Transistor Biasing

The transistor was invented in 1948 by John Bardeen, Walter Brattain, and William Shockley at Bell Laboratory in America. The invention of the transistor completely revolutionized the electronic industry. Since then, there has been a rapidly expanding effort to utilize and develop many types of semiconductor devices such as FET, MOSFET, UTJ, SCR, etc. transistors have replaced bulky vacuum tubes in performing many jobs.

When a third doped element is added to the crystal diode in such a way that two P-N junctions are formed, the resulting device is known as a transistor. The transistor━ an entirely new type of electronic device is capable of achieving amplification of weak signals. As the transistor has two P-N junctions, one junction is forward biased and the other junction is reversed biased. The forward-biased junction has a low resistance path whereas a reverse-biased junction has a high resistance path. The weak signal is introduced in the low-resistance, circuit, and output is taken from the high-resistance circuit.

Transistors can operate in three regions namely cut-off, active, and saturation regions. To operate the transistor in the desired region we have to apply the external DC voltages of correct polarity and magnitude to the two junctions of the transistor.

What is Transistor Biasing ?

Transistor biasing is defined as the proper flow of zero signal collector current and the maintenance of proper collector emitter voltage during the passage of a signal. Transistors can operate in three regions namely cut off, active and saturation region. To operate the transistor in the desired region we have to apply the external dc voltages of correct polarity and magnitude to the two junctions of the transistor. The basic purpose of transistor biasing is to keep the base-emitter junction properly forward biased and collector-base junction reverse biased during the application of signal.

Need for Transistor Biasing

• For proper working it is essential to apply voltages of correct polarity across its two junctions.
• If it is not biased correctly, it would work inefficiently and produce distortion in the output signal.
• Operating point should bot be affected due to temperature changes and device variations.
• Further for various applications , transistor is biased as shown in table.

Region of operation	Base Emitter Junction	Collector Base Junction	Application
Cut off	Reverse biased	Reverse biased	As a switch
Active	Forward biased	Reverse biased	As an amplifier
Saturation	Forward biased	Forward biased	As a switch

Factors Affecting the Operating Point

The point obtained on the dc load line by the value I_c and V_CE when no signal is applied at the input is known as operating point.

The following are the factors that affect the stability of the operating point-

• Temperature dependence of I_C .
• Change of V_BE and β due to replacement of transistor.
• Thermal runway.

What is Stabilization?

The process of making operating point independent of temperature changes or variations in transistor parameters is known as stabilization. When the temperature changes or when the transistor is replaced by same type, the operating point (i.e., zero signal I_CQ and I_CEQ ) also changes. For faithful amplification, it is essential that operating point remains fixed. This necessitates to make the operating point independent of these variations. This is known as stabilization.

Need for Stabilization

The need for stabilization lies in various dependencies:

• Temperature Dependence of I_C : We know the collector current is given as -

I_C=βI_B+(1+β) I_CBO

where, β= Common emitter current gain

I_B = Base current

I_CBO= Leakage current.

The above three variables are strongly temperature dependent. Hence, with the increase in temperature, if any of these parameters changes, then collector current changes and so operating point is shifted to any other region.

• Variation in Parameters between Transistors of Same Type : Even after the tremendous advances in semiconductor technology, the transistor parameters such as β vary widely between transistors even of same type. Change in β will shift the operating point in any undesired region.
• Thermal Runway : The collector current is given as

I_C= βI_B + (1+β) I_CBO

If temperature changes then I_CBO changes which turn changes I_C and operating point. Flow of collector current in the collector circuit produces heat at the collector junction. This raises the temperature. Hence I_CBO increases which in turn increases collector junction and the whole process repeats again. Such successive increase in I_C,will drive the operating point into saturation region. This process is also called thermal runway. This is a very dangerous situation because due to excessive heat the transistor may burn out.

• Bias Stability : It is necessary that I_C should be kept constant in spite of variations of I_CBO or I_CO. the extent to which a biasing circuit is successful in maintaining this is measured by Bias Stability Factor.

By definition,

S=dI_C/dI_CO : at constant I_B and β

Hence we can understand that any change in collector leakage current changes the collector to a great extent. The stability factor should be as low as possible so that the collector current doesn't get effected.

• Negative Feedback: If the original input signal and feedback signal are out of phase then it is called negative feedback. It is used in amplifiers. We have seen that negative feedback reduces the gain of the amplifiers but still it is employed in almost all the amplifier circuits. It is because the negative feedback improves the overall performance and stability of an amplifier.

Types of Transistor Biasing

The following are the most commonly used types of transistor biasing are as below :

• Fixed bias circuit (Single base resistor biasing) or base bias.
• Collector to base bias circuit.
• Voltage divider bias circuit (V.D.B.) or self bias.
• Emitter bias or modified fixed bias circuit.

Fixed Bias Circuit

In this, a resistance R_Bis connected between supply V_CC and base terminal of the transistor. The required zero signal base current I_B is provided by V_CC and a single V_CC keeps the base emitter junction forward biased and the collector base junction reverse biased.

👁 Fixed Bias Circuit

Base Circuit (at the input end)

Apply K.V.L. to the base circuit-

V_CC-I_BR_B-V_BE=0

I_BR_B=V_CC-V_BE

I_B=(V_CC-V_BE)/R_B

I_B≅ V_CC/R_B (∵V_CC>>V_BE)

• For silicon transistors V_BE is 0.7 V and for germanium transistors V_BE is 0.3 V.
• In this circuit, the supply V_CC is of a fixed value. As the resistance R_B is selected, I_B is also fixed.

Hence this circuit is called as fixed bias circuit.

Collector Circuit (at the output end)

Now apply K.V.L. to the collector circuit.

V_CC-I_CR_C-V_CE=0

V_CE=V_CC-I_CR_C

I_CR_C=V_CC-V_CE

I_C=V_CC-V_CE/R_C

The collector current in CE configuration is given as,

I_C=βI_B+I_CEO

where I_CEO→ Leakage current

I_C=βI_B (∵βI_B>>I_CEO)

Advantages of Fixed Biased Circuit

• It is a simple circuit.
• The operating point can be fixed anywhere in the active region of the characteristics by simply changing the value of R_B.
• Biasing conditions are set easily.
• No loading effect as no resistor is present at base-emitter junction.
• Due to their simplicity fixed bias circuits are cost effective.

Disadvantages of Fixed Biased Circuit

• Poor thermal stability.
• Since I_C=βI_B is already fixed. I_C depends on β which is different for same type of transistor and shifts the operating point.
• Adjusting the operating point over a wide range may be challenging with fixed bias, limiting its suitability for certain applications.
• Potential for thermal runaway.
• Limited temperature compensation leads to changes in bias point.

Collector Biased Circuit

In this, one end of R_B resistor is connected to the base and the other end to the collector. here, the base current is not determined by V_CC.

👁 Collector Biased Circuit

Base Circuit: Apply K.V.L. to it:

V_CC=(I_C+I_B)R_C+I_BR_B+V_BE

V_CC=I_CR_C+I_BR_C+I_BR_B+V_BE

But I_C=βI_B,

∴ V_CC=βI_BR_C+I_BR_C+I_BR_B+V_BE

V_CC=I_B[(1+β)R_C+R_B]+V_BE

∴ I_B=(V_CC-V_BE)/{(1+β)R_C+R_B

Collector Circuit

Apply K.V.L. to this circuit,

V_CC=(I_C+I_B)R_C+V_CE

V_CC=I_CR_C+I_BR_C+V_CE

∴ V_CE=V_CC-(I_C+I_B)R_C

I_C=βI_B

∴ I_C=β[(V_CC-V_BE)/(1+β)R_C+R_B

Advantages of Collector to Base Bias Circuit

• It is a simple circuit.
• It provides some stabilization of the operating point.
• It needs only one resistor at base.
• Reduced sensitivity to transistor variations.
• Lower sensitivity to the power supply changes.

Disadvantages of Collector to Base Bias Circuit

• It does not provide good stabilization because stability factor is high.
• It provides a negative feedback which reduce the gain of the amplifier.
• Dependency on component matching.
• More complex in comparison to fixed bias circuits.
• Restricted utilization in certain application with specific voltage requirements.

Voltage Divider Bias Circuit

The voltage divider is formed by R₁ and R₂. The voltage drop across R₂ forward biases the base emitter junction. The R₁ and R₂ resistor act as a voltage divider giving a fixed voltage at point B which is base. This is the most widely used biasing method that provides biasing and stabilization to a transistor.

👁 Voltage Divider Bias Circuit

Base Circuit: Voltage across R₂ is the base voltage V_B.

Apply voltage divider to this circuit

∴ V_B=(V_CCR₂)/(R₁+R₂)

Collector Circuit: Voltage across R_E is V_E and can be obtained as

V_E=I_ER_E=V_B-V_BE

I_E=(V_B-V_BE)/(R_E)

Apply K.V.L.

V_CC=I_CR_C+V_CE+V_E

V_CE=V_CC-I_CR_C-I_ER_E

Advantages of Voltage Divider Base Circuit

• I_C in this circuit is almost independent of transistor parameters.
• The resistor employed in the emitter provides stabilization.
• It is the most prominent one.
• Flexibility in adjusting operating point.
• Moderate sensitivity to power supply changes.

Disadvantages of Voltage Divider Base Circuit

• The signals tend to get mixed up.
• It requires more components.
• Due to the increased complexity, it tend to be more expensive.
• It can lead to reduction in overall circuit efficiency due to the power dissipation.
• Dependency on component matching.

Modified Fixed Bias Circuit (Fixed Bias With Emitter Resistor)

In this, resistor R_E has been added from emitter to the ground terminal of the fixed bias circuit. This is done to improve the stability of the circuit.

👁 Modified Fixed Bias Circuit

Base Circuit

Apply K.V.L. to this circuit

V_CC=I_BR_B+V_BE+I_ER_E

But I_E=I_B+I_C

and I_C=βI_B

On substituting the value of I_C in I_E, we get

I_E=I_B+βI_B

Now substituting the value of I_E in V_CC

V_CC=I_BR_B+V_BE+(βI_B+I_B)R_E

=I_B(R_B+βR_E+R_E)+V_BE

∴ I_B=(V_CC-V_BE)/(R_B+βR_E+R_E)

Collector Circuit

Apply K.V.L. to this circuit,

V_CC=I_CR_C+V_CE+I_ER_E=I_CR_C+V_CE+(I_B+I_C)R_E

V_CE=V_CC-I_BR_E-I_C(R_C+R_E)

I_C=βI_B

∴ I_C=β[(V_CC-V_BE)/(RB+βR_E+R_E)]

Advantages of Modified Fixed Bias Circuit

• The addition of emitter resistor provides improved stability.
• It provides a high voltage gain.
• It is a simple circuit, with enhanced stabilization.
• Reduced sensitivity to transistor variations.
• It can strike a balance between simplicity and performance.

Disadvantages of Modified Fixed Bias Circuit

• Its operating point can be effected by change in temperature.
• It requires the use of dual polarity power supply.
• Limited voltage range.
• Potential for component matching.
• Complex circuit.

Solved Example on Transistor Biasing

1. For the fixed bias circuit determine I_B, I_C, V_CE, V_B, V_C and V_BC for the following parameters R_B = 240 KΩ , R_C= 2.2 KΩ , V_CC= 12 V and β=50.

As it is fixed bias circuit

We have I_B = (V_CC – V_BE)/ R_B

∴ I_B= (12-0.7)/ 240 KΩ = 47.08μA

∵I_C= β I_B

∴ I_C= 50 x 47.08μA = 2.35mA

∵V_CE = V_CC- I_CR_C

∴V_CE = 12 – 2.35mA x 2.2 KΩ= 6.83 V

∵Emitter terminal is grounded

∴ V_B= V_BE= 0.7V

V_C = V_CE= 6.83 V

∵V_BC= V_B- V_C

∴V_BC= 0.7V – 6.83V = 6.13 V

👁 Transistor Biasing

Biasing in Amplifier Circuits

In all amplifier circuit, the DC bias is provided by the voltage divider circuit of resistances R₁ and R₂ and emitter resistor R_E provides stabilization. the value of R₁ and R₂ are such that the current through them is 10 times the base current (I_B) and the value of R_E is in between 500 and 1000 Ω.

👁 Common Emitter Amplifier

Applications of Transistor Biasing

Type	Parameter Analysis	Application
Fixed Bias Circuit	β dependent, unstable Q point	Digital Switch
Collector To Base Bias	Greater stability	Amplifier
Voltage Divider Bias Circuit	β independent, stable Q point	Amplifier
Emitter Bias Circuit	Greater stability	Ic driver amplifier

Conclusion

Transistors are one of the most widely used semiconductor devices which are used for a wide variety of applications, including amplification and switching. However, to achieve these functions satisfactorily, a transistor must be supplied with a certain amount of current and/or voltage. Biasing is necessary to keep the transistor in its active region, where it can amplify signals without distortion. A Stability factor is a measure of the stability of a transistor amplifier circuit with respect to changes in transistor parameters like input and output current and voltages due to temperature, aging, or other factors.