Voltage Divider Bias Calculator for BJT Biasing

Voltage divider for BJT and diode bias is used to bias BJT (bipolar junction transistor) and diode. Enter the input voltage, and resistance to find the voltage across the base.

Circuit Diagram

voltage-divider-bias-calculatorWhat is biasing

The biasing is the application of dc voltage to transistors, diodes and other electronic components. In case of BJT the purpose is biasing is to establish the proper linear operating point where the transistor operates properly in a linear method.

The easiest method for biasing a BJT is to connect an external battery (voltage source) at its base which supplies the voltage to the base terminal. However, such an approach is not practical. Since we have to add an extra component. The problem can be solved by employing a separate voltage divider circuit which uses a combination of two resistors to step down the collector supply.

Stiff vs NonStiff divider

While analyzing above figure and equation one might notice that symbol ≅ is used instead of =. In the majority of cases, the approximately calculated value is quite enough. However one might obtain wrong readings when the loading effects are higher.

The knowledge of stiff divider and RIN(Base) helps us to understand the behavior of bias. RIN(Base) is the dc input resistance of the transistor which loads the resistor R2.

A stiff voltage divider the one where RIN(Base) can be ignored and we can use our previous equation. For a divider to be stiff its the internal resistance of transistor should be higher than 10 times the resistor R2. Mathematically, RIN(Base) ≥ 10 R2

A nonstiff divider is the one where RIN(Base) < 10 R2. In this case, the equation used should be:

equation for nonstiff divider

You can use the Loaded voltage divider calculator to solve an equation like this.

How to find RIN(Base)

RIN(Base) depends on the βDC, VB, IE.


RIN(Base) = [βDC * VB]/ IE.


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