OPERATIONAL AMPLIFIERS Part II LAB

OBJECTIVES

1. inverting amplifiers;
2. current to voltage converters;
3. integrators.

DISCUSSION

APPARATUS

• DC power supply with multiple outputs
• 741 operational amplifier integrated circuit chip
• oscilloscope
• prototype board, capacitors and resistors
• function generator

PROCEDURE

1. There is one less resistor in this basic circuit than in the inverting amplifier. Build the circuit as shown in figure 1 below:
   
   Note: the schematic is less detailed than last week's, since you are now experienced in properly biasing an op amp. If in doubt, look at last week's schematic. Note and record the output voltage. (It probably won't be zero!)
   The gain is much higher in this circuit, and therefore any small residual voltage that appears at the input terminals will be amplified much more. There are two techniques for dealing with any offset voltage:
   
   • (a) Use an external resistor between V2 and ground; or
   • (b) Use the offset features built into the Op Amp itself.
   We will use both methods in this experiment.
2. Add a resistor to the non-inverting input as shown in figure 2.
   
   Try different resistors for the additional resistor Rb. Suggested values are 500 kOhm -- 1 MOhm. See if the output comes closer to zero.
3. Then add a potentiometer as shown in figure 3:
   
   Its important that the center contact of the potentiometer be connected to pin 4, i.e. the -15 Volt side of the 741 Op Amp. Now you should be able to zero out the rest of the output voltage by adjusting the "pot".
4. Check to verify that your output voltage = I x R2. You can do this by generating a 1 microamp with external resistors and the seperate power supply. See the suggested circuit in figure 4:
   

After verifying that you have an amplifier that allows you to measure 1 microamp with 0.001 microamp resolution, you can applyh this circuit to a typical measurement problem in physics: measuring the "photo current from a photo-tube" as a function of the voltage across the tube.


5. Connect the input of your Op Amp current converter to the Photo-electric effect module as shown in figure 5:
   
   By applying a +5V to the Photoelectric module, the anode is positive, thus attracting the electrons and increasing the current.
   
   When you reverse the polarity to -5V, the anode can be made negative, the electrons are repelled and eventually stopped when the voltage reaches a great enough value.
6. With +5 V connected to the module, adjust V2, the voltage across the phototuve. Do this with no light on the tube. Then allow the room light to fall on the phototube. Adjust V2 from 0 to maximum and record both V2 and V1, the output of the Op Amp.
7. Repeat with -5V connected to the module. This time you only need to adjust V2 from 0 to approximately -1V.
8. Plot your data to see how the photo-tube behaves. Be sure to label your photo-tube current in microamps.
9. Take another few data points with twice the light falling on the photo-tube. Comment on the value of V2 when both curves intersect or closely approach I = 0 microamps! What is the significance of this?