What is NPN Transistor? BJT Construction, Working & Applications

NPN Transistor – BJT Transistor Construction, Working & Applications as Inverter, Switching & Amplifier

When a third doped element is added to a diode in such a way that two PN-junctions are formed, the resulting device is called a transistor. Transistors are smaller than vacuum tubes, and were invented by J. Barden and W.H. Brattain of Bell Laboratories, USA.

What is NPN Transistor?

The construction of a NPN transistor is done as the name suggests by taking a P-type semiconductor material and sandwiching that in between two N-type semiconductor materials. In reality, the P-type layer is thin. This transistor has there terminals known as emitter, base and collector.

A BJT (Bipolar Junction Transistor) is device of two back to back connected PN junctions diodes having three terminals namely Emitter, Collector and Base. 

The name transistor is derived from the “Transfer of Resistance” i.e. it converts and transfers the internal resistance from low resistance of emitter – base to the high resistance of collector – base circuit.

The schematic diagram of a NPN transistor is shown in the above figure. The voltage between the baes and emitter terminals is called V_BE, and is more positive at the base than the emitter because for NPN transistor, base terminals should be higher in potential than the emitter.

Also, there is representation of the voltage between the collector and emitter, it is denoted by V_CE, and this voltage is positive with respect to emitter. In short, the NPN transistor conducts when the collector terminal is at a higher potential than both the base and the emitter.  Like a PNP transistor, this NPN transistor is also a current controlled device.

A transistor has three sections of doped semiconductors. With one side emitter and another is collector while the base lies in between. The middle portion is called the base. It forms the two aforementioned PN-junctions between the emitter and collector.

Emitter:

One section which supplies charge carriers is called the emitter. To supply a large number of charge carriers, the emitter is always in forward biased when compared to the base.

Base:

The middle section of the transistor, which forms the two PN-junctions between emitter and collector, is called the base. The base-emitter junction is forward biased which allows a low resistance for the emitter circuit. The base-collector junction is reverse biased which provides a high resistance in the collector circuit.

Collector:

The section other than emitter which collects the charges is called the collector. The collector is always reverse biased.

Identification of the transistor can be done using the resistance of the diodes present inherently inside the NPN transistor. Also we can take the knowledge we know of the diodes to analyse the internal diodes of the transistor.

Emitter-Base terminal: There is a diode in between the emitter-base terminals so those two terminals should function as a normal diode and conduct in only one direction.

Collector-Base terminal: There is a diode similar to the collector-base terminals. These terminals again should act as the terminals of a normal diode, and conduct only in one direction.

Emitter-Collector terminals: The Emitter-Collector terminals are not connected internally and hence will not conduct in either direction.

A table showing the resistance values across the different terminals of the transistor is given below.

Similar to the PNP transistor, the NPN transistor has three modes of operation, namely cutoff, active and saturation regions.

The following fig shows the symbol and pins of BC547 NPN Transistor

The following table shows the measuring points and result of a BC 547 NPN transistor while checking and measuring the NPN transistor values through DMM.

BC 547 NPN	Measuring Points	Result
	1-2	0.717 VDC
	1-2	OL
	1-3	OL
	1-3	OL
	2-3	OL
	2-3	0.711 VDC

Biasing & Working of NPN Transistor

The working a NPN transistor is more complex than that of the PNP transistor. The transistor is in OFF state if; the voltages at base and emitter are same. As the base voltage increases above the emitter voltage, the device shifts to its ON state.

In ON state, there is sufficient voltage difference with base terminal being the higher one, there is a flow of electrons generated from collector to the emitter which in turn causes a current to flow from emitter to collector. In this transistor, based on the working we can divide the three terminals it has into input and output ports. The input terminal is the base terminal, and the output terminal is the collector-emitter region.

The voltage at collector terminal should be greater than the emitter and must be positive with respect to the emitter voltage of the transistor, this ensures the current flow between emitter and the collector. There is also a positive potential drop across the base emitter junction, which is to no surprise the voltage drop across a silicon diode, i.e. 0.7V.

This voltage drop hast to be accounted for, hence the difference between base voltage and the collector-emitter region should be more than 0.7V. If this is considered, then the transistor will not operate.

To use the NPN transistor as a signal amplifier, it has to be operated in the active region. In a configuration where the emitter has common connection, the transistor total current flow is defined as the ratio of the collector current to the base current, and this ratio is called DC current gain.

It doesn’t have any units as it is the ratio of two current values. This ratio is given a symbol and is represented by β. The maximum value of the DC current gain is generally near 200. There is another quantity which is defined by the ratio of collector current to the emitter current. This ratio is given a symbol and is represented by α. This value is equal to unity generally because the collector current is almost equal to the emitter current.

Transistor Current

The following equation shows the relation between emitter current, base current and collector current in a biased transistor.

I_E = I_B + I_C

Where:

• I_E = Emitter Current
• I_B = Base Current
• I_C = Collector Current

It shows the the emitter current is equal to the sum of base current and collector current. The value of base to emitter current is 2 – 5% while the collector current is almost 95 – 98%. That’s why both the base and collector current is equal to the emitter current.

The transistor gain equation can be written as follow as well by using Kirchhoff’s current law.

• I_E + (- I_B) + (-I_C) = 0

or

• I_E – I_B – I_C = 0

or

• I_B = I_E – I_C

or

• I_C = I_E – I_B

or

• I_E = I_B + I_C

Transistor Gain, Current Gain, Voltage Gain & Power Gain (α, β and γ)

• Transistor Gain

Transistor Gain is known as the ration between circuit output and input.

Transistor Gain = Output / Input

• Current Gain:

The ratio between collector current and emitter current is known as current gain of a transistor represendt by the greek symbol alpha “α” or h_FE

Current Gain; α_DC = A_i = -I_C / I_E = I_OUT / I_IN

or

α_DC = I_C / I_E = Collector Current / Emitter Current

The DC written with alpha “α” symbol is for DC values. The more the value of α, the better of a transistor will be as it shows the quality of the transistor.

α_DC can be written as simple “α” and it is known as forward current transfer ratio or amplification factor which is also represented by h_FB. In h_FB, the “F” represents the “Forward” and “B” represents the “Common Base” where the the alpha (α) is generally derived from transistor’s common base circuits.

The AC alpha (α_AC) of a transistor:

α_AC = ΔI_C / ΔI_E = Change in Collector Current / Change in Emitter Current

α_AC is also known as the short circuit gain of a transistor which can be represented by h_fb.

Good to know:

• h_FB = DC alpha α_DC
• h_fb = AC alpha α_AC

Also, the ratio between DC collector current and DC base current is known as current gain which is represented by the Greek symbol Beta “β“.

β_DC = -I_C / -I_B = I_C / I_B 

or

I_C = βI_B

It is also known as common emitter DC forward transfer ratio represented by H_FE.

We use AC beta “β_AC” when analyzing a transistor for AC operations

β_AC = ΔI_C / ΔI_B

β_AC can be also represented by h_fe.

Lastly, the ratio between emitter current and base current in common collector is also known as current gain and represented by the Greek symbol Gamma “γ”

γ = I_E / I_B

or

putting the value of I_E in the above equation from “I_E = I_C + I_B“

γ = β +1

• Voltage Gain

The ratio between input and output voltage is known as the voltage gain of a transistor.

Voltage Gain = A_V = α I_E R_CB / I_E R_EB

Voltage Gain = A_V = Voltage across R_CB / Voltage across R_EB

or

A_V = α x (R_CB / R_EB)

or

A_V = V_OUT / V_IN

• Power Gain

Transistor Power gain can be calculated by the following equation.

Power Gain = A_P = P_OUT / P/_IN

A_P = α² x A_R

Where:

• A_P = Power Gain
• α = Current Gain
• A_R = Resistance Gain

The overall expressions for relation between alpha, beta and gamma (α β & γ) are given below:

• α = β / ( β + 1 )
• β = α / (1-α)
• γ = β +1

Characteristics Curve & Operation Regions of NPN Transistor

There are mainly four regions of operations of a BJT transistor namely:

• Active Region
• Cutoff Region
• Saturation Region
• Breakdown region

Active Region: It is known as the normal operation of a transistor. Or the region between saturation and breakdown region is known active region.

Cutoff Region: A region where the value of  base current I_B becomes zero and make the first (or lower) curve is known the cutoff region of a transistor. In this region, both the emitter-base diode and collector-base diodes become and operate in reverse bias.

Saturation Region: In this region, both the emitter-base diode and collector-base diodes become and operate in forward bias. It is the initial slop (or almost perpendicular) area near to the origin (in the curves) when the initial voltage increases from zero to 1 and so on.

Breakdown Region:

When the collector voltage increases too much crossing the rated value, it leads to breakdown the collector diode. For this reason, a transistor should not be operated in breakdown region as it will damage and destroy the transistor circuit.

The graph between the collector current and the collector-emitter voltage with varying base current is called the output characteristics curve of bipolar transistor. The figure below show the output characteristics curves of a NPN transistor.

The transistor is ON when there is a small current through the base terminal and small positive voltage relative to the emitter terminal. Otherwise the transistor is OFF. This is reflected in the graph as well. The collector current depends on the collector voltage only until the collector voltage reaches 1V level.

There is also a straight line joining points A and B. This straight line is called ‘Dynamic Load Line’. This line connects the points where V_CE = 0 and Ic = 0. The straight line and the region around it is the active region of the transistor.

Given the base current and the collector voltage, the common emitter configuration characteristics are used to calculate the collector current.

Common Emitter Configuration of NPN Transistor

There are three possible configuration for the transistor, and one in the discussion is the common emitter configuration. The circuits that use common emitter configuration are generally used in voltage amplifiers. As the name suggests, we take one of the three terminals, in this case the emitter as common. This common terminal will act as both input and output to the transistor.

The voltage amplification which is achieved using the common emitter configuration can be done only in one step. So these circuits are also called single stage common emitter amplifier circuit. The input terminals as we discussed earlier is the base terminal, collector as output terminal.

The emitter remains as common terminal. The amplification process starts with biasing the base-emitter junction forward. This means that there is a greater positive potential at the base terminal than at emitter terminal. This process will enable us to control the current flow in the transistor.

As the output required has to have amplification, we use the common emitter amplifier which has a very high gain even though the output is inverted. Due to the dependencies of the diode characteristics on the ambient conditions, the gain is very much influenced by the surrounding temperature and the bias current.

This is the most commonly used configuration of the NPN transistor because it has a very low output impedance and it also provides a high input impedance. This configuration also has a very high power and voltage gain.

The typical value of the current gain for this configuration is around 50. This configuration is generally used where there is a low frequency amplification to be done. The radio frequency circuits also use this configuration. The common emitter amplifier configuration is shown below.

NPN Transistor in Amplification Circuits

Like in the push pull transistor circuit. Although all the configurations of the push-pull amplifier can technically be called push-pull amplifier, only the Class B amplifier is the actual push-pull amplifier. In contrast to Class A amplifier, Class B amplifier has two transistors for the push-pull electrical action of which one is NPN and the other is PNP.

Each transistor will work for one half of the cycle of the input producing the necessary output. This improves the efficiency of the Class B amplifier many times higher than the Class A amplifier. The conduction angle for this amplifier is 180 degrees, because each transistor works for one half only.

NPN Transistor as a Switch & Inverter

in digital logic circuit, a BJT transistor works in saturation and cut-off region as bias voltage is not provided to the base of the transistor in logic circuits. This way, the low or high output voltage are gain which can be used for switching purposes in those digital logic circuits.

The following circuit shows an inverter NPN circuit used for switching purpose.

It clearly shows that there is no input biasing voltage at the base but a square wave form as input provided through a resistor connected in series to the base of the the NPN transistor which working as an inverter.

The circuit shows that both the V_CC and input high level value si +5V where the voltage between collector and emitter V_CE is output voltage.

When the input voltage is high i.e. +5V:

• The base – emitter junction is forward bias.
• Current flows to the base through the series resistor R_B.
• The value of R_B and R_C provide I_B current which situates the circuit i.e transistor operates in situation region.

In other words, when the input to the inverter is high “+5V”, the transistor saturated and its output is low “≈0V”. When the input to the inverter is low, the transistor is cut-off and its output us high. In short,

• In saturate region, it is “ON”
• In cut-off region, it is “OFF”.

As shown by the circuit input and output i.e when the input is low, the output is high, that’s why it is also known as BJT inverter circuit.

The above explanation of ON and OFF operation of transistor inverter circuit is similar to the close and open switch connected between collector and emitter. That’s the reason a transistor inverter circuit is also known as a transistor switch.

When the transistor is in saturation mode, the voltage between collector and emitter is zero same like the voltage across the closed or ON switch and the amount of current is maximum.

Similarly, In cut-off region (when transistor is Open of OFF), the value of flowing current from collector to the emitter becomes zero same like an open or OFF switch and the voltage across the switch is maximum.

The ON-OFF operation of the circuit depends on the values of input voltages i.e. when

• Input Voltage is High = +5V = Switch is ON
• Input Voltage is Low = 0V = Switch is OFF

The following fig shows the switching operation in cutoff and saturation regions of BJT (NPN transistor).

NPN Transistor Applications

• Mostly used in switching purposes.
• They are used in Darlington pair configuration to amplify weak signals.
• In circuits where we need to sink the current.
• They are also used in circuits for temperature sensing.
• They can perform well in very high frequency applications.
• Used in logarithmic converters.

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