Introduction to Biasing

This experiment is designed to introduce the concept of Biasing. Biasing in this context is the selection of appropriate circuit elements in order to create a desired voltage at a specific point in the circuit. In the case of the two LEDs pictured, each is rated to operate optimally at a different voltage. The green one is designed to operate at 2 Volts and the yellow one is designed to operate at 12 Volts. If we try to power both LEDs in parallel, as shown in the picture, appropriate resistors need to be chosen to bias the LED's input voltage.

The experiment originally called for the yellow LED to be of the 5 V type, so the results may be different than originally predicted.

In order to properly Bias the circuit, it is important to know how much current is dropped across the LED when it is operating properly. From now on, the Yellow LED will be referred to as LED 1 and the green will be LED 2. LED one is rated for 22.75 mA at 5V and LED 2 is rated for 20.00 mA at 2 Volts. Based on this information, it is determined that each LED has an effective internal resistance of 424, and 100 Ohms respectively. Based on this data, 2 resistor values were chosen to drop 4 volts and 7 volts from a 9 volt power supply. The result can be found in the schematic seen above.

A keen eye will notice that the current through the left branch cannot possibly be near the 22.75 milliamps requested in the ratings, and this is due to LED 1 not being of the proper type. However, the node voltage between LED 1 and Resistor 1 is still 5 Volts as desired.

The Current in the left branch is 10.7 mA and the right branch has current of 20.00 mA. The resistors are disipating .043 and .14 Watts respectively. Both of these values are less than the maximum power rating for the chosen resistors so our circuit is safe for construction.

Once the circuit has been constructed, it is desirable to see if our calculations actually created the desired bias voltage.

A multimeter was used to determine the current and voltage for each LED in three configurations. The first with both LEDs in the circuit, the second with only LED 1 connected and the third configuration with only LED 2 connected.The results can be found in the following table.

It is evident in the data that biasing is independent of the load as long as the power supply is operating at much lower than its maximum range (which for the chosen supply was 2 Amps).

Questions:
How long could your circuit operate if running off of a 9-Volt battery if the supply voltage begins to drop at .2A-hr?

.2A-hr/.0292A = 6.6 hours

What is the percent error between the achieved LED current and the desired current?

The green LED has a percent error of -4% but the yellow LED has percent error of -56%. This large error is due to the yellow LED not conforming to the specifications outlined in the laboratory report. They were in fact rated for 12 Volts instead of 5 Volts.

From your Data determine the circuit efficiency.

Power out = Pled1 +Pled2 = 10mA*5.64V + 19.2mA*2.18V = .0982 W
Power in = 29.2 mA * 9.09 V = .2654 W

efficiency = Pout/Pin = .0982/.2654 = 37%

If the battery voltage was changed to 6 Volts how would the efficiency change? What is the ideal voltage to achieve maximum efficiency?

If the voltage was lowered to 6 volts the efficiency would go up because smaller resistors could be used to bias. Thus less power would be wasted into heat.

The ideal configuration, as far as efficiency is concerned, would be to connect a 30 Ohm resistor, LED 1 and LED 2 in series. Drive the entire circuit at 7 Volts.

Power out = 20mA*4.4 V + 20mA*2V= .088 + 04 = .128 W
Power in = 20 mA * 7 V = .14 W

Efficiency = 88.57%

A higher theoretical efficiency is possible, however it requires a higher current in series, which would overdrive the green LED and possibly damage the circuit.