Prelab 8

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 V_C 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 V_C = 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 1μA 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 R_f unchanged and choose R_c so that the sensor current can be canceled at room temperature using the mid-range of V_p. 

• Precisely zero V_out by adjusting the potentiometer. Is V_p what you expected?
• Calculate the feedback resistance R_f  required to obtain a sensitivity of 0.5V per degree C and change R_f 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 R_f 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.)