PC/CP320 Physical Computing

Introduction to Operational Amplifiers

Objectives

An operational amplifier or op-amp is a high gain DC amplifier; the operational portion of the name derives from its original application for performing mathematical operations in analog computers. The op-amp is an analog device that is very widely used in both analog and digital design because its operating characteristics can be significantly changed by connecting different external components.

The objectives for this lab are:

to investigate the behaviour of an op-amp in an open-loop configuration - the comparator
- operation of inverting and non-inverting inputs
- impact of op-amp gain
- limitations imposed by supply voltages
to investigate the behaviour of an op-amp in one of the many closed-loop configurations - the inverting amplifier
- how to adjust gain
- limitations imposed by supply voltages

Equipment

textbook
bench supply, oscilloscope, function generator
breadboard, variable resistor, resistors
for open-loop circuit and inverting amplifier use the LM741:
data sheet from local server.

Procedure

The comparator - the op-amp in an open-loop configuration

Operational amplifiers are differential amplifiers that amplify the difference in voltage between the two input connections or V_o = A_v (V₊ - V_- ) where A_v is the voltage gain.

Using the LM741, construct the open-loop comparator circuit shown.
Since you will be having negative supply voltages that are not the same as ground, use white wires for the negative supply voltage. There are white alligator clips to help with this.

Use ±12V bench (i.e. fixed) supplies for the supply voltages (note the above caution about polarity).
(Note that the diagram does not explicitly show the supply voltages for the op amp.)

Don't confuse the positive supply and the non-inverting input, or the negative supply and the inverting input. Always ensure you know which one you are talking about.

Precautions should be taken to insure that the power supply to the operational amplifier never becomes reversed in polarity. The input voltage at the positive supply pin must be greater than the input voltage at the negative supply pin. If polarity is reversed to the IC, the internal conductors can fuse and destroy the chip.

Use the two variable dc supplies for V₁ and V₂ (from the supply on top of the bench).
Never apply inputs before establishing supply voltages.

Don't confuse the positive supply and the non-inverting input, or the negative supply and the inverting input. Always ensure you know which one you are talking about.

Monitor V_out with a digital voltmeter.

Sketch the circuit in your lab notebook. Since you will be doing a lot of circuit variations, it is important to get into the habit of always sketching the circuit in your lab notebook. Always include the supply voltages.
1. Set V₁ to 1 volt, and V₂ equal to 0 volts. Note V_out.
2. Increase V₂ slowly to 2 volts, while watching V_out. Don't change V₁.
  If you notice anything happen to V_out, record what happens and the value of V₂.
3. Now leave V₂ at two volts, and increase V₁ slowly to 3 volts, while watching V_out.
  If you notice anything happen to V_out, record what happens and the value of V₁.
4. Repeat the pattern of the last two steps, leapfrogging V₁ and V₂ until they reach the positive supply voltage or until you see a pattern emerge. What pattern did you observe in V_out?
Since V_o = A_v (V₊ - V_ ), what output would you expect if you had unity voltage gain, i.e. A_v = 1? Explain.

Knowing this, what can you say about A_v for this op-amp, e.g. is it much less than 1, is it less than 1, equal to 1, greater than 1, much greater than 1? Explain your reasoning.

Demonstrate your results (i.e. explain why this is called a "comparator") to the lab supervisor.

The inverting amplifier - one of the many closed-loop configurations

An inverting amplifier circuit produces an amplified output signal that is 180° out of phase with the input signal.

Using an LM741, construct the closed-loop inverting amplifier circuit shown to the left. Use ±12V for the supply voltages. Use a feedback resistor, R_f, of 10 kOhms and an input resistor, R_in, of 1 kOhms. When you sketch the circuit in your lab notebook, note the feedback and input resistor values.

In order to show that the circuit works as expected, you'll need to display both the input and output at the same time.

Drive the amplifier with a 1 kHz sine wave with a DC offset of zero. Start with a small amplitude and increase the amplitude; note the signal behaviour as the amplitude increases. Sketch input and output; include measurements of amplitude.

At what point does the signal saturate (output signal will no longer look like a sine wave)? For the remainder of this exercise, use a reasonable amplitude (not too small but do not saturate the output).
Theoretically, V_o = A_v (V₊ - V_ ) where A_v is the open-loop voltage gain. The closed-loop gain is a function of the feedback resistor and the input resistor. For the circuit in the previous question (nonsaturated state), what closed-loop gain did you observe? How does this relate to the resistors used in the circuit?
What maximum output swing did you observe? What is this as a function of the rails?
Since you know the equation (question 2) and you know the supply voltages you used, you can calculate the point at which saturation would occur, i.e. for what value of V_in? Show the calculation.
Measure the voltage at the inverting input. Sketch the circuit and indicate how you are doing this measure.

The inverting input is said to be at virtual ground. A virtual ground is a voltage ground because the point is at 0V; however, it is not a current ground because it cannot sink any current.
Change the supply voltage to +5V/-12V. Sketch your circuit and show both the input and output voltages. Do you understand why the supply voltages are called the rails? Note the relationship of Vo to the supply voltages; does Vo reach the rails?
Change the supply voltage to +5V/0V. (What is this configuration called?) Sketch your circuit and show both the input and output voltages. Is this consistent with what happened in the previous item? What does this illustrate about using inverting amplifiers in single supply configurations?
Return to a supply voltage of ±12V. Try sine waves of different frequencies; at some point the amplifier will stop working properly. Note what happens and when it happens.
What happens if you try different resistor values? Hold either the feedback or input resistor constant and change the other resistor. What is the relationship between the resistors and the gain? Do you see the voltage divider formed by R_in and R_f?

Demonstrate the inverting amplifier circuit to the lab supervisor. Be prepared to explain the circuit's behaviour.
In order to show that the circuit works as expected, you'll need to display both the input and output at the same time.
You do not have to keep the circuit past the demonstration.

Have the lab supervisor sign your lab notebook before you leave the lab.