Working with transistor - Part 2 (bias a common emitter circuit)

This is a quick summary of how to bias a common emitter circuit.

Assuming we have a common emitter circuit based on 2N3904 as illustrated below and we want the output to be 10V, 10mA.

Step-1 Find the value for RE

Assuming we want to have 1V, 10mA at the point showed below, RE will be 1V / 10mA = 100 Ohm.

When IC = 10 mA, VCE = 1 V, the hFE of 2N3904 is between is between 100 to 300.

Excerpt from 2N3904 Datasheet

Step-2 Find the voltage at the point where R1, R2 and Base are connected.

Because there is an approximate 0.6V forward voltage between Base and Emitter, to get 1V at Emitter, the voltage at the point where R1, R2, and Base connected needs to be around 1.6V.

Step-3 Find the value for voltage divider resistor R1 and R2

Assuming there is 1mA current flowing through R1 and R2, the value for R1 is (20V - 1.6V) / 1mA = 18400 Ohm and the value for R2 is 1.6V / 1mA = 1600 Ohm.

Step-4 Find the value for RC

When there is a 10m IC flowing through RC, RC is (20 - 10) / 10mA = 1000 Ohm.

Note, In the above example, the input signal gain is RC / RE = 1000 / 100 = 10.

Important!!

The maximum total power dissipation for 2N3904 is around 625mW.

If IC is set to 50mA and VCE is set to 10A, the total power flowing through the transistor will be around 500mW. Althought this is lower than the 625mW limit, the transistor will burn out eventually as it cannot handle it for a long period of time.

Built and measure the circuit

Below are the measured results of the circuit.

One important thing to note, while measuring IC and IE, I noticed that the measured figures were drifting upward (but the figures went down when I blew air to the transistor). When I measured the case of the transistor, I could see the temperature steadily increase.

Although the transistor is only taking about 78mW and the 2N2222 has an absolute maximum power dissipation of 500mW, care needs to be taken to make sure the transistor is properly cooled so as to keep it operating within the designed range and from burning out.

Excerpt from 2N2222 Datasheet

References:

SparkFun According to Pete #37: Transistor Biasing Configurations Part 2 <-- Good source!!
https://www.youtube.com/watch?v=IkabRft5Sdk

NPN Transistor
http://www.electronics-tutorials.ws/transistor/tran_2.html