Objective: The purpose of this lab is to further
understanding of the operational amplifier by building and studying
amplifier circuits that convert currents into voltages. The National
Semiconductor LF356 operational amplifier will be used as the basis
for circuits that measure light and temperature. The sensors of
interest and their connections are shown in Figure 1.
Pre-lab:
For the current-to-voltage converter shown in Figure 2, find the
relationship between the output voltage vo and input
current iin.
Laboratory Procedure:
Optical Detector
Construct the optical detector circuit shown in Figure 3 using a
356 FET-input amplifier and the PN168 photo-transistor. (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 when VC is
around 8V, making sure to note the location of 0V.
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 = 8V.
Use this to estimate the incident light intensity. Compare
this with the intensity of direct sunlight (sunlight intensity =
10,000 to 100,000 lux).
Temperature Sensor
Construct the temperature sensing circuit shown in Figure 4
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.
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?
Temperature Sensor with Bias Adjustment
Greater sensitivity can be obtained using a summing
configuration of the op-amp circuit to cancel out the sensor's
current at room temperature, as shown in Figure 5. Add the +15V
input section to your circuit keeping Rf unchanged
and choose Rc so that the sensor's current can be
cancelled at room temperature using the mid-range of the
potentiometer's voltage Vp.
Precisely zero vo by adjusting the potentiometer. Is
Vp what you expected?
Calculate the feedback resistance Rf required 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 updated 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 may say something about how
relaxed you are in doing this lab, or how cold the room is.)