Basics of BJT | Analog Circuits - Electronics and Communication Engineering (ECE) PDF Download

What is a Bipolar Junction Transistor (BJT)?

• A bipolar junction transistor is a three-terminal semiconductor device that consists of two p-n junctions which are able to amplify or magnify a signal. It is a current controlled device. 
• The three terminals of the BJT are the base, the collector, and the emitter.
• A signal of a small amplitude applied to the base is available in the amplified form at the collector of the transistor. This is the amplification provided by the BJT. Note that it does require an external source of DC power supply to carry out the amplification process.

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Construction of Bipolar Junction Transistor

• BJT is a semiconductor device that is constructed with 3 doped semiconductor Regions i.e. Base, Collector & Emitter separated by 2 P-N Junctions.
• Bipolar transistors are manufactured in two types, PNP and NPN, and are available as separate components, usually in large quantities. The prime use or function of this type of transistor is to amplify current. This makes them useful as switches or amplifiers. They have a wide application in electronic devices like mobile phones, televisions, radio transmitters, and industrial control.

Modes of Operation & Characteristics: 

1. Active mode: Emitter-base junctions are forward biased and the collector-base junction is reverse biased. The BJT amplifier operates in this mode.
2. Saturation mode: In this mode, both emitter-base and collector-base junctions are forward-biased. The closed switch operates in this mode.
3. Cutoff mode: Both emitter-base and collector-base junctions are reverse-biased. An open switch operates in this mode.
4. Inverted mode: This mode is the inverse of the active mode i.e., emitter base junction is reverse-biased, and collector base junction is forward-biased. In this mode transistor ceases its action of the transistor. Not used practically.

Operation of Bipolar Junction Transistor

• Active region: The region in which the transistors operate as an amplifier.
• Saturation region: The region in which the transistor is fully on and operates as a switch such that the collector current is equal to the saturation current.
• Cut-off region: The region in which the transistor is fully off and the collector current is equal to zero.

Types of Bipolar Junction Transistor

• PNP bipolar junction transistor
• NPN bipolar junction transistor

1. PNP BJT
In PNP BJT, the n-type semiconductor is sandwiched between the two p-type semiconductors. The two p-type semiconductors act as emitter and collector, respectively, while the n-type semiconductor acts as a base. This is shown in the figure below.

The current enters the transistor through the emitter such that the emitter-base junction is forward-biased and the collector-base junction is reverse-biased.

2. NPN BJT
In NPN BJT, p-type semiconductor is sandwiched between the two n-type semiconductors. The two n-type semiconductors act as emitter and collector respectively while the p-type semiconductor acts as a base. This is shown in the figure below.

Current entering the emitter, base, and collector has the sign convention of positive while the current that leaves the transistor has the sign convention of negative.

Function of Bipolar Junction Transistor

• BJTs are of two types namely NPN and PNP based on doping types of the three main terminals. An NPN transistor consists of two semiconductor junctions that have a thin p-doped anode region and PNP transistor also consists of two semiconductor junctions that have a thin n- doped cathode region.

• The flow of charge in a Bipolar transistor is due to the diffusion of charge carriers between the two regions belonging to different charge concentrations. Regions of BJT are known as the base, collector, and emitter.
• The emitter region is highly doped when compared to other layers. Both collector and base layers have the same charge carrier concentrations. Among these junctions, the base-emitter junction is forward biased, and the base-collector junction is reverse biased. Forward biased means p-doped region has more potential than the n-doped side.

Voltage, Charge Control and Current

• The base-emitter current is controlled by the collector-emitter current. This conclusion is drawn by the current-voltage relation of the base-emitter junction. Collector current has a base region where minority carriers are concentrated.
• Transistor models such as the Glenn poon model are responsible for the distribution of the charge which explains the behaviour of a transistor.

Configuration of Bipolar Junction Transistors

• The Common Emitter Configuration: It has both voltage and current gain.
• The Common Collector Configuration: It has no voltage gain but has a current gain.
• The Common base configuration: It has no current gain but has a voltage gain.

The Common Emitter (CE) Configuration

• In the Common Emitter or grounded emitter configuration, the input signal is applied between the base, while the output is taken from between the collector and the emitter as shown. 
• This type of configuration is the most commonly used circuit for transistor-based amplifiers and which represents the "normal" method of bipolar transistor connection. 
• The common emitter amplifier configuration produces the highest current and power gain of all the three bipolar transistor configurations. 
• This is mainly because the input impedance is LOW as it is connected to a forward-biased PN-junction, while the output impedance is HIGH as it is taken from a reverse-biased PN-junction.

• In this type of configuration, the current flowing out of the transistor must be equal to the currents flowing into the transistor as the emitter current is given as Ie = I_c + I_b. 
• Also, as the load resistance (R_L) is connected in series with the collector, the current gain of the common emitter transistor configuration is quite large as it is the ratio of I_c/I_b and is given the Greek symbol of Beta, (β). 
• As the emitter current for a common emitter configuration is defined as Ie = I_c + I_b, the ratio of I_c/I_e is called Alpha, given the Greek symbol of α. 
• Note that the value of Alpha will always be less than unity. 
• Since the electrical relationship between these three currents, I_b, I_c and I_e is determined by the physical construction of the transistor itself, any small change in the base current (I_b), will result in a much larger change in the collector current (I_c). 
• Then, small changes in current flowing in the base will thus control the current in the emitter-collector circuit. Typically, Beta has a value between 20 and 200 for most general purpose transistors.

By combining the expressions for both Alpha, α and Beta, β the mathematical relationship between these parameters and therefore the current gain of the transistor can be given as:

Where: "I_c" is the current flowing into the collector terminal, "I_b" is the current flowing into the base terminal and "I_e" is the current flowing out of the emitter terminal. 

• Then to summarise, this type of bipolar transistor configuration has a greater input impedance, current and power gain than that of the common base configuration but its voltage gain is much lower. 
• The common emitter configuration is an inverting amplifier circuit resulting in the output signal being 180 degree out-of-phase with the input voltage signal.

The Common Collector (CC) Configuration

• In the Common Collector or grounded collector configuration, the collector is now common through the supply. 
• The input signal is connected directly to the base, while the output is taken from the emitter load as shown. This type of configuration is commonly known as a Voltage Follower or Emitter Follower circuit. 
• The emitter follower configuration is very useful for impedance matching applications because of the very high input impedance, in the region of hundreds of thousands of Ohms while having a relatively low output impedance.

The common emitter configuration has a current gain approximately equal to the β value of the transistor itself. In the common collector configuration the load resistance is situated in series with the emitter so its current is equal to that of the emitter current. As the emitter current is the combination of the collector AND the base current combined, the load resistance in this type of transistor configuration also has both the collector current and the input current of the base flowing through it. Then the current gain of the circuit is given as:

The Common Collector Current Gain:

This type of bipolar transistor configuration is a non-inverting circuit in that the signal voltages of Vin and Vout are inphase. It has a voltage gain that is always less than "1" (unity). The load resistance of the common collector transistor receives both the base and collector currents giving a large current gain (as with the common emitter configuration) therefore, providing good current amplification with very little voltage gain.

The Common Base (CB) Configuration

• As its name suggests, in the Common Base or grounded base configuration, the BASE connection is common to both the input signal and the output signal with the input signal being applied between the base and the emitter terminals. 
• The corresponding output signal is taken from between the base and the collector terminals as shown with the base terminal grounded or connected to a fixed reference voltage point. 
• The input current flowing into the emitter is quite large as its the sum of both the base current and collector current respectively therefore, the collector current output is less than the emitter current input resulting in a current gain for this type of circuit of "1" (unity) or less, in other words the common base configuration "attenuates" the input signal

• This type of transistor arrangement is not very common due to its unusually high voltage gain characteristics. Its output characteristics represent that of a forward biased diode while the input characteristics represent that of an illuminated photo-diode. 
•  This type of amplifier configuration is a non-inverting voltage amplifier circuit, in that the signal voltages Vin and Voutare in-phase. 
• Also this type of bipolar transistor configuration has a high ratio of output to input resistance or more importantly "load" resistance (R_L) to "input" resistance (R_in) giving it a value of "Resistance Gain". Then the voltage gain (A_v for a common base configuration is therefore given as:

Common Base Voltage Gain:

Characteristics of different transistor configurations are given in the following table:

Applications of BJT

• BJT is used as a detector or also known as a demodulator.
• BJT finds application in clipping circuits so that the waves can be shaped.
• Logic circuits and switching circuits use BJT.