INDIANA UNIVERSITY, DEPT. OF PHYSICS, P400/540 LABORATORY FALL 008 Laboratory #: Operational Amplifiers Goal: Study the use of the operational amplifier in a number of different configurations: inverting amplifier, non-inverting amplifier, summing amplifier, follower, current source, and current to voltage converter (transconductance amplifier) 1. Introduction This is the first integrated circuit ("IC") that we will be using. The particular package style is called "DIP" for "dual in-line package." We will be using the, the "original" low-cost operational amplifier. There are now op amps with lower noise, faster response ("slewing"), etc., but this one illustrates generic op amp behavior. The chip should "straddle" the trench in the breadboard, making connections to each pin very easy. The pin numbering scheme for all chips is as shown: 8 7 5 3 + 7 4 1 3 4 15 V The "dot" always designates pin 1, and the pins are numbered counter-clockwise looking down on the chip. Note that in the above, not all pins are used ("NC" or "not connected"). Remember to use a DVM to set the adjustable supply to and ditto for the 15 V supply. Connect these to "bus bars" on the breadboard, with the across the top and the 15 V across the bottom. Immediately make the power connections to the IC chip, and before any other connections, use a DVM right at the pins to make sure that the +15 and 15 Volts are present. This is one of the dominant reasons for IC circuits not working: lack of proper power. Open-loop gain Wire up the circuit below. Watch the put voltage with a DVM as you slowly twiddle the potentiometer, trying to apply zero volts (when the "wiper" is exactly at the center and you have exactly the midpoint between +/ 15 V).
Describe the behavior. Is it consistent with gain claims of "100's of volts/mv'? 7 4 15 V 15 V 3. Inverting Amplifier in 1k Construct the inverting amplifier drawn above. Drive the amplifier with a 1 khz sine wave. Measure the gain? What is the maximum put swing? How ab linearity (try a triangle wave)? Try sine waves of different frequencies. At some high frequency, the amplifier fails to work well. Figure a way to estimate the slew rate, i.e., the fastest "slope" in V/ms or V/ns to which the amplifier can respond (N.B. if your signal is Asin(πft), differentiating this once gives the slope of the signal (in volts per unit time) for any time, and you should be able to find the maximum slope. Alternatively, try inputting a square wave and see how the device tries to duplicate the "infinite" input slope.). Drive the circuit with a sine wave at 1 khz again. Measure the input impedance of this amplifier circuit by adding a 1 kω resistor in series with the input.
4. Summing Amplifier in 15k 15 V Adjustable "DC Offset" Let's get the "operational" of the operational amplifier: adding signals (currents really): Build the circuit above that allows you to sum a DC level with an input signal. For the input, choose a small amplitude sine wave. Describe its operation and the range of DC offsets you were able to achieve. 5. Non-inverting Amplifier Wire up the non-inverting amplifier below. Measure the voltage gain and compare it to prediction. Try to measure the circuit's input impedance, at 1 khz, by putting a 1 Meg resistor in series with the input. How is this different from the inverting amplifier? in 3 + 1k
. Follower in Build the follower shown above. Input a sine wave of some frequency and examine the put and measure the gain. The ultimate boring circuit What is it good for? Different Op Amp Uses Because they can accept either voltages or currents as inputs, and produce voltage or current puts, we can label amplifiers as one four types: Input Output Amplifier Name Voltage Voltage Voltage Voltage Current Transconductance Current Voltage Transresistance Current Current Current In the next section, you will construct a current source that is controlled by an input voltage (a "transconductance" amplifier). It is similar to the non-inverting amplifier, with the input voltage set with a voltage divider.
7. Current Source 15k 1k 3 + 180 A "load" Try the op-amp current source shown above. What should the current be, and what do you measure it as? Vary the load potentiometer and watch the current using the DVM. Determine the range of load resistances over which the current source varies by less than 10%. This is called its compliance. This current source, although far more precise and stable than our simple transistor current source, has the disadvantage of requiring a "floating" load (neither side connected to ground). It also has significant speed limitations when the put current or load impedance varies at microsecond speeds. 8. Current to Voltage Converter Phototransistor (no collector connection) light 10 M "
(a) Photodiode: Use the supplied phototransistor (feel free to peel back some of the rubber coating if not enough light is getting into the fiber optic lead) as a photodiode in the circuit above. To determine the pins, try using the DVM to distinguish them as learned previously. Examine the put signal (if the DC level is more than 10 V, reduce the feedback resistor to 1 MΩ). If you see fuzz on the put (oscillations), put a small capacitor (~100 pf) in parallel with the feedback resistor. What is the average DC put level, and what is the percentage "modulation"? This can be quite large due to the fluorescent lights in the lab... What input photocurrent does the put level correspond to? Try covering the phototransistor with your hand. Look at the "summing junction" (the inverting input pin) with the scope, as the put voltage varies. What should you see? (b) Phototransistor: Now connect the device as a phototransistor, as shown below (the base is to be left open, as shown). What is the average input photocurrent now? What ab the percentage modulation? Look again at the summing junction. light 100k (or more) Phototransistor If you are curious and/or interested, try the "game" on pg. 18 of your manual