Pre-Lab for Lab 2

Every Student must complete this before coming to lab.You must submit this to the TA as you come into the lab…

If have not completed the Pre-Lab you will not be able to do the Lab!!!

Read the entire lab write-up and then answer the following questions:

(1) Why does an op amp need a power supply to be connected? (Don’t just say that it doesn’t work without it!)

(2) What is the range of op amp input resistances mentioned in the write up?

(3) How does one tell which pin is the first pin of the 741 IC?

(4) Make a sketch similar to that in Figure 4 for the inverting op amp circuit you’ll be building in this lab.

Name: ____________________________ Section:_________Klotzkin or Zhou

Name(s)___________________________________________________ Section:_____Klotzkin or Zhou

LAB 2: THE OPERATIONAL AMPLIFIER

OBJECTIVEIn this lab you will build and test amplifiers based on the 741 integrated-circuit op-amp.

AMPLIFIER BASICSA voltage amplifier is a circuit that takes an input voltage and increases it to a larger value. We usually want to do this to an information-carrying voltage – one that wiggles as a function of time (like a voltage signal coming from an electric guitar). We often need to amplify a signal in electronic circuits because we need the signal bigger for some reason… (i) for a guitar signal we may need to make the signal voltage bigger to be able to drive some electronic device we want to plug the guitar into (e.g., a tape recorder), (ii) for the voltage signal picked up by your cell phone’s antenna we need to amplify the voltage signal to make it big enough for the rest of the cell phone’s circuits to work properly so they can turn the received signal into a good-sounding voice signal. Our view of a general voltage amplifier can be shown in Figure 1. The supply voltages (often one positive (+Vcc) and one negative (-Vcc)) provide the energy needed to boost the signal levels.

Figure 1: Typical scenario using a voltage amplifier. The left-hand box represents some circuit or device that "creates" a signal (e.g., an electric guitar) that we would like to connect to the right-hand box (e.g. a tape recorder); but the signal voltage from the left-hand device is too small to directly drive the right-hand box. The amplifier boosts the signal voltage vin(t) by a gain of A so that it will be able to properly “drive” the right-hand circuit.

There are many ways to implement a voltage amplifier. It is possible to build one from a single transistor and some resistors (and capacitors). It is also possible to build one using integrated circuits (ICs) that are widely available. An integrated circuit can contains hundreds or more transistors and is typically designed to operate with only a few external components needed. Figure 2 shows piles of integrated circuits… note that each of them has several “leads” coming out of the black body that houses the electronics inside. This lab will explore the use of an integrated circuit type called an operational amplifier (or op amp for short)… they are ICs that are very easy to use to build high-performance voltage amplifiers. It should be no surprise that if you open up many modern electronic gizmos that you would find lots of op amps!

Figure 2: Piles of integrated circuits.

OPERATIONAL AMPLIFIER BASICSAn operational amplifier (“op-amp”) is an integrated circuit device that has two inputs and a single output, as well as connections for voltage sources to be connected; some have additional connections. The circuit diagram symbol for the op amp is shown in Figure 3. Note that all the voltages shown in the figure are referenced to ground. The op amp is designed to do one fundamental thing: Create an output voltage that is a large multiple, A, of the difference between the two input voltages: the output of the amplifier is given by vo = A (vin+ – vin–). The multiplying number A is called the “open-loop voltage gain,” vin+ is called the non-inverting input voltage, and vin– is called the inverting input voltage. (Both are node voltages with respect to ground.) Typically, the open-loop voltage gain A is on the order of 103 to 106.

Figure 3: Circuit diagram symbol for an operational amplifier (Op Amp). All voltages shown are referenced to ground. Vcc+ is a positive supply voltage and Vcc– is a negative supply voltage. The input voltages are vin+ and vin– and the output voltage is vout.

Clearly, with such a large open loop gain, even a small difference (vin+ – vin–) gives a large value for vo = A (vin+ – vin–). However, the output of the op amp is not capable of going more positive than +Vcc+ or more negative than –Vcc. Therefore, if the difference (vin+ – vin–) is more than a VERY small value the output will be either +Vcc or –Vcc. So, we need to put some external components around the op amp to make it work like a reasonable voltage amplifier.

One important feature of op-amps is that a resistor can be placed between the output node and the inverting input to provide feedback and adjust amplification. While operating in

its linear region (+Vccvout –Vcc), the op-amp adjusts its output voltage such that the voltage difference between the two inputs is nearly zero (i.e. vin+ = vin–). When the output swings to Vout = Vcc , however, the op-amp is operating in its saturation region, and it cannot force vin+ = vin– . (Note: Vcc is the supply voltage specified by the designer of the op-amp. Our general-purpose 741 op-amp requires two power supply voltages, 15V, for normal operation.)

Another important feature of the op-amp is that its input resistance is very large and can be taken as infinite in many applications. The 741 op-amp is built using a type of transistor called a bipolar-junction-transistor (BJT) and has an input resistance of about 2 M. (This is large enough to be considered infinite in many applications but not all scenarios. There are op amps built using another kind of transistor called field-effect-transistors (FET) that have input resistances of 106 M… that is 1012 !!!!) The input resistance limits how much current can flow into the op amp’s vin+ and vin– terminals. Because of an op amp’s high input resistance, only a very small current flows into either input of the op-amp. Thus, in many practical op-amp circuits, the current flowing into either of the inputs is usually on the order of A.

OPERATIONAL AMPLIFIER CIRCUITS

As discussed in class, there are many ways to configure components around an op amp to get it to do something useful. Two such useful configurations provide voltage amplification of an applied signal. These two configurations are the “non-inverting amplifier” and the “inverting amplifier”

Non-Inverting Amplifier

The operational amplifier configuration in Figure A (in the appendix) is called a non-inverting amplifier. The operation of this circuit was described in class where we derived (see also the appendix of this lab write-up) the input-output relationship for this configuration:

Note that in Figure A there are numbers shown next to each terminal of the op-amp; these are the pin numbers of the integrated circuit package. The last page of this write-up shows the pin arrangement for the IC package for the 741 op amp (notice the “741” in the HA17741 part number at the top of the page).

Important: Notice that there is a little “half-circle” on the edge between pins 1 and 8; that is how we know where to start counting as pin 1!!!

Pin 1: We won’t need this pin. It is used in some more advanced circuits.Pin 2: This is the pin that corresponds to the “vin–” input terminal of the op-amp.Pin 3: This is the pin that corresponds to the “vin+” input terminal of the op-amp.

Pin 4: This is where you connect the negative voltage supply of -15V.Pin 5: We won’t need this pin. It is used in some more advanced circuits.Pin 6: This is the pin that corresponds to the output terminal vout of the op-amp.Pin 7: This is where you connect the positive voltage supply of +15V.Pin 8: Is not used for anything… it is not even connected to anything inside the IC!!

Inverting AmplifierThe operational amplifier configuration in Figure B (in the appendix) is called an inverting amplifier. The operation of this circuit was described in class where we derived (see also the appendix of this lab write-up) the input-output relationship for this configuration:

The negative sign here means that a positive voltage at vin is inverted when creating vout.

PROCEDURE

Part 1: Non-Inverting Amplifier

(1) Obtain R1 = 10 kand R2 = 10 k. Measure the values of these resistors and record their values.

(2) Calculate the gain of a non-inverting op-amp circuit (with your measured resistor values) and record the value.

(3) Set the dual power supply outputs to provide +15V and -15V. You learned how to do this in Lab #0. After doing so…turn off the power supply (you’ll turn it back on after you’ve connected the circuit).

(4) Build the circuit of Figure A with a 741 op-amp and these resistors. See the figures in the appendix on breadboard connections and op-amp connections. Also, refer to the datasheet in the appendix for the op-amp pin diagram. (Note that -Vcc is VEE on the datasheet.)

See the breadboard picture in Figure 4 to see how to place the op-amp across the gap in the terminal strip such that you don’t short any of the op-amp’s pins together by placing them in the same column of 5 holes. (Refer to Lab #1 for a refresher on how the breadboard’s holes are connected!) Note: You must apply the correct polarities to the op-amp pins or you will damage the op-amp.

Figure 4: Breadboard connections for the non-inverting op amp circuit.

(5) Verify the following connections have been made:

(a) resistor 1 op-amp pin 2(b) resistor 1 ground bus (one of the blue outer rails)(c) op-amp pin 3 Signal Generator positive (red) lead (d) op-amp pin 4 -25V-supply terminal(e) op-amp pin 7 +25V-supply terminal(f) resistor 2 op-amp pin 2(g) resistor 2 op-amp pin 6(h) ground bus Signal Generator negative (black) lead

& 25 supply’s common terminal

(6) Turn on the power supply, the signal generator, and the oscilloscope.

(7) Connect Channel #1 of the oscilloscope to measure the input voltage (between pin 3 and ground… see Figure 4). Press the channel #1 enable button (see the posted write-up on the oscilloscope) to turn on Channel #1. Set the signal generator to give a 1V p-p, 1kHz sinewave. Verify on the scope that you indeed have this (remember… don’t trust the amplitude reading on the Signal Generator, read it off the scope!).

(8) Connect Channel #2 of the oscilloscope to measure the output voltage (between pin 6 and ground… see Figure 4). Press the channel #2 enable button (see the posted write-up on the oscilloscope) to turn on Channel #2. Hitting Autoscale now should be helpful. You should see both channels displayed on the scope’s screen… if not, use the scope controls to achieve this (see the posted write-up on the oscilloscope).

(9) Measure the p-p value of the output signal and verify that its value matches (within reasonable tolerances) the input p-p value times your computed gain value. This is voltage amplification!! Your output signal should be bigger than the input signal by a factor equal to the gain of the amplifier… measure and record the output p-p voltage.

(10) Repeat this measurement of the output p-p voltage for input p-p values of 2Vp-p, 3Vp-p, 4Vp-p, and 5Vp-p and record them in the table in the results section. Compute and record the actual gain value the circuit provided at each of the input levels tested.

(11) In the results section, write answers to the questions posed there.

(12) Shut off the power supply. Obtain a 1 k resistor, measure its value and record it. Replace R1 in the circuit with this 1 k resistor.

(13) Re-compute the expected gain and record its value.

(14) Turn the power on again, and measure and record the output p-p voltage for each of the following input p-p values: 0.2 Vp-p, 0.4 Vp-p, 0.6 Vp-p, 0.8 Vp-p, 1.0 Vp-p. Compute and record the actual gain for each of these cases.

(15) In the results section, write answers to the questions posed there.

(16) Turn off the power supply.

Part 2: Non-Inverting Amplifier with Voltage Divider at Input

(1) Disconnect the signal generator from the previous circuit but leave the rest of the circuit intact.

(2) Connect two 10k resistors on the breadboard and apply the signal generator across them – as shown below – to form a voltage divider. Note: the ground symbol represents your ground bus on your breadboard. Put your channel #1 scope probe across the bottom resistor – as shown below – and adjust the signal generator until you observe 0.5V p-p; record the measured value.

(3) Connect the input of your non-inverting op-amp configuration (but with R1 = R2 = 1k) to the voltage divider point as shown below and turn on the power supply.

Figure 5: Driving a non-inverting op amp configuration with a voltage divider.

(4) Measure and record the vout, vin (as shown in Figure 5) and the resulting gain.

(5) Turn off the power supply, replace R1 and R2 with 1 M resistors.

Figure 6: Driving a non-inverting op amp configuration with a voltage divider.

(6) Turn on the power supply and measure and record the vout, vin (as shown in Figure 6Figure 5) and the resulting gain.

(7) Answer the questions in the results section.

(8) Shut off the power supply.

Part 3: Inverting Amplifier

Repeat all the steps in Part 1… except now use the Inverting amplifier configuration shown in Figure B in the appendix.

In this section you should notice that the output signal is inverted with respect to the input: when the input signal goes positive the output signal goes negative, and vice versa.

Part 4: Inverting Amplifier with Voltage Divider at Input

Repeat all the steps in Part 2… except now use the Inverting amplifier configuration shown in Figure B in the appendix. The circuit you’ll need to test is shown below:

Results Section

Part I

For R1 = R2 = 10k

Measured R1 Value:_______________

Measured R2 Value:_______________

Predicted Gain:_________________ Show computation here:

Measured vin (Vp-p) Measured vout (Vp-p) Measured Gain

What happens as you increase the input p-p value further?

At what p-p input voltage does the output stop looking like a sinewave?

Why does that happen?

For R1 = 1k & R 2 = 10k

Measured R1 Value:_______________

Measured R2 Value:_______________

Predicted Gain:_________________ Show computation here:

Measured vin (Vp-p) Measured vout (Vp-p) Measured Gain

What happens as you increase the input p-p value further?

At what p-p input voltage does the output stop looking like a sinewave?

Why does that happen?

Why does this happen at a different input level than before?

Part II

Measured Voltage Divider Voltage:_______________________ (before Op Amp Circuit is connected)

For R1 = 1k & R 2 = 1k

Measured R1 Value:_______________

Measured R2 Value:_______________

Measured vin (Vp-p) Measured vout (Vp-p) Measured Gain

For R1 = 1M & R 2 = 1M

Measured R1 Value:_______________

Measured R2 Value:_______________

Measured vin (Vp-p) Measured vout (Vp-p) Measured Gain

Did the voltage at the voltage divider point change when the non-inverting op-amp configuration was connected? Explain why or why not?

Did it matter if the op amp resistors were 1k or 1M? Why?

Did the non-inverting op-amp configuration still provide the same expected gain from vin to vout?

Part III

For R1 = R2 = 10k

Measured R1 Value:_______________

Measured R2 Value:_______________

Predicted Gain:_________________ Show computation here:

Measured vin (Vp-p) Measured vout (Vp-p) Measured Gain

What does it mean for the gain to be negative? Use sketches of what you observed to illustrate your answer.

For R1 = 1k & R 2 = 10k

Measured R1 Value:_______________

Measured R2 Value:_______________

Predicted Gain:_________________ Show computation here:

Measured vin (Vp-p) Measured vout (Vp-p) Measured Gain

Part IV

Measured Voltage Divider Voltage:_______________________ (before Op Amp Circuit is connected)

For R1 = 1k & R 2 = 1k

Measured R1 Value:_______________

Measured R2 Value:_______________

Measured vin (Vp-p) Measured vout (Vp-p) Measured Gain

For R1 = 1M & R 2 = 1M

Measured R1 Value:_______________

Measured R2 Value:_______________

Measured vin (Vp-p) Measured vout (Vp-p) Measured Gain

Did the voltage at the voltage divider point change when the inverting op-amp configuration was connected?

Was the voltage at the voltage divider point different when the inverting op-amp configuration was connected with different R values?

What can you say about any cautions you should have about connecting an inverting op amp circuit to a driving circuit? Can you say the same thing about the non-inverting op amp circuit?

Figure A

Figure B

Appendix

Signal Generator

Signal Generator