Manual

A
fter you finish the wiring, set the switch to position B.
LEDs 1 and 2 indicate the output voltage of the
operational amplifier IC. An LED lights if it is
connected to 1.5V or higher. In this experiment, we
connect the two LEDs in series, so they only light
when connected with about 3V. When they are off,
the output voltage of the operational amplifier must
be less than 3V.
View the schematic diagram. With the switch at
position B, the 1.5V battery voltage is connected to
two 10kW resistors, with the positive terminal of the
operational amplifier connected between the
resistors. These two 10kW resistors divide the 1.5V
supply voltage in half. This signifies the positive
input terminal receives an input voltage of only
0.75V.
To total the output voltage of the operational
amplifier you multiply its input voltage by the
amplification factor (R1/R2) + 1. So, the output
voltage is 0.75V x ((220kW / 100kW) + 1) = 2.4V.
Slide the switch position A. This eliminates the 10kW
resistors from the circuit, so the amplifier’s positive
input terminal receives the full 1.5V input voltage.
Using the above equation, you can see that the
output voltage of the operational amplifier is now
1.5V x ((220kW / 100kW) +1) = 4.8V. Because the
voltage supplied to them is more than 3V, the LEDs
light dimly.
Let’s alter the amplification factor. Slide the switch
to position B again and press the key. This adds the
47kW resistor to the 100kW resistor in parallel,
making total resistance of R2 about 32kW. Now the
output voltage is 0.75V x ((220kW / 32kW) +1) =
5.9V, enough to light the LEDs brightly.
Now slide the switch to position A again (to connect
1.5V to the amplifier’s positive (+) input terminal),
a
nd press the key. The LEDs light brightly.
Calculating the output voltage gives 1.5V x ((220kW
/
32kW) +1) = 11.8V. However, the actual output
voltage will be limited by the available battery
voltage, which is 1.5V + 3.0V + 3.0V = 7.5V.
Notes:
EXPERIMENT #71: CHANGING INPUT VOLTAGE
Wiring Sequence:
o 31-67-92
o 32-34
o 81-89-88-70-36-121
o 63-122
o 68-90-91-138
o 69-132
o 82-84-133
o 83-131-120
o 87-137
o 119-124
-90-
Schematic
EP-130_62315RevC.qxp_EP-130_062812 6/23/15 11:17 AM Page 90