CTU: EE 415 – Advanced Electronics: Lab 2: Oscillators 1

Colorado Technical University EE 415 – Advanced Electronics

Lab 2: Oscillators August 2010

Loren K. Schwappach

ABSTRACT: This lab report was completed as a course requirement to obtain full course credit in EE415, Advanced Electronics at Colorado Technical University. This report introduces two resonating oscillators built using three resistors, a capacitor, and an Op-Amp.

If you have any questions or concerns in regards to this laboratory assignment, this laboratory report, the process used in designing the indicated circuitry, or the final conclusions and recommendations derived, please send an email to LSchwappach@yahoo.com.

I. INTRODUCTION

Operational amplifiers (Op-Amps) in feedback circuitry can be utilized for advanced signal conditioning as well as linear amplification. Their performance is generally locked upon their frequency linearity and feedback design. An oscillator utilizes positive feedback and a triggering to produce a square wave output.

II. OBJECTIVES

This lab uses an operational amplifier (Op-Amp) to design and build two oscillators. The first Op-Amp resonates at 200 Hertz and the second resonates at 25k Hertz.

III. DESIGN APPROACHES/TRADE-OFFS

In order to simplify the design of each oscillator hand calculations were simplified by ensuring the values of each resistor were identical (R1=R2=R3=Rx).

IV. PROCEDURES / RESULTS

This section outlines the procedures required to reproduce this lab and obtain similar results.

A. PART 1 – 200 HZ OSCILLATOR

To design the 200 Hz oscillator using a 1n Farad capacitor, a resistance value of 2.275M ohms was calculated using equation (6). After verifying the output frequency with Multisim this resistance value was increased to 2.365M ohms producing a better frequency result.

i. CALCULATIONS:

Schmitt Trigger Circuit:

Oscillator Circuit:

ii. EQUIPMENT:

To effectively reproduce the circuits built in this lab

you will require the following components/parts/software.

+/- 5 Volts Direct Current (VDC) Power Source

Signal Generator

Breadboard

Three (3) 2.365M Ohm Resistors

741 Op-Amp

Multisim Version 11, by National Instruments

Oscilloscope

CTU: EE 415 – Advanced Electronics: Lab 2: Oscillators 2

iii. CIRCUIT DIAGRAM:

Figure 1: Multisim design of 200 Hertz oscillator.

iv. RESULTS:

Figure 2: Multisim transient analysis results of 200 Hertz oscillator.

From Figure 2 it is observed the 200 Hertz oscillator correctly produced the 200 Hertz square wave. This was further verified by the oscilloscope.

Figure 3: Oscilloscope results of 200 Hertz oscillator circuit. .

B. PART 2 – 25 KHZ OSCILLATOR

After recalculating the resistor values using equation (6) a value of 18.2k Hertz was chosen. However, after simulating the circuit in Multisim it was discovered that a resistor value of 5.5k ohms produced a frequency very close to 25k Hertz. However, due to the slow switching speed of the 741 Op-Amp, due to its parasitic resistance and capacitance, the output waveform appeared more like a triangle wave than a square wave. Thus the 25k Hz Op-Amp design performed very poorly as an oscillator circuit.

i. CALCULATIONS:

The equations for the 25k Hertz oscillator were the same as

the 200 Hertz oscillator.

Schmitt Trigger Circuit:

Oscillator Circuit:

CTU: EE 415 – Advanced Electronics: Lab 2: Oscillators 3

ii. EQUIPMENT:

+/- 5 Volts Direct Current (VDC) Power Source

Signal Generator

Breadboard

Three (3) 5.5k Ohm Resistors

741 Op-Amp

Multisim Version 11, by National Instruments

Oscilloscope

iii. CIRCUIT DIAGRAM:

Figure 4: Multisim design of 25k Hertz oscillator.

iv. RESULTS:

Figure 5: Multisim transient analysis results of 25k Hertz oscillator. From Figure 5 it is observed the 25k Hertz oscillator produced a 25k Hertz signal, however as a square wave the signal was very distorted demonstrating the slow frequency response of the oscillator due to parasitic resistance and capacitance. This was further verified by the oscilloscope.

Figure 6: Oscilloscope results of 25k Hertz oscillator circuit.

C. PART 3 –200 HZ WORST CASE (-20%R AND +20%R)

The next stage in the lab was to verify the worst case behavior of oscillator with resistors lower (-20%) than the calculated value, and with resistors higher (+20%) than the calculated value. The 200 Hertz oscillator resistance value of 2.28M ohms was used as the base resistance. Using this values for the worst high and low case were obtained.

i. CALCULATIONS:

CTU: EE 415 – Advanced Electronics: Lab 2: Oscillators 4

ii. EQUIPMENT:

+/- 5 Volts Direct Current (VDC) Power Source

Signal Generator

Breadboard

Three (3) 1.82M Ohm Resistors

Three (3) 2.73M Ohm Resistors

741 Op-Amp

Multisim Version 11, by National Instruments

Oscilloscope

iii. CIRCUIT DIAGRAM:

Figure 2: Multisim design of 200 Hertz oscillator, Worst case scenario, low resistance, -20%.

Figure 3: Multisim design of 200 Hertz oscillator, Worst case scenario, high resistance, +20%.

iv. RESULTS:

The worst case low scenario produced a 164 Hertz oscillating signal, while the worst case high scenario produced a 175 Hertz signal. The calculated resistance produced a 169 Hertz signal.

CTU: EE 415 – Advanced Electronics: Lab 2: Oscillators 5

Figure 4: Multisim transient analysis of 200Hz worst case low circuit.

Figure 5: Multisim transient analysis of 200Hz worst case high circuit.

Figure 6: Oscilloscope results of 200 Hertz oscillator circuit displaying worst case low results. .

Figure 7: Oscilloscope results of 200 Hertz oscillator circuit displaying worst case high results.

V. CONCLUSIONS

The Op-Amp oscillator circuit utilized a hysteresis loop to create oscillation from the positive feedback of a Schmitt trigger. This coupled with slow negative feedback created oscillation. Conditions for oscillation include a charged storage device (capacitor/inductor) and a resistor to control the oscillation frequency.

REFERENCES

[1] Neamen, D. A., “Microelectronics Circuit Analysis and Design 3

rd Edition” John Wiley & Sons, University of New

Mexico, 2007.