ECE20B Lab, Fall, 2003

Lab Prelab Assignment (Week 2, October 6 – 10)

Reading Assignment: Lab Book (Carley and Khosla) Chapter 7 (Operational Amplifiers) 1. (1 point) Label the input, output and bias (power supply) ports of an op-amp, give the

equation governing Vout in terms of other voltages. Briefly describe this equation. 2. (1 point) Describe the operation of the comparator circuit shown in your Lab Book.

Calculate the threshold voltage at the “+” (non-inverting) input terminal of the op-amp.

3. (1 point) Prove that the gain (Vout/ Vin) of a unity gain buffer is approximately 1. Do not use the virtual short approximation in your proof.

4. (1 point) Calculate the gain for the non-inverting amplifier shown in Fig. 7.7 in your Lab Book. What is the output impedance of the circuit (Vout/ I, where I is the current through the resistors)?

5. (1 point) Calculate the gain for the inverting amplifier shown in Fig. 7.4 in your Lab Book. What is the input impedance of the circuit (Vin/ I, where I is the current through the resistors)?

Experiment 1: (Exercises 1-4, chapter 7) Comparator 1) Follow the pinout diagram (see Fig 7.5 in chapter 7) of LM741CN chip. Place the chip on the

protoboard and complete the voltage comparator circuit as shown in Fig 2.1 below. Power the op-amp to + (-) 5V using the + (-) 20 V power supply with the protoboard ground connected to the common of the power supply.

2) Measure the (threshold) voltage at the “+” terminal of the op-amp and compare with the analysis in the Prelab. Explain any discrepancies.

3) Use the DC offset on the function generator as Vin (Note: Operate the generator in ‘continuous’ mode) Vary its value from –5 to 5 V. Record your data pairs (input and out voltages) in your lab notebook. Remember to take several readings close to threshold in order to be able to construct a more accurate graph of Vout versus Vin. Make a linear plot of Vout versus Vin. (Be sure you label the axes (with units), mark the scale, and give a title to the graph! Be sure to use graph paper for the figure if your notebook is not ruled for computation and graphing). Compare the result of this plot to the threshold voltage measured in step (2). What is the difference in the measured saturation voltages to the values of the + (-) 5V power supply voltages?

Fig. 2.1 Comparator Circuit

Experiment 2: (Exercise 5, chapter 7) Buffer 4) Construct the buffer circuit shown in Fig 2.2. 5) Vary Vin from –5V to 5V. Again be sure to record your data in the notebook. Plot Vout

versus Vin. What is Vout when Vin is zero? Define gain of the amplifier as the ratio of Vout to Vin (or the slope of the Vout versus Vin curve). What is the amplifier gain in the non-saturated region? How does it compare to your analysis in Prelab Question 3. How do the saturated voltages compare with those obtained for the comparator?

Fig. 2.2 Buffer circuit

V in +

_

V out

-5v

+5v

V in

+

_

+V power =5V

V out

-V power =-5V

R2 = 6.8 k Ω

R1 = 1.5 k Ω

Experiment 3: (Exercise 6, chapter 7) Inverting Amplifier

6) Construct the inverter circuit shown in Fig. 2.3 using R1 = 1.5 kΩ and RF = 6.8 kΩ. Power the op-amp to + 15 and –15 V respectively. Vary the Vin from –5 V to 5V. Plot Vout versus Vin. What is the actual amplifier gain? (Slope of curve) How does it compare with your analysis in Prelab Question 5? What is the input voltage range over which the op amp is not saturated? Explain your observations.

Fig. 2.3 Inverting Amplifier circuit

Experiment 4: (Exercise 7, chapter 7) Non-inverting Amplifier 7) Construct the non inverting circuit shown in Fig. 2.4 using the same R1 and RF as in (6)

above. Power the op-amp to + 15 and –15 V respectively. Vary the Vin from –5 V to 5V. Plot Vout versus Vin. What is the actual amplifier gain? How does it compare with your analysis in Prelab Question 4? What is the input voltage range over which the op amp is not saturated? Explain your observations.

Fig. 2.4 Non-inverting amplifier

Vin

+

_

Vout

R1

RF

Vin +

_

Vout

R1

RF