Design and Construction of a Variable Gain Amplifier for Tunable - - PowerPoint PPT Presentation

Design and Construction of a Variable Gain Amplifier for Tunable Diode Laser Spectroscopy

Robert Moffatt

Thomas Jefferson High School for Science and Technology Laboratory:NRL

Mentor: Dr. James W. Fleming

Introduction

• My project supported

research done by Dr. James Fleming and

• Dr. Andy Awtry.
• They are investigating

how the displacement

• f oxygen by the

evaporation of water mist can be used to put

• ut fires.

Experiment Setup

• 760nm laser beam

sent through air near flame.

• Photodiode

measures light intensity

• Signal amplified

and sent to computer Laser Control Laser

Amplifier Photodiode

The Signal

• Laser current

follows a 2KHz triangle wave.

• Wavelength and

intensity sweep up and down with current.

Light Output from Laser: Light Input to Photodiode: 760nm

Attenuation by Mist

• Relative amplitude of

absorption line unaffected by mist.

• Mist does affect

absolute amplitude of signal.

• Amplification is

required to keep the signal above the noise level.

Without Mist: With Mist:

Attenuation by Mist

• Attenuation caused by

mist varies over three

• rders of magnitude.
• No single gain setting
• n the amplifier

allowed the entire range to be measured.

• I designed and

constructed circuits to solve this problem: an amplifier with multiple gains, and an amplifier with automatically controlled gain.

Multiple-Gain Amplifier

• This multiple-gain

amplifier circuit was designed by Dr. Volker Ebert.

• One input and three
• utputs.
• The outputs are

monitored simultaneously, so the computer is always provided with at least

• ne acceptable signal.

Laser Control Laser

Amplifier Photodiode

The Schematic

• The gain of each
• utput can be

switched between two settings: 3 or 6, 10 or 40, and 30 or 250.

• The circuit uses

three OP27 and OP37 op amps

Auto-Gain Amplifier

• Gain controlled

automatically

• Output measured
• nce per cycle
• Circuit

determines if its gain should increase, decrease, or remain the same

Laser Control Laser

Amplifier Photodiode

Block Diagram:

• The circuit consists of

three modules: – A digitally controlled amplifier – A sampling circuit – Control circuitry

Logic + - + - Vref1 Vref2 Logic 1MHz Demultiplexer & Inverters 8 4 8 Sync. Sample-and-Hold

In Sample Out

74LS169 Counter

ENP U/D

74LS163 Counters

MR

Digitally Controlled Amplifier Analog Input Analog Output Digital Gain Output

Results: Multiple-Gain Amplifier

• A LabView program was used to record and analyze the data.
• % O2 is constant; fluctuations are due to noise

Comparison of Fixed Amplfications

5 10 15 20 25 30 35 40 45 50 15:50.4 16:07.7 16:25.0 16:42.2 16:59.5 17:16.8 17:34.1 17:51.4 18:08.6

time (min:sec) Oxygen (%) No Gain 3x Gain 10x Gain 30x Gain

Water Mist On Decreasing Mist Mist Off

Results: Auto-Gain Amplifier

• Input signal varied over 2.5 orders of magnitude.
• Signal level maintained within predetermined limits

Signal Voltage for Variable Gain and Fixed Gain

1.E-02 1.E-01 1.E+00 1.E+01 30:00.0 30:43.2 31:26.4 32:09.6 32:52.8 33:36.0

Time (min:sec) Signal to ADC (volts)

Auto Gain Fixed Gain

Increasing Water Mist Density Decreasing Mist Mist Off

2.2V 0.7V

Results: Auto-Gain Amplifier

• Allowed % O2 measurements to be made in high mist densities.

Oxygen Concentration Measured by Fixed Gain and Auto Gain

0.0 10.0 20.0 30.0 40.0 50.0 30:00.0 30:43.2 31:26.4 32:09.6 32:52.8 33:36.0

Time (min:sec) Oxygen Concentration (%) Fixed Gain Auto Gain

Increasing Water Mist Density Decreasing Mist Mist Off

Conclusion

• The Multiple-Gain Amplifier was
• successful. It allowed valid O2

concentration measurements to be taken during periods of high mist density.

• The Auto-Gain Amplifier was successful in

controlling the gain and providing valid O2 concentration measurements.

Acknowledgements

• Dr. Fleming (NRL Mentor)
• Dr. Awtry (NRC/NRL Postdoctoral Fellow)
• Dr. Ebert (Visiting Professor, University of

Heidelberg)