How to Calculate Currents in Transistors

In order for transistors to operate correctly, the right biasing voltage must be applied at the correct points. This voltage that is required to jump-start a transistor varies on the type of transistor and the construction materials used. The function of the transistor, either as a switch or an amplifier, will also determine the amount of voltages required to deliver the expected results. Transistor configurations also play a part in determining the amount and direction of voltage required for normal transistor operation to take place.

Instructions

Switch Currents

1. Determine the maximum load current (Lc) of the device. This will determine if the transistor will be turned fully on, saturation state, and fully off, cut-off state.

2. Determine the HFE of the transistor. The HFE is the ratio of the current across the collector (Ic) to the current across the base (Ib), which is typically in the range of 50 to 100.

3. Calculate the minimum base current required to saturate the transistor by dividing the load current with transistor HFE as follows:

   Lc / HFE

4. Calculate the emitter current. This is done by taking the negative supply voltage (Vee) that forward biases the emitter junction through a resistor (Re) and the positive supply voltage (Vcc) that reverse-biases the collector junction and performing the calculation as follows:
   Vbe = Vb -- Ve
   Ie = (Vee -- Vbe) / Re

Amplifier Currents

1. Calculate the base voltage (Ib). This voltage is required to forward-bias the transistor to start conducting. To calculate the base voltage, you will need the base bias voltage (Vb), the base-emitter volt drop (Vbe, generally 0.7v for silicon devices) and the base input resistor (Rb) and arrange the formula as follows:

   Ib = Vb -- Vbe / Rb

2. Calculate the collector current (Ic, current entering the transistor). This is current flowing through the load resistor (RL) and corresponds to the supply voltage (Vcc) and their relationship can be given as:

   Ic = Vcc -- Vce / RL

3. Construct a Load line to determine the Q-point (suitable operating point) for the circuit. This can be calculated by determining the "Cut-off" and the "Saturation Point" of the transistor using the calculation below:

   When Vce = 0, Ic = Vcc -- 0 / RL, Ic = Vcc / RL

   When Ic = 0, 0 = Vcc -- Vce / RL, Vcc = Vce

4. Plot a graph of Ic Vs. Vce and draw aLoad line connecting the points where:

   Vce = 0, Ic = Vcc / RL
   Ic = 0, Vce = Vcc
   The center point becomes the Q-point where the circuit operates at maximum efficiency.

Tips & Warnings

• Ensure your bias voltage polarities are correct otherwise you run the risk of damaging the transistor permanently.

References

• transistor calculations
• transistors as switches

Resources

• transistor circuits