CHEMISTRY 425
Instrumental methods of analysis

INTRODUCTION TO OPERATIONAL AMPLIFIERS

Purpose:

To become familiar with common operational amplifiers packages, and pin-out configuration. A circuit using an operational amplifier as an inverting amplifier will be constructed.

Introduction:

An operational amplifier is a very high gain amplifier which commonly comes in three packages - the 8-lead mini-DIP (dual in-line package, used in the laboratory), standard 16-lead DIP and the TO-99 metal can. They have a more-or-less standardized pin-out arrangement which is as shown:

Sorry for the poor quality of the figure. Pin 4 is V_CC- and pin 7 is V_cc+

Equipment and materials:
Oscilloscope, dual trace, two sets of oscilloscope leads
Sine wave signal generator
Operational amplifier, 8-lead mini-DIP (type P in the diagram)
Digital multimeter (DMM)
Resistors
Capacitors, two 0.1 µF
Power supply, ± 15 V
Bread board
Jumper wires

Procedure:

1. Set up the circuit as shown in figure:

Bypass the supply potential pins directly on the board to the ground with 0.1 µF capacitors. Carefully sketch the circuit diagram for the circuit including ± 15V source in your notebook. In this diagram show the meaning of pin numbers.

2. Test for open circuits or short circuits within the operational amplifier. This can be done by measuring the current supplied to the operational amplifier by the positive lead of the power supply. It should be 1mA < I_input < 5 mA. Lack of current usually means improper connection or problem with a power source. The power source is overload-protected and if it was accidentally shorted, it need to be turned off and back on to reset.

3. Feedback resistance. Choose R_f (the feedback resistance) so that the amplification (A_f) is about 20. Show your calculations and record your results. Be certain to show the value of R_f on your circuit diagram. The choice of the resistance should not result in amplification that would require over 10 V on the output.

4. Amplification (A_f) and phase (Greek phi). Adjust the signal generator for <0.4 V_pp (pp = peak-to-peak) sine wave of about 1 kHz. Adjust the offset of the signal generator so that the output of the operational amplifier (U_o) is symmetric with respect to ground. By using the dual trace capability of the oscilloscope, observe the U_i and U_o simultaneously. Carefully sketch the waveform. From your sketch, determine:

i) Amplification (A_f),

ii) calculate the gain in dB (Gain = 20 log U_o/U_i),

iii) phase shift of U_o with respect to U_i.

5. Frequency effects. Measure the amplification and phase shift at various frequencies going to the highest frequency the signal generator can provide. Record your data in a table. Plot  graphs of amplification, gain and phase shift vs. log frequency.