of a negative feedback circuit as: A = = = (Eq. 3) This means that the circuit amplification will not depend on non-idealities inherent in the Op Amp, which will influence A o. This is not an unusual circuit configuration. We will construct two negative feedback op amp circuits and will measure their amplifications A. These are the Voltage Feedback and the Operational Feedback circuits. Each is pictured below. Voltage Feedback Amplifier Operational Feedback Amplifier Without much trouble, it can be shown the amplification for the above circuits is given by: Voltage Feedback Amplifier A = (Eq. 4)

5 P a g e 5 Operational Feedback Amplifier A = (Eq. 5) You should note the Voltage Feedback Amplifier is non-inverting whereas the Operational Feedback Amplifier is inverting. Meaning, a non-inverting circuit leaves positive input signals positive at the output and negative input signals negative at the output. The opposite behavior is observed at the output of an inverting circuit. Finally, you should understand that we cannot get something for nothing. We cannot amplify a signal infinitely; the output voltage is limited to a minimum or maximum value close to that of the power supply for the Op Amp. If we use a Bipolar Power Supply (+V CC to -V CC ), the voltage gain is limited to somewhat less than ±V CC. If we use a Single Voltage Power Supply (+V CC to GND), the output is limited to a gain in the +V CC to GND range. In the oscilloscope trace below, the input voltage (yellow) is amplified by an Inverting Amplifier powered by a bipolar power source of ±14 V. Note the output signal (green) is clipped at 27 V peak-to-peak. 9/9f/Inverting_Amplifier_Signal_Clipping.png Operational Amplifiers are useful in many, many other circuit configurations and are ubiquitous in modern electronic instrumentation. The above circuits give us examples of how an instrumental signal may be amplified and give us an opportunity to gain some experience working with Op Amps. However, one note of caution. Certainly the frequency of these circuits is important, but it will remain unexamined here.

6 P a g e 6 Procedure You will be given an LM324 Integrated Circuit that contains four Operational Amplifiers in a DIP package. These will be internally compensated high-gain Op Amps. You should consult the pin diagram for this IC in the Appendix. 1. You should locate the IC pins that represent V +, V - and V out for each Op Amp. You should also locate the pins that allow you to power the Op Amps. Finally, you locate the pin for GND. Place the IC on the breadboard such that it straddles the central notch. [Note the notch in the surface at one end of the DIP package. All pin locations are given relative to this notch.] 2. Obtain an Enercell 3-12V DC power source with hobby leads. Set the source to +7.5 V. Measure the source's voltage with the Digital Voltmeter. Note which lead represents the source's GND. Connect the Power Source for your Op Amp to the power busses on the breadboard. Note which bus is the GND. You should note this power source is a Single Voltage Supply Source. Now connect the appropriate power bus to V + and GND on the IC. Use a Digital Voltmeter to test the voltage across the DIP's V + and GND pins. 3. Obtain two 1 k resistors. Measure the actual resistance of each using a Digital Voltmeter. Use these resistors to construct a Voltage Feedback Amplifier. Each pin on the DIP should be wired to a separate wire. 4. Use a sinusoidal input for V in supplied by a Function Generator. The generator should be set at 100 mv and 100 Hz. Measure both V in and V out simultaneously the two input channels of an oscilloscope. (Be sure to use a common ground for everything.) Trace or record the scope traces. Do you observe saturation? Is the output inverting or noninverting? Note V o,in and V o,out.

7 P a g e 7 5. Next construct an Operational Feedback Amplifier. Use a 2 k resistor for R f and a 1 k resistor for R i. Make the same notations. Also, try input voltages of 100 mv, 1V and 10V. Note when saturation sets in.

8 P a g e 8 Data Analysis You should submit a Memo Style report of your findings. 1. Calculate the measured amplification for each circuit. Compare your measured results with those that are expected. Make note of any saturation or inversion for the circuit.

9 P a g e 9 Appendix Pin Diagram for the LM324

10 P a g e 10 Resistor Color Codes Resistor Color Code taken from They have a nice discussion on how to read the color code as well.

11 P a g e 11 Breadboard Configuration The following schematic is taken from

12 P a g e 12 References "Amplifier" accessed September 3 rd, Braun, Robert D. "Operational Amplifier Experiments for the Chemistry Laboratory" J. Chem. Ed. 73 (1996) 858. Skoog, Douglas A., Holler, F. James and Crouch, Stanley R. (2007) " Principles of Instrumental Analysis, 6 th Ed. " Thomson, Belmont, California. Sprott, Julien C. (1981) "Introduction to Modern Electronics" John Wiley & Sons, New York.