How to Calculate Voltages in Transistors

How to Calculate Voltages in Transistors
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In order for transistors to operate correctly, the right biasing voltage and current must be applied at the correct points. This biasing voltage varies depending on the type of transistor and the construction materials used. The function of the transistor, either as an amplifier or as a switch, will also determine the amount of voltages required to deliver the expected results. The many transistor configurations used, either to act as switches or amplifiers, also play a part in determining the amount and direction of voltage required for normal transistor operation to take place.

Feedback and Bias

    Determine the base bias voltages by measuring the voltage difference between the two ends of the base resistor (Rb). This should be equal to the supply voltage (Vcc).

    Determine the voltage drop between the collector and emitter junctions (Vce) of the transistor using the formula Vce = Vcc - IcRc, where "Vce" is the collector emitter voltage; "Vcc" is the supply voltage; and "IcRc" is the voltage drop across the base resistor (Rb).

    Determine the Vcc in a feedback-biased circuit. This can be done using the formula: Vcc = Vrc + Vrb + Vbe + (Ic + Ib)Rc + IbRb + Vbe, where "Vrc" is the voltage across the collector resistor; "Vrb" is the voltage across the base resistor (connected across the base) and the junction between the collector resistor and the transistor collector; and "Vbe" is the voltage across the transistor base and emitter.

Switching Voltages

    Determine the cut off and saturation voltages. The saturation voltage corresponds to the maximum voltage passing the transistor while the cut off voltage is zero, as the following calculation for saturation shows: Vbb > IcRb / (Ic / Ib) + 0.7v

    Determine the cut off voltage. The base current must be zero, and hence the collector current must be zero to make this statement true: Vce = Vcc.

    Plot a Load line graph, with "Ic" against "Vce," to determine the optimum operating voltage using the values:

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

    The middle point determines the optimum voltage for the transistor operation.


    • The "Vce" will determine the power rating of a transistor. This is displayed on the casing. Use Ohm’s Law to determine simple voltage differences, such as across a collector resistor, using the formula V = IR.


    • Always use base and collector resistors to avoid damaging the transistor.

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