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The forward voltage of an LED is in the range of 1. Build both of the versions of the circuits in figure 1 a and 1 b as shown in figure 2 on your solder-less breadboard.
Use two LEDs preferably of the same color. Green LEDs will have a higher forward voltage drop than red or yellow. We want the diode current, ID, to be about 1 mA and the as close to this same value in both versions of the circuit.
Also for case a we need to pick values for R1 and R2. We want the current in R1 to be much smaller than the current in R3. Figure 2, LED based volt regulator example Hardware setup: Open the voltage supply control and the voltmeter instrument windows from the Scopy software. A DMM, if available, could be useful to more accurately measure the DC voltages in the circuit than the Scopy voltmeter instrument. Figure 3 LED based volt regulator breadboard connections Procedure: Turn on both the positive and negative power supplies.
What voltage did you measure at the LED? Are these the correct expected values and why? Directions Step 2: Modify your breadboard setup from step 1 as shown in figure 3.
Be sure to turn off the power supplies before making any modifications to your breadboard. Replace the LED diode with the shunt regulator stage from earlier lab . Resistors R1, R2 and transistor Q1 are connected as the zero gain amplifier from the earlier lab . Resistor R3 and transistor Q2 are added as in the stabilized current source lab .
Q3is added as common emitter stage, its base connected to the collector of Q2 and collector connected to the combined node of R1, R3 R4. Figure 6 LED based volt regulator example Procedure: Turn on both the positive and negative power supplies. Observe the voltage at - V REF, pin 14 of the op amp and across the band-gap shunt regulator collector and emitter of Q3. You can adjust potentiometer R3 to produce a Be sure to turn off the power supplies before making any changes or additions to your breadboard.
Set AWG 1 to trapezium trapezoid waveform at Hz. Set the amplitude to 5V peak-to-peak with a 2. Use the oscilloscope instrument in the XY mode, scope channel for X and scope channel 2 for Y.
Start AWG 1 and turn on the fixed negative 5V power supply. Disconnect the fixed negative power supply from - V SS and remove any supply decoupling capacitors.
Set the amplitude to 5V peak-to-peak with a Start AWG 1 and turn on the fixed positive 5V power supply. Repeat your measurements of pins 14 of the OP recording the lowest value for - V SS where the reference voltage is constant. Directions Step 3: Modify your breadboard setup from step 1 as shown in figure 4.
Change the two terminal, shunt, regulator used in step 2 to the three terminal reference  by adding emitter follower stage Q4, and compensation capacitor C1.
Figure 7 NPN three terminal band-gap reference example Hardware setup: The setup is the same as step 1. Figure 8 LED based volt regulator example Procedure: Turn on both the positive and negative power supplies. Observe the voltage at - V REF, pin 14 of the op amp and across the band-gap three terminal regulator emitter of Q4 and emitter of Q3.
Repeat the supply headroom tests you did in Step 2 for this configuration. Are there any differences?
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