EE 212 Lab
Lab 8: Operational Amplifiers - Part II, Current-to-Voltage Amplifiers

Data Sheet: LF356 JFET Input Operatonal Amplifiers (Please don't print whole data sheet)

In last week's lab, operational amplifier circuits were used to amplify voltages, i.e., convert a voltage into a bigger voltage. In this lab operational amplifier circuits will be utilized to convert a current to a voltage. The current of interest will come from optical and temperature sensors that produce an output current proportional to the incident light intensity or temperature. This sensor current is converted to a low impedance output voltage by means of a current-to-voltage amplifier configuration. It is noted that both sensors will need a bias voltage across them to operate properly.

1. Optical Detector

• Construct the optical detector circuit shown in figure 1 using a 356 FET-input amplifier and the PN168 phototransistor. (The 356 more closely approaches an ideal op amp but has the same pin connections as the 741.)

• Vary the collector voltage VC to determine whether the PN168 acts more like a current source controlled by the incident light or variable resistor whose resistance changes with light.
• Observe and sketch the output waveform, making sure to note the location of 0V DC.
• Confirm that the output is caused by light incident upon the phototransistor. What is the source of the time varying light incident on your detector? (Hint: Measure its frequency.) Test your hypothesis by disabling the offending light source. Is there still a residual light level? What is its source?
• The spec sheet for the PN168 says that 500 lux incident light intensity produces 3mA collector current with VC = 8 volts. Use this to estimate the incident light intensity. Compare this with the intensity of direct sunlight (sunlight intensity = 10,000 to 100,000 lux).

2. Temperature Sensor

• Construct the temperature sensing circuit shown in figure 2 using an Analog Devices Model AD590 temperature sensor. This is a two-terminal device that behaves as an ideal current source of 1mA per degree Kelvin over the temperature range -55 to +150 degrees C. Pin connections are as shown.
• View the output voltage on your scope to verify that it is a DC level only. Use a multimeter to accurately measure the output DC voltage.
• What room temperature does your reading imply? Compare with the room temperature reading from a thermometer.
• Check to see how much of the output voltage is due to the amplifier itself by replacing the sensor with a source of zero current.
• What is the sensitivity of the above circuit (Volts per degree Kelvin)?
• How could the sensitivity be increased?
• Approximately how much could the sensitivity be increased without saturating the amplifier output?

3. Temperature Sensor with Bias Adjustment

• Greater sensitivity can be obtained using a summing configuration of the op-amp circuit to cancel out the sensor current at room temperature, as shown in figure 3. Add the +15V input section to your circuit keeping Rf unchanged and choose Rc so that the sensor current can be canceled at room temperature using the mid-range of Vp

• Precisely zero Vout by adjusting the potentiometer. Is Vp what you expected?
• Calculate the feedback resistance Rrequired to obtain a sensitivity of 0.5V per degree C and change Rf to this value.  Make sure that the output is still adjusted for 0V DC at room temperature. Why should the output stay near 0V DC even though Rf has been changed?
• Check the operation of the above circuit by holding the transducer between your thumb and forefinger. What temperature increase do you register? Normal body temperature is 37 degrees C, and a person with 'warm' hands registers about 28-30 degrees C. Do you have warm or cold hands? (This will say something about how relaxed you are in doing this lab or, how cold the room is.)

© Copyright 2003 New Mexico Institute of Mining and Technology