Explained: Bipolar Junction Transistors (BJTs) and Configurations

If we look at the diodes, their internal construction look fairly straightforward. They are internally built using two sections of semiconductor material, forming what is called a PN-junction.

But when we talk about a bipolar junction transistor or a BJT, things get a little more advanced. It involves adding an extra layer of semiconductor material which gives it unique properties, particularly its ability to amplify signals.

Now if we connect two individual signal diodes in reverse to one another, what we get is two PN-junctions in series, which shares a common positive or negative terminal.

This combination forms a device with three layers, two junctions, and three terminals. This becomes the main foundation of what we call a Bipolar Junction Transistor, or BJT for short.

Transistors are actually highly flexible components. They have three terminals and are made from different types of semiconductor materials.

If we subject them to a small voltage we can make them act like either a conductor or an insulator. This ability to shift between these states makes them incredibly useful.

Transistors perform two main roles: they can either function as switches which is critical for digital circuits, or they can amplify signals which is important in analog circuits.

Bipolar transistors in particular, operate in three specific modes:

• Active Mode: In this mode, the transistor functions like an amplifier where the current flowing through the collector (Ic) is directly proportional to the base current (Ib) multiplied by the transistors current gain (β) that is Ic = β*Ib.
• Saturation Mode: Here the transistor is fully switched on, functioning like a switch and the current through the collector reaches its maximum or saturation point.
• Cut-off Mode: In this state the transistor is fully turned off, also behaving as a switch but here no current flows through the collector.

The term “Transistor” comes from the blending of two words, which are “Transfer” and “Varistor.” This name indicates how these little devices worked in the early days of electronics.

Basically, the transistors help to control the electrical signals in any circuit.

When we talk about the basic types of bipolar transistors, we are mainly looking at two configurations, which are PNP and NPN. These names indicates how the P-type and N-type semiconductor materials are arranged in the device.

Now, let us break down what a bipolar transistor actually looks like.

It has a structure made up of two PN junctions which terminate into three pinouts.

We call these terminals as the Emitter (E), Base (B), and Collector (C). Each terminal has a specific role and is named to help us distinguish it from the others.

• Emitter (E): This part is where the majority of charge carriers come from. It pushes charges towards the base.
• Base (B): This is a thin layer that acts as a gate for controlling the flow of current. It connects the emitter to the collector.
• Collector (C): This terminal collects the charge carriers that pass through from the emitter.

We can compare the BJTs to be like little traffic controllers for electrical current. They manage how much current moves from the Emitter to the Collector based on the voltage we apply to the Base terminal.

So in a way, we can think of them as switches that we control with a smaller current at the base, which then influences a much larger current to flow through the collector.

Heres how it works. when we send a tiny amount of current into the base terminal, it allows a much bigger current to flow from the emitter to the collector.

This is what makes transistors so useful, because they can amplify signals and control power in circuits.

Now whether we’ are dealing with a PNP or an NPN transistor both types operate on the same basic principles.

The main difference between them lies in how we apply voltage and the direction of the power supply for each type.

So while they might look different on paper, they practically do the same job, just through a few tweaks in how we set them up.

Internal Construction of Bipolar Junction Transistors

P-N Junction Internal Structure of BJT

The Diode Analogy of BJT

BJT Symbols

The way we represent PNP and NPN bipolar transistors is pretty straightforward. In the bipolar transistor circuit symbols, you will always find an arrow that shows us how current flows in the circuit, specifically from the base terminal to the emitter.

This arrow points from the positive side (the P-type material) to the negative side (the N-type material), just like it does in a regular diode symbol.

How We Connect Bipolar Transistors

Since a bipolar transistor has three terminals, we have a few different methods to connect it in an electronic circuit. Each setup has one terminal that serves as a common point for both the input and output signals.

Depending on how we connect it, the transistor will react differently to the input signal because its characteristics change based on the arrangement.

Here are the three main ways we can connect them:

1. Common Base Configuration: In this setup we get a voltage gain, but we do not see any current gain. It is like having a strong signal but not being able to push much current through.
2. Common Emitter Configuration: This is where things get interesting! In this arrangement, we get both current gain and voltage gain. It’s like having a powerful amplifier that boosts both the strength of the signal and the amount of current.
3. Common Collector Configuration: Here we have current gain without any voltage gain. It is great for boosting current but does not help with increasing voltage.

So depending on how we decide to connect our transistors in a circuit, we can achieve different results based on what we need for our project!

Understanding the BJT Common Base (CB) Configuration

As indicated by the name itself, in the Common Base (CB) setup, the Base terminal becomes the shared point for both the input and output signals.

So, when we connect things up, we apply the input signal between the base and the emitter terminals.

Then we take the output signal from between the base and the collector terminals. In this arrangement we usually ground the base or connect it to a fixed voltage reference.

Now let us talk about what happens with current in this configuration. The current that flows into the emitter is pretty substantial because it includes both the base current and the collector current.

However when we look at the collector current that comes out, it is actually less than what goes into the emitter.

This means that for a common base setup, we end up with a current gain of “1” or even less.

In simpler terms, this configuration tends to reduce or “attenuate” the input signal rather than to amplify it.

So even though using a common base setup can be useful in certain situations, it does not boost our current like some other configurations do!

Analyzing The Common Base BJT Circuit

This configuration is known as a non-inverting voltage amplifier, which means that the input voltage (Vin) and the output voltage (Vout) move together in sync, or they are “in-phase.” However we do not see this type of BJT confiiguration very often because it has some pretty unique characteristics, especially when it comes to its high voltage gain.

When we look at the input side, it behaves a lot like a forward-biased diode, which means that it allows current to flow easily. On the output side, it functions jut like a lit photo-diode which is sensitive to light.

Another cool thing about this configuration is that it has a high ratio of output resistance to input resistance. This is important because it gives us what we call “Resistance Gain.”

So we can say that the load resistance RL is much higher compared to the input resistance Rin, which is a important feature of this setup.

Now, when we calculate the voltage gain Av for this common base configuration, we can see how effectively it amplifies the signal. Overall, eventhough this type of amplifier circuit is not super common, it definitely has its own special traits that can be useful in certain situations!

Formula for Calculating BJT Common Base Voltage Gain

Av = Vout/Vin = (I_C * R_L)/(I_E * R_IN)

In the above formula I_C/I_E refers to the current gain, also indicated by the symbol alpha α, and R_L/R_IN refers to the resistance gain of the BJT.

Therefore, when we talk about the common base circuit, we usually see it being used in simple amplifier setups, for example like those for microphones or radio frequency (Rf) stuff. This is mainly because it does a really great job at handling high frequencies.

Understanding The Common Emitter (CE) BJT Configuration

Now let us look into the Common Emitter (CE) configuration. In this setup, we put the input signal between the base and the emitter and then we derive the output from between the collector and the emitter.

This is actually the most popular way to connect transistors for amplifiers and it is pretty much what we think of as the standard method for bipolar transistors.

What is cool about the common emitter amplifier configuration is that it gives us the best current and power gain compared to the other two configurations we have with bipolar junction transistors.

The reason for this is that it has a LOW input impedance because it connects to a forward biased PN-junction.

On the flip side, it has a HIGH output impedance since that comes from a reverse biased PN-junction. So in simple terms, this setup really maximizes the gain of the amplifier design!

Analyzing The BJT Common Emitter Amplifier Circuit

In a BJT common emitter amplifier circuit, we have to keep in mind that the current that is flowing out of the transistor has to be perfectly equal to the currents that are entering into the transistor.

We can think of it this way: the emitter current, which we call Ie, is actually made up of two parts, and we can write it down as Ie = Ic + Ib.

Now when we connect a load resistance which we refer to as R_L in series with the collector, what happens is that the current gain for this common emitter transistor setup turns out to be pretty substantial.

This gain is basically the ratio of Ic (the collector current) to Ib (the base current) and we usually represent this current gain with a Greek letter called Beta, which looks like this: β.

So since we have already established that the emitter current for our common emitter configuration is expressed as Ie = Ic + Ib, we can also define another ratio called Alpha, represented by another Greek letter α.

This ratio is actually Ic/Ie. It is really important to note that Alpha will always be less than one, like it will never hit that magical number one.

Now here is where it gets interesting: the relationship between these three currents, Ib, Ic, and Ie, is all tied to how the transistor is physically built.

This means that if we make even a tiny little change in the base current (Ib) of the BJT, we will see a much bigger change in the collector current (Ic).

So basically, when we tweak the current flowing into the base just a little bit, it ends up controlling how much current flows through the emitter-collector circuit.

Typically speaking, for most general-purpose transistors out there, Beta usually hangs out somewhere between 20 and 200.

So let us say we have a transistor with a Beta value of around 100, what this tells us is that for every single electron that flows from the base terminal, there are about 100 electrons flowing between the emitter and collector terminals.

Finally, if we decide to combine our expressions for both Alpha (α) and Beta (β), we can come up with a mathematical relationship that ties these parameters together and gives us an idea of the current gain of our transistor, as depicted below:

Alpha, (α) = I_C/I_E and Beta, (β) = I_C/I_B

∴ I_C = α * I_E = β * I_B

Since we have, α = β/(β + 1), and β = α/(1 – α)

∴ I_E = I_C + I_B

In the above formula, the Ic indicates the current flowing into the collector terminal, the Ib represents the current flowing into the base terminal, and the Ie denotes the current flowing out of the emitter terminal.

So, when we are talking about this particular type of bipolar transistor configuration, we can say that it has a very impressive input impedance, and along with this, it also has a high current and power gain compared to what we see in the common base configuration. But one bad thing is that its voltage gain is actually quite a bit lower.

Another point is that, the common emitter configuration acts like an inverting amplifier circuit.

What this means is that the output signal that we get will have a phase shift of 180 degrees compared to the input voltage signal. Meaning if we supply an input signal to this configuration, the output will get flipped upside down or be inverted!

Understanding The BJT Common Collector (CC) Configuration

In this configuration, which we can also call as the grounded collector configuration, we connect the collector pin of the BJT with the ground supply line of the power supply. This means that the collector terminal ends up being common for both the input signals and the output signals, which looks great.

Now when we look at the connection details we find that the input signal goes straight to the base terminal.

Next, we take the output signal from across the emitter load resistor, just like it is shown in our diagrams.

This particular type of configuration is very often referred to as a Voltage Follower or Emitter Follower circuit.

One of the coolest things about this common collector or emitter follower setup is how incredibly useful it is for impedance matching applications.

It boasts a super high input impedance which can be in the range of hundreds of thousands of Ohms! At the same time, it keeps a relatively low output impedance.

So in a nutshell, a common collector BJT circuit becomes a fantastic choice when we are trying to match impedances in our electronic circuits!

Analyzing a Common Collector BJT Circuit Design

When we talk about the common emitter configuration, we find that it has a current gain that is roughly equal to the β value of the transistor itself. That is pretty straightforward, right?

As we can see that, for any common collector configuration, the load resistance gets connected in series with the emitter terminal of the BJT. This means that the current flowing through this configuration is actually equal to the emitter current.

Now, here is where it gets a little more interesting, the emitter current is actually made up of both the collector current and the base current all mixed together.

So, in this type of transistor configuration, the load resistance ends up having both the collector current and the input current from the base flowing through it.

Using all the above facts, we can define the current gain of the circuit through the following formulas:

I_E = I_C + I_B

A_i = I_E/I_B = (I_C + I_B)/ I_B

A_i = I_C/I_B + 1

∴ A_i = β + 1

The common collector current BJT configuration is considered a non-inverting circuit, which implies that the input signal voltages Vin, and the output signal voltages Vout, are actually “in-phase.” So when we look at the input and output voltages, we find them move together in harmony!

Now, when we investigate more about the common collector configuration, we find that it has a voltage gain that is pretty much around “1,” which is what we call unity gain.

Because of this characteristic, we can think of it as a voltage buffer since it keeps that voltage gain right at unity.

Now, the load resistance in the common collector transistor receives both the base current and the collector current flowing through it.

This gives us a large current gain, similar to what we see in the common emitter configuration. So, with this type of configuration we end up with good current amplification, but with very little increase in voltage gain.

Now that we have taken a good look at these three different types of bipolar transistor configurations, we can go ahead and summarize all the various relationships between the individual DC currents flowing through each leg of the BJT and their respective DC current gains.

We can present all of this information in the following table for clarity!

Table Showing the Relationship between the DC Currents and the Gains of BJT Configurations

When using a transistor (whether it is an NPN or a PNP), the way you connect it to other components determines how the signal is amplified. The calculations for how much the signal is amplified are the same for both NPN and PNP transistors.

The only difference between NPN and PNP transistors is the direction of the current flow and the polarity of the voltages. So even though the calculations are the same, the actual voltages and currents will be opposite in direction for NPN and PNP transistors.

Conclusions

How a transistor behaves in a circuit depends on how it is connected. Different connections can change how well the circuit amplifies signals, how much resistance it has at the input and output, and how much power it can gain.

The table below lists these differences for different transistor configurations. So, depending on what we want our BJT circuits to do, we could choose a configuration that gives us the right combination of input resistance, output resistance, and gain.

Table showing Different types of Bipolar Junction Transistor Configurations and their Characteristics

References:

Bipolar junction transistor

Bipolar Junction Transistor