THE TRANSISTOR AS AN AMPLIFIER AND AS A SWITCH
THE BJT TRANSISTOR AS AN AMPLIFIER
In electronics, the bipolar junction transistor (abbreviation commonly used: BJT , from the English bipolar junction transistor ) is a type of transistor widely used in the field of analog electronics mainly as a current amplifier and switch. It constitutes the most widespread family in electronics together with the field effect transistor with respect to which it is able to offer a greater output current with the disadvantage, however, of not having an isolated control terminal ( gate ). It is composed of three layers of doped semiconductor material, usually silicon, in which the central layer is doped oppositely to the other two, so as to form a double pn junction. Each layer is associated with a terminal: the central one is called the base , the outer ones are called collector and emitter .
PRINCIPLE OF OPERATION
The operating principle is based on the possibility of controlling the electrical conductivity of the device, and therefore the electrical current flowing through it, by applying a voltage between its terminals. This device involves both electrons and holes (majority charge carriers), and therefore this type of transistor is called bipolar . The bipolar junction transistor can classically be used in three different configurations known as common base , common collector or common emitter : these terms refer to the signal-free terminal (usually because it is connected to the reference potential, either directly or through a capacitor bypass).
JUNCTIONS OF A BJT TRANSISTOR
This transistor essentially consists of two anti-series pn junctions: the base terminal is the “connection between the heads”, while the ends are the two emitter and collector terminals: its graphic symbol precisely reflects this characteristic. Its operation is based on the distance between the two opposite junctions, which must be much smaller than the diffusion length of the majority charge carriers of the emitter and collector areas: this means that when the emitter-base junction goes into conduction, the carriers of charge from the emitter diffuse in the majority in the collector, remaining captured by the base-collector junction, instead of reaching the base terminal as they should (it can therefore also work by exchanging collector and emitter, albeit with much less effectiveness).
GAIN OF A BJT TRANSISTOR
The ratio β between the current that is captured by the collector and that which reaches the base is one of the fundamental parameters of any bipolar junction transistor; generally it is 50 to 800 or more, i.e. the current at the collector is 50 to 800 times greater than that at the base. The small symbol of the diode on the emitter terminal, which identifies the polarity of the transistor (pnp or npn) indicates how the internal junctions are oriented and therefore the type of majority charge carriers: pnp have collector and emitter of p type, therefore the the majority bearers are the gaps; npn have n-type, so they are electrons. In the design and construction of electronic circuits, pnp and npn transistors are almost completely identical in characteristics, but the bias voltages across them must be of opposite sign. Therefore, depending on the polarization, the transistor (apart from the configuration), has four regions of operation for increasing base voltages:
- Reverse active region
- Region of interdiction
- saturation region
- Direct active region
THE BJT TRANSISTOR USED IN SWITCH MODE
The transistor is often used as a switch (switching transistor) suitable for activating or deactivating circuits, transducers, etc. In all these applications, operation is linked to two particular states of the bipolar junction transistor: the saturation one (ON) and the interdiction one (OFF). In saturation it is essential that the two junctions are forward biased. For low values of VCE (voltage between collector and emitter) the base current IB loses control over the collector current IC and lacks proportionality IC = H FE * I B ( where H FE stands for DC current gain ). In interdiction the transistor does not conduct (OFF) and this condition occurs if both junctions are reverse biased.
VOLTAGE LEVELS VBE
An npn transistor can be considered disabled if the voltage VBE is less than or equal to zero, while a pnp transistor can be considered disabled if VBE is greater than or equal to zero. Considerable importance assumes the time taken by the device to pass from one state to another. In the ideal case, the transistor switches to the OFF and ON states and vice versa instantaneously. If this phenomenon were to occur we would not have heat losses from the bipolar junction transistor because it does not absorb power in the cut-off and saturated state. In fact, in the interdiction state almost no current passes through the transistor and in the saturation state it has almost zero voltage.
BJT TRANSISTORS AND DIGITAL ELECTRONICS
In digital electronics the transistor certainly plays a very important role. The figure above represents the symbol of a BJT transistor. To understand the inner workings of digital integrated circuits, it is essential to know how they work. I will tell you briefly about the transistor used as an amplifier, but what interests us is understanding how it works when the component acts in a switch configuration. When configured as an amplifier a certain base current IB controls the collector current IC, and therefore taking into account the resistance between collector and emitter, we can say that a certain base current IB controls the current flowing between collector and emitter .
AMPLIFICATION COEFFICIENT
Since the base current controls the collector current, there is a link between the two currents which takes the name of the amplification coefficient (indicated with the letter ß beta).
ß= IC / IB
For example, if a manufacturer sells us a transistor having a gain of 100 (ß) we will have that: assuming that the base current IB is 1mA:
IC = ß X IB = 100mA. As you can see, the current has been amplified 100 times on the collector.
Depending on the bias voltage, i.e. the voltage applied to the base, a transistor can operate in three characteristic areas:
- Interdiction maximum resistance between collector and emitter and therefore does not allow the passage of current between these two terminals ( open switch )
- Saturation minimum resistance between collector and emitter, passage of the maximum current between these two terminals ( switch closed ).
- Linear this area is used in the amplification of signals.
So the areas that interest us are Saturation and Interdiction which allow the transistor to be turned on or off in order to obtain two logical states.
FURTHER LINK WHEN THE TRANSISTOR IS USED AS AN AMPLIFIER
TRANSISTOR BJT AS A SWITCH
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