Transistors are electronic semiconductor devices that can be used to design electronically controlled switch or signal amplifiers. A transistor controls a large current flow in a circuit using a small current flow, just like a huge water pressure gushes out of a faucet of a tap by applying gentle force to the tap’s knob.
Applying small base current causes large collector current to flow to the emitter, and applying small gate voltage causes large source current to flow. Transistors are used in most electronic circuits, like switching circuits, amplifier circuits, oscillator circuits, current-source circuits, voltage-regulator circuits, power-supply circuits, digital logic ICs, and almost any circuit that uses small control current or voltage to control larger currents.
When a transistor is configured in a circuit to work as electronically controlled switch, the transistor can be either normally OFF or normally ON. Normally OFF means the transistor switch is OFF pending till when the right amount of current or voltage is sent to it to turn it ON.
Families of transistor
We have two major families of transistors
1. Bipolar junction transistors (BJT)
2. Field effect transistors (FET)
The major difference between the two families is that BJTs are current driven, while FETs are voltage driven. In the case of FET, the switching can occur with little or no current at all being delivered to the control lead of the transistor. Since little or no current is required to drive the FET, FETS have very high input impedance (~1014Ω). Because the FET’s control lead does not draw current from the circuit while in operation, it means that the circuit’s operation is not influenced by this control lead. Unlike the BJT. In the case of the BJT, the control lead which is the base draws a certain amount of current to carry out its operation, and this current must be accounted for in the circuit analysis.
Difference between BJT and FET
The difference between FET and BJT is tabulated below
|Driven by current||Driven by voltage|
|Draws reasonable amount of current||Draws little or no current|
|Not easy to manufacture||Easy to manufacture|
|Not so cheap to make||Cheaper to make|
|Used less in making IC’s||Used more in making IC’s|
|Can be made small||Can be made extremely small|
|High transconductance||Low transconductance|
|High voltage gain||Low voltage gain|
|Used more in amplifiers||Used more in switching|
Some transistors require a negative voltage and/or output current at their control lead (relative to one of their other two leads) to function, whereas others require a positive voltage and/or input current at their control lead.
Bipolar Junction Transistors
Bipolar junction transistor, A.K.A BJT, is a three terminal electronic component that can amplify signal or act as an electronic switch. The bipolar junction transistor comes in either NPN or PNP configurations.
The NPN type uses a little input current or positive voltage sent to the base of the transistor with respect to the emitter to drive a large current to flow from the collector to the emitter. At the other hand, a PNP type uses a little output or negative voltage sent to the base with respect to the emitter to drive large current to flow from the emitter to the collector.
The ability of bipolar junction transistors to control large current flow in a circuit using little base current make them very useful in the design of electrically controlled switching like amplifier circuits, current and voltage regulator circuits, oscillators, memory circuits.
How a Bipolar Junction transistor (BJT) Works
The semiconductor that forms a transistor is tweaked in such a way that supplying little current at one terminal of the transistor (BASE) causes large current to flow throw the other two terminals (COLLECTOR to EMITTER for NPN and EMITTER to COLLECTOR in PNP). Below is a depiction of what happens in an NPN transistor to bring about the transistor action that brings about switching. It should be noted that same thing happens for a PNP, only that the polarities and currents are reversed.
An NPN transistor is formed by sandwiching a p-type semiconductor in-between two n-type semiconductors. Remember that an n-type semiconductor has excess electrons and p-type has excess holes. In the NPN type, the two n-types are the collector and emitter, while the base is the p-type. Sandwiching the two creates a p-n junction. This junction prevents easy passage of electrons from the first n-type semiconductor (collector) to the second n-type (emitter). With this situation, we will have a condition where current is not flowing from the collector to the emitter.
This is the process of supplying the appropriate voltage to the base of a transistor causing it to go into operation. What biasing does is: it breaks down the depletion layer or region of the p-n junction thereby allowing electrons to move freely between the collector and emitter.
The method of biasing an NPN transistor is different from the method of biasing a PNP transistor.
To bias an NPN transistor, positive voltage is supplied to the base of the transistor. Normally, in an NPN transistor, the electrons on either of the collector or emitter will not be able to flow into each other because of the depletion layer, an external energy is required to create the push. This external energy comes in form of a positive voltage. For an NPN transistor, when a positive voltage is supplied to the base to bias it, it is called forward-bias and when a negative voltage is supplied to the base to bias it, it is called reverse-bias. Reverse bias increases the depletion layer by pushing the electrons in the n-type regions of the transistor semiconductor setup far away. On the other hand, forward bias causes the electrons in the n-type semiconductors to be drawn to the positive base where the positive biasing voltage is supplied. As the electrons are finding their way to the base, because the p-type depletion layer is thin, compared to the n-type regions, the depletion layer varnishes and at this point, the electrons from the collector eventually flow into the emitter.
Characteristic curve of a transistor
A transistor (BJT in this case) is a special electronic component, in that it enables you to control current flow at one point in the circuit by varying current flow at another point in the circuit.
The driving current is called the base current IB, the base current controls the flow of the collector current IC at a certain collector-emitter voltage VCE. Approximately, the collector current is equivalent to the emitter current.
IC ~ IE. In transistor operation, biasing is the process of sending the right amount of current to the base of the transistor to set it into action.
Transistor Behavior and operation
There are three important terms used to describe a transistor’s operation. Thy are shown on the transistor characteristic curve shown below:
- Cut off region
- Saturation region
- Active/Linear region
- Quiescent point, Q-point
- Cut-off region: This is the region where the biasing base current is very insufficient to cause collector current to flow, thereby resulting to an open circuit scenario between the collector and emitter. What this means is that, current will not flow from the collector to the emitter.
- Saturation region: This is the region where there is very enough biasing current at the base of the transistor causing maximum collector current to flow. When a transistor operates in this region, a closed circuit exists between the collector and emitter. This means that all the current that find their way at the collector of the transistor will flow down to the emitter.
- Active/Linear region: This region in the transistor operation is the region in-between the cut-off and saturation regions, where there is a linear relationship between base current IB, collector current IC and emitter current IE.
From the explanation of what the cut-off and saturation regions are and saying that active /linear region is the region in-between the two, means that biasing a transistor is simply a process of manipulating the resistance that exists at the junction between the collector and emitter of the transistor to create varying collector emitter-voltage VCE.
When the transistor is in the cut-off region, VCE is equal to VCC, but when the transistor is in the saturation region VCE is equal to zero. However, when the transistor is in the active /linear region, VCE is some value less than VCC and greater than zero. A transistor’s VCE value determines to a reasonable extent how well signals are amplified by the transistor.
4. Quiescent point: Any selected point along the linear/active region that does the desired signal amplification for any given transistor is called the quiescent point Q-point.
The transistor formula
The basic formula used to describe the behaviour of a bipolar junction transistor is IC = hFEIB,
Where hFE is the transistor Current gain called “hybrid parameter forward current gain, common emitter.” Often symbolized as β, hence, we have that IC = βIB
IC is the collector current, while IB is the base current. Every BJT has its unique hFE it’s a constant for the transistor but can vary in during transistor operation due to variation in temperature and collector-emitter voltage, the value this can be ranges from 50-500. So, if we have a transistor with hFE of 200 and a base current of 2mA flows into the base of a transistor to bias the transistor, the collector current will be equal to IC = 200 x 2 = 400mA.
I have made video tutorial on rules of electronic circuit design and current flow rules, you can watch the video below:
One of these rules is that, “current flows from a region of high potential to a region of lower potential,” hence, for collector current to flow from collector to emitter, collector voltage VE must be higher than emitter voltage VE by at least a few tenths of a volt, otherwise no collector current will flow no matter the base current value.
Again, for a transistor with 0.6V barrier voltage, the base voltage must be 0.6V greater than the emitter voltage for collector current to flow. Do not forget that there are limits to the collector current that can flow in a given transistor as well as voltages.
So, because both the collector and base currents flow into the emitter, we have that IE = IC + IB, however, because the base current is always very small, we can approximate IE ~IC.
Combining the equations IE = IC + IB and IC = hFE + IB, we get
IE = (hFE + 1) IB for the NPN.
NB: same thing can be done for the PNP.
From the equation above, we can see that the difference between IE and IC is the 1 term, however, when the value of hFE is large, the one can be negligible, which is always the case. Hence, we have IE~IC.
Again, we have that:
VBE = VB- VE = +0.6V for NPN and VBE = VB-VE = -0.6V for NPN
Calculating collector voltage
To calculate collector voltage VC, see image below:
we must put into consideration the resistor R in order to obtain the collector voltage.
NB: Care should be taking while using the formulas listed earlier, one should always check the operating characteristics of the transistor before applying the formulas because the formulas give idealistic results. when real results are expected, the transistor’s operating characteristics should be put into consideration.
The Concept of Transresistance rtr
This is the resistance that is inherent in the emitter junction region of the transistor. It is determined by temperature and emitter current; the formula is as shown below:
The transresistance of a transistor is somewhat below 1000Ω, hence, it can be neglected in some designs, however, in some cases, it is the transresistance that is the main player in the transistor operation, in such situations, a very reasonable attention is given to the transresistance.
Transistor parameter calculations
I hope you learnt a lot from this tutorial, in the next tutorial we will discuss application of transistors in electronic circuit design. in the meantime, check out these video tutorials: