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 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.
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
- What is the sensitivity of the above circuit (Volts per
- 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 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
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.)
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