OpAmp Basics

This project is a brief introduction to build a simple signal processing circuit. It uses two LM741 Operational Amplifier ICs (opamp). It will accept a signal from the function generator and will couple it via a first op amp in a buffer or impedance coupler configuration. Then the resulting signal is fed into an inverting amplifier. The circuit is powered by a bipolar power supply build upon two single-supply power sources connected in series to provide a +V and -V symmetric voltage to the opamps. To compare input and output, two oscilloscope channels are used to show these signals.

Students are required to design this circuit for a voltage gain (G) of 3 V/V to amplify a sine wave with 1v peak to peak and a frequency of 1kHz. Output must have the same waveform (which means no harmonic distortion nor saturation, which are defined as non-linearities). G is defined as the ratio of output to the input.

Before starting the construction, students must be acquainted with the use of power supplies, function or signal generator, and oscilloscope.

The aims of this project is to: a) design an inverting amplifier circuit, b) simulate the circuit using Thinkercad, c) build the circuit at the laboratory, and d) test it to compare practical results versus simulated results and expected design results.

1 Breadboard

Resistors according to design

2 LM741 IC

Dupont jumper cables

Connecting cables for each instrument and power supply

Design the circuit. First stage is a buffer or impedance coupler. Look into your text book and use a design that leads to a circuit with negative feedback from output (Vo) to the inverting input of op amp. Signal coming from the generator (Vi) will be connected to the inverting input. Design equation is Vo = Vi

Second stage is an inverting amplifier. As it may be found in the appropriate textbook, input signal (Vi) is fed into the inverting input of op amp via an input resistor (R1). Non-inverting input is connected to ground. A feedback resistor (R2) is connected between output (Vo) and the inverting input of opamp. Design equation is Vo = -R2/R1 Vi

When both stages are interconnected total transfer of the circuit is Vo=-R2/R1 Vi with an input impedance greater than 10 Megaohm and an output impedance of 50 ohm.

Choose the values of R2 and R1 to achieve a G=3

In Tinkercad start a new circuit and place an empty breadboard. Remember that sockets (holes) are connected horizontally for the top two rows and the bottom two. Anytime placing the cursor over any socket will highlight in green where it is connected to.

Get two power supplies from Thinkercad right menu. Set the voltage of each to 15V. Put these power supplies in series as shown in figure. One extreme of the series will be +Vcc voltage, the other will be -Vcc voltage. Connect these to the top inner row and the lower inner row respectively. Connect ground (middle of the series) to the outer row of top and bottom (this would provide some shielding to reduce interference in the circuit. Briefly turn on the circuit (play button - right top) check for any short circuit which may show as power consumption (more than 1 or 2 mA).

Place two 741 operational amplifiers in the middle of the breadboard, allow enough space to let conductors pass and resistors to be connected. It is important to keep circuit as "clean" as possible. Tip. If you have two circuits to be placed, put the first at 25% of the length of the breadboard starting from left and the other at the 75% of such a length.

Place wires from appropriate terminals to +Vcc bus and -Vcc bus. Briefly turn on the circuit (play button - right top) check for any short circuit which may show as power consumption (more than 1 or 2 mA). If there is no short circuit we can assume circuit is working perfectly.

Place the rest of components (R1, R2) and connecting cables. Use appropriate colors to allow you to "read" the implemented circuit and compare to your design (checking). Try to be as neat as possible i.e. avoid diagonal wires and over-the-IC wires. Always remember how "holes" are connected in the breadboard.

Place a function generator. Select a sine wave with 1kHz frequency and 1V peak. Make any necessary conversions such as period to frequency (T = 1/f) or Vp = 1/2 Vpp. Connect the function generator to the appropiate input and ground in the breadboard. Again, Briefly turn on the circuit (play button - right top) check for any short circuit which may show as power consumption (more than 1 or 2 mA). If there is no short circuit we can assume circuit is working perfectly.

Place two oscilloscopes. One will be connected to input (from signal generator), the other will be connected to IC-2 (inverting amplifier) output. Instruments work in autoscale mode so no further adjustment is needed. Axes show the full length of scale (vertical - voltage, horizontal time).

Press the play-button (top right). If circuit was well-implemented you must see both signals. If not check connections. If you place 2nd oscilloscope screen to the output of IC-1 Buffer you should see the same signal as input. Measure and record input and output signals (period and amplitude). Verify frequency and calculate gain (G). Compare to your design calculations. Are these the same? If not check resistor values and function generator settings.

Follow steps 2 to 9 now with physical components from the lab stores. Remember to keep power supplies turned off until you are checking or testing. Always follow turn on order: a) power supply, b) function generator. Turn off order is inverse. Measure on oscilloscope screen (remember Auto-button is your best friend). Record the results in your logbook. Compare them to your design and simulation results ¿any errors? If so, write them in your logbook.

Follow your teacher's instructions and submit a report with photographs and discussion of results.