3rd Lab

We have our successfully simulated circuit shown as below. The output voltage range of this circuit is around 0 to 12.5 volts, and it can provide an appropriate current for a load resistor of 50 ohms.

Component List
D1: 6.8V Zener Diode
C1: 1mF
C2: 1uF
C3: 1uF
T1: BC107
OP1 and OP2: LM741CN
R1 and R2: 1k ohm
R3 and R4: 10k ohm
R5: 1k ohm
R6: 4k ohm
R7: 1k ohm potentiometer
R8 and R9: 1k ohm

We built the circuit according to the circuit diagram above.

• First test:

1. Progress: The output voltage is steady, it is a DC output.
2. Problem1: The output voltage cannot be adjusted when we change the resistance of the potentiometer.
3. Problem2: The output voltage is not zero, but quite low compared to the input voltage.
4. Actions taken: We used a multimeter to measure the voltage drop accross each component, to see whether it was working properly.
5. Discovery: We find out that the voltage drop across R4 is too large. It was strange for the voltage to drop from 5V  to 1V after passing R4. The voltage signal was not amplified by the operational amplifier. Aactually, the voltage signal drops from 1V to 0.2V.

The supervisor told us there exists some potential problem in our design. But he could not tell where is the mistake. To keep in process, I decided to build the circuit from google, while my partner continues with the designing part.

I finished building the circuit from google.

• Second test:

1. Progress: The output voltage is DC and the amplitude of it is much larger than that of the designed circuit.
2. Problem: The output voltage does not change with the variation of potentiometer.
3. Actions taken: I suspect there is some connection mistake in the circuit. So I check the connection line by line.
4. Discovery: I find out there is one connection error around the potentiometer. The third pin of the potentiometer is not connected to the circuit at all. It means that the whole potentiometer is connected to the circuit, it functions as a normal resistor. As a result, the output voltage will not change with the potentiometer.
5. Result: After the correction of the mistake, the output voltage can response to the change of potentiometer.

Here is the plot of the input and output voltege.