NPN Transistor

In the previous course, we found that the standard Bipolar transistor, or BJT, is in two basic forms. An NPN (Negative-Positive-Negative) type and a PNP (Positive-Negative-Positive) type.

The most commonly used transistor configuration is the NPN Transistor. We also learned that the connections of the bipolar transistor can be biased in one of three different ways: the common base, the common emitter and the common collector.

In this tutorial on bipolar transistors, we will take a closer look at the "common emitter" configuration using the Bipolar NPN Transistor. Below is an example of the construction of an NPN transistor along with the current flow characteristics of transistors.

Bipolar NPN Transistor Configuration

NPN Transistor
Bipolar NPN Transistor Configuration

(Note: Describes emitter and conventional current flow "out" for bipolar NPN transistor.)

The structure and terminal voltages for a bipolar NPN transistor are shown above. The voltage between the base and emitter (VBE) is positive in the base and negative in emitter. Because the base terminal for an NPN transistor is always positive compared to the Emitter. In addition, the collector supply voltage is positive according to emitter (VCE). Therefore, the transmission of the Collector by a Bipolar NPN transistor is always more positive than both base and emitter.

NPN Transistor

Voltage sources are connected to an NPN transistor as shown. The collector is connected to the VCC supply voltage via RL, a moving load resistance to limit the maximum current flowing from the device. Base supply voltage VB is connected to the reused base resistance RB to limit maximum base current.

Therefore, in an NPN Transistor, it is the movement of negative current carriers (electrons) along the base area that constitutes the transistor effect. Because these mobile electrons provide the connection between the collector and emitter circuits. This connection between the input and output circuits is the main feature of the transistor action. Because the amplifier properties of transistors come from the result control that Base applies on the collector-to-transmitter current.

Next, we can see that the transistor is a current-powered device (Beta model), and when the transistor is "completely on", a large current (Ic) flows freely from the device between the collector and emitter terminals. However, this only happens when a small biased current (Ib) flows into the base terminal of the transistor at the same time, thereby allowing the base to act as a kind of current control input.

The current in the bipolar NPN transistor is called the DC current gain of the device. Hfe or beta (β) symbol is the ratio of these two currents (Ic/Ib). The Β value can be up to 200 for standard transistors.

Relationship between α and β in the NPN Transistor

NPN Transistor
Relationship between α and β in the NPN Transistor

By combining two parameters β Α and Α, we can produce two mathematical expressions that give the relationship between the different currents flowing in the transistor.

NPN Transistor

Beta values range from about 20 for high-current power transistors and over 1,000 for high-frequency low-power bipolar transistors. For most standard NPN transistors, the Beta value can be found on manufacturer datasheets. However, it usually ranges from 50 to 200.

The collector current resulting from zero base current ( Ib = 0 ) will be zero in Ic (β*0). In addition, when the base current is high, the corresponding collector current will also be high, which will cause the base current that controls the collector current. One of the most important features of the bipolar junction transistor is that a small base current can control a much larger collector stream. Consider the following example.

NPN Transistor Example

A bipolar NPN transistor has a value of 200 DC current gain (Beta). Calculate the base current Ib required to replace a 4ma-resistant load. Calculate the base current Ib required to replace a resistant load of 4mA.

NPN Transistor

Therefore, the β = 200, Ic = 4mA and Ib = 20μA.

The collector voltage (Vc) must be larger and more positive than the emitter voltage (And) to allow the current to flow through the transistor between the collector-emitter connections. In addition, the input characteristics of an NPN Transistor are approximately 0.7 V (a diode volt drop) voltage drop between the base and emitter terminal for silicon devices, as it is an advanced biased diode.

The base voltage (Vbe) of an NPN transistor must be greater than this 0.7 v. Otherwise, the transistor is not transmitted by the given base current.

NPN Transistor

Here Ib is the basic current. Basic biased voltage etc. Vbe is the basic emitter voltage drop (0.7 v) and the Rb is the basic input resistance. As the Ib increases, Vbe gradually rises to 0.7 V, but Ic rises exponentially.

Common Emitter Configuration

Bipolar NPN Transistors are used as a semiconductor switch to "turn on" or "turn off" load currents by controlling the basic signal to the Transistor in saturation or cutting zones. In addition, it can also be used to produce a circuit in its active zone that will strengthen any small AC signal applied to the base terminal with emitter grounded.

First, if a dc "biased" voltage suitable for the base terminal of transistors is applied, if it is always allowed to operate in the linear active zone, an inverted amplifier circuit called a single-stage common emitter amplifier is produced. Such a common emitter amplifier configuration of an NPN transistor is called a Class A amplifier.

As a result, the transistor always works 50/50 between the cutting and saturation zones, allowing the transistor amplifier to accurately reproduce the positive and negative half of any AC input signal that is superimposed on this DC bias voltage.

Without this "Bias Voltage", only half of the input waveform will be strengthened. This common emitter amplifier configuration using an NPN transistor has many applications. However, it is widely used in audio circuits such as the front amplifier and power amplifier stages.

Referring to the common emitter configuration shown below, a family of curves known as output properties curves associate the output collector current with the emitter voltage (Ic) and (VCE) with different base current values (Ib). The curves of the output characteristics are applied to the transistor for transistors with the same β value.

A DC "load line" can also be drawn on the output characteristics curves to indicate all possible working points when different base current values are applied. It is necessary to correctly adjust the starting value of the Vce to allow the output voltage to change both up and down when upgrading AC input signals.

Single Stage Joint Emitter Amplifier Circuit

NPN Transistor
Single Stage Common Emitter Amplifier Circuit

Output Properties Curves of a Typical Bipolar Transistor

NPN Transistor

The most important factor to consider is the effect of Vce on collector current Ic when Vce is larger than about 1.0 volts. We can see that IC is not greatly affected by changes above this value in Vce and is instead almost entirely controlled by the basic current Ib. When this happens, we can say that the output circuit represents a "constant current source".

From the common emitter circuit above, it can also be seen that the emitter current is the sum of Ie, collector current, Ic and base current. IB is added together, so we can also say that it is Ie = Ic + Ib for the common emitter (CE) configuration.

NPN Transistor