Egr 2105 - Signal Amplification ,Op Amp

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ENGR2105_Lab6_OperationalAmplifiers1.pdf

© N. B. Dodge 01/12

ENGR 2105 – Signal Amplification

1. Introduction and Goal: Amplifiers increase the power (amplitude) of an electrical signal. They are used in audio and video systems and appliances.

Amplifiers are designed to amplify signals within a frequency range. Today,

we study an operational amplifier and use it to amplify a sinusoidal signal.

2. Equipment List: Required instruments and components are shown below.

• Multisim

3. Experimental Theory: Amplifiers increase signal strength (Figure 1) due to power from an external source, unlike the passive circuit elements that we

have studied so far. We won’t cover amplifier internals but instead

concentrate on the fundamentals of operation.

3.1 Theory of the operational amplifier: Figure 2 shows a basic “op amp.”

3.1.1 The op amp has inverting and non-inverting inputs (─ and +), an output, and two power inputs, +V and ─V (DC

voltages). It has no ground (or 0 V) input, but you can use

power supply ground to attach to the ground lead of the

oscilloscope probe.

3.1.2 An op amp cannot have an output larger than its power supply voltages. If the +V DC voltage is, for example,

+15V., then the output cannot possibly exceed +15V. If the

output would have a swing of more than ±15 V or more, the

output voltage swing will be clipped (Figure 3). In practice,

clipping occurs when the output of the op amp is much less

© N. B. Dodge 01/12

than the ± supply voltage. A good rule of thumb is that the

op amp output should limited to about 60-70% of the supply

voltage limits.

3.1.3 The amount of amplification of an op amp is called gain. The maximum gain of most operational amplifiers is very

large (usually > 100,000). Op amp gain can be reduced to a

useful range by “negative feedback,” which is discussed

below.

3.1.4 If op amp output is between ±0.6V, (± V = the DC power levels), it is operating in its “linear gain region.” Thus,

amplification is constant and linear. Output is K times the

input, where K a constant, as shown in Figure 4.

3.1.5 Important op amp characteristics: (1) high input resistance, (~ 1 MegΩ), and (2) low output impedance (a few hundred

Ω or less).

3.1.6 With its high gain, low output resistance and high input impedance, the op amp is easy to analyze if we assume: (1)

Input impedance is infinite (→∞), (2) output impedance →

0, (3) gain →∞.

3.2 Negative Feedback: With such high K, the op amp would only be

useful to amplify only tiny inputs. To amplify larger inputs, we can

use negative feedback to lower K.

3.2.1 In Figure 5, the input is vi(t) and the output signal is vO(t). The circuit resistors are Ri (input resistor), Rf (feedback

resistor), and RL (load resistor). Input voltages are ±V.

3.2.2 Using assumptions of 3.1.6: Since amplifier input impedance is large, we assume input current is negligible: in

= 0. Since vp = 0 (ground = 0V), and since in is 0, then vn =

vp = 0. These are approximations, but they are close enough

for our analysis.

© N. B. Dodge 01/12

3.2.3 By Kirchoff’s current node law ∑𝑖𝑛𝑜𝑑𝑒 = 0, “Node n” (Figure 6). Then 𝑖𝑖 + 𝑖𝑓 = 𝑖𝑛. Since in = 0, then 𝑖𝑖 + 𝑖𝑓 = 0.

From the Figure 5, 𝑖𝑓 = (𝑣𝑂 − 𝑣𝑛) 𝑅𝑓⁄ )and 𝑖𝑖 = (𝑣𝑖 − 𝑣𝑛) 𝑅𝑆⁄ . But 𝑣𝑛 = 𝑣𝑝 = 0, so that 𝑖𝑓 = 𝑣𝑂 𝑅𝑓⁄ and

𝑖𝑖 = 𝑣𝑖 𝑅𝑖⁄ .

3.3 Discovery Exercise: In your worksheet, use the information above to

develop a formula for the gain, which you will use in the exercises

below.

4. Pre-Work: Prior to lab, watch the lecture (link on eCampus) and complete the worksheet.

5. Experimental Procedure: 5.1 Negative Feedback Amplifier:.

5.1.1 In Multisim, select the “UA741” op amp from the “Analog → OpAmps” menu.

5.1.2 For the input resistor, use a resistor of resistance Ri = 1 kΩ 5.1.3 Set up the feedback resistor for an amplification of K=10 (technically

─10). Based on the 1kΩ value of Ri, use your formula for K to select

the feedback resistor, Rf, and connect it as shown in the video.

5.1.4 Use a 1kΩ resistor for the load resistor. (Since there is very little current in this circuit, your choice for load resistor does not matter.)

5.1.5 Place your ground connection and a voltage probe to measure the voltage across the Load Resistor.

5.1.6 Place a voltage probe to measure the voltage from the voltage source.

© N. B. Dodge 01/12

5.1.7 Set the AC Voltage source to to 1 Vp = 0.5 Volts at 1000 Hz. 5.1.8 Start the simulation. 5.1.9 Use the “Grapher” to see the input and output AC signals. 5.1.10 If resistor selection is correct, the output should be ~ 5 Vp. Is your

output that value? Note input and output peak voltages on your data

sheet. Calculate gain using selected resistor values. Are they close?

5.1.11 Take a screen shot of your Multisim circuit and include it in your lab report.

5.1.12 Change feedback resistor for a gain of ~ 50. 5.1.13 Change the AC Voltage Source to a peak voltage of 100 mV. Start the

simulation and check the output In the grapher. Record second

resistor value calculated for K=50.

5.1.14 Take a screen shot of your Multisim circuit and include it in your lab report.

5.1.15 The op amp is an inverting amplifier in negative-feedback mode. Looking at the Grapher, you should see that the phase of the output is

180 degrees from the input.

5.2 Design Exercise (Hint: Related to the activity you did in the

Worksheet )– A Non-Inverting Amplifier: What if you need a non-

inverting amplifier (one that does not produce an output that is 180

degrees out of phase with the input)?

5.2.1 Design a non-inverting op amp circuit with a gain of 100, using resistors and a second op amp. You can still use the

equation that you developed for the earlier design. Note: to

preserve the condition that the op amp output voltage be no

more than about 60-70% of the power supply maximum

input voltages, the input should have a peak voltage of no

more than 50 mV.

5.2.2 Hints: (1) When amplifiers are cascaded (the output of the 1st op amp is fed into the input resistor of the 2nd op amp),

their gains multiplie Total Gain = (Gain from Op Amp 1) x

(Gain from Op Amp 2).

5.2.3 Hints: (2) Each negative-feedback op amp inverts the signal.

5.2.4 After demonstrating amplifier circuit, the experiment is complete.

5.2.5 Take a screen shot of your Multisim circuit and include it in your lab report.

© N. B. Dodge 01/12

6. Laboratory Area Cleanup: Wash your hands! 7. Writing the Laboratory Report: In your report, do the following:

7.1 Discuss gain characteristics of the op amp. Did your amplification formula work?

7.2 How did the calculated gain and actual gain compare? 7.3 How accurate was the amplification in your non-inverting circuit? 7.4 What, if any, problems did you encounter?