How to bias a Bipolar Junction Transistor using Voltage Divider Biasing Technique

Transistor biasing is required when we do not want varying output current and voltage due to changes in temperature or transistor changes. A transistor is usually biased when the transistor is used to operate as an amplifier. There are different transistor biasing method such as self bias, base bias, emitter bias, voltage divider bias and others. Here we show how to bias a Bipolar Junction Transistor using Voltage Divider Biasing Technique. 

For implementation and real time testing see How to Build BJT amplifier and test with Soundcard based PC Oscilloscope using VDB

When we want to use transistor to amplifier signal we need to bias the transistor. Biasing a transistor means to set up input and output current and voltage of the transistor to a pre-defined values. This is required so that the output current and voltage from the transistor does not vary with environmental changes such as temperature and changing the transistor. The current gain of transistor is what is important to stabilize because when temperature changes the current gain changes. Also different transistor of same model can have slightly different current gain(DC beta). For example different transistor of same transistor model 2N3904 can have different current gain. 

There are different types of biasing techniques but the most effective is the voltage divider biasing method. In this method, we use two resistor(R₁ and R₂) in resistor divider configuration to set fixed base current and voltage. This is why the biasing circuit is called voltage divider biasing circuit. In voltage divider biasing we also use resistor at the collector(R_C) and emitter(R_E). 

Following picture shows a voltage divider biasing circuit for Bipolar Junction Transistor(BJT).

 

To start biasing the transistor, we need to decide upon the output current and voltage. The output current is the collector current(I_C) and the output voltage is the collector voltage(V_C). The values for output current(I_C) and output voltage(V_C) depends upon the transistor capabilities. Because of this we need to refer to the transistor datasheet. For example we should not exceed the collector current and collector voltage value rated in datasheet when deciding upon the output current and voltage. Also we need to be within the power dissipation rating of the transistor which is the product of collector current and collector voltage.

Let assume that we are using 2N2222A transistor and that we want the output current(I_C) to be 5mA and output voltage(Vc) to be 3V. To get this output current and voltage regardless of changes in beta(temperature changes or transistor changes), we need to bias the transistor. To bias the transistor for this output current and voltage we need to use the resistor R₁, R₂, R_C and R_E to bias the circuit.

 To start with, we assume the emitter voltage(V_E) to be 1.2V which is around 1/4th of Vcc(5V). Also we assume that emitter current(I_E) is approximately equal to collector current(I_C). With these assumption we can calculate the emitter resistor(R_E) which is as follows,

\[R_{E}=\frac{V_{E}}{I_{E}}\]

with, V_E = 1.2V and I_E=5mA we get R_E=240Ω

 

Next we calculate the resistor values of R₁ and R₂. To obtain these resistor values we must first determine the base voltage(V_B). From the circuit we can see that the dc voltage at the base is given by,

\[V_{B}=V_{E}+V_{BE}\]

We have V_E=1.2V and V_BE=0.75V, substituting above, we get, V_B=1.95V

The resistor divider equation is given by,

\[V_{B}=\frac{R_2}{R_1+R_2} V_{CC}\]

Let use R₂=2.2KΩ and with V_B=1.95V and V_CC=5V. 

Rearranging the above equation,

\[R_{1}=\frac{V_{CC}}{V_{E}+V_{BE}}*R_{2}-R_{2}\] we get R₁=3.44KΩ

 The resistor Rc can be calculated using the following equation.

\[R_{C}=\frac{V_{CC}-V_{C}}{I_{C}}\] With  V_CC=5V, V_C=3V and I_E=5mA we get R_C=400Ω

 

To calculate the resistor values we can also use the online BJT biasing calculator.

 The following shows the current and voltages using the resistor values calculated above in Multisim electronics circuit simulation software.

In this electronics tutorial we thus showed how to calculate the resistor values to bias a BJT transistor with voltage divider biasing circuit. Here we showed only the DC bias of BJT using Voltage Divider Biasing. For Amplifier Design using BJT we need to also include coupling and by capacitor and include the effect of ac input source, load resistance to the amplifier and perform AC analysis of voltage divider biasing circuit. For a complete BJT amplifier design using VDB see the tutorial How to bias a BJT using voltage divider biasing. The BJT Amplifier Design with Voltage Divider Biasing Online Calculator can be used to calculate the resistor and capacitor values.

See previous tutorials how to attenuate Signal with Voltage Divider Circuit and How to remove DC bias from Signal.