A transistor is a very old device. Nowadays we don’t prefer using it and instead, we choose to use IC. Here it must be kept in mind that a transistor has its own importance. It plays role in general electronic circuits. Why? The reason is that a transistor is large, durable and can drive high current. It might take a little time to get used to working with transistors just like me but no matter what the reason is, you can understand its working in a simple way.

In this article, it is briefly explained and it includes how the transistor circuit works in a simple way. I will also be learning it with you.

Are you ready? Let’s proceed.

How does a transistor circuit works

A transistor is an active device as it amplifies the current. A transistor has many types which are over 20,000 obtained from hundreds of manufacturers.

Types of Transistors:

Transistors can be put into two types for a standard method; NPN and PNP. They also have different symbols.

Type of Transistor

The figure above shows the class of semiconductor materials that are used to make the transistor.

Currently, we mostly use the transistor of NPN type. The reason is that it is made easy from silicon materials. The following article will be covering the NPN type transistor more.

If you are new to electronics, this might be a great start for you to have good learning. You must learn about the use of transistors first.

The transistor consists of a pin. This pin has a base (B), collector (C) and an emitter (E). These parts are called the leg of a transistor which represents the functioning of the transistor.  Learning these parts won’t help you to understand how to use transistors. This is only knowing the lead of a transistor.

In addition to standard transistors (bipolar), there is another type of transistor called the fields to effect transistor. They are often represented by acronyms FET. They have different symbols and properties but are not discussed in detail in this article.

Learn the Basics of How Transistor Circuit Works; Transistor Current:

This type of transistor is also called a small-signal transistor. We may call it a TO-92 model. Look at the Figure. We often use this transistor in the number 3 group.

How Transistor Circuit Works :different Small transistor legs

The legs of this transistor are used differently. You must be careful when using such a device.

  • BC547, BC337, etc.
  • CS9013 or 2N3904
  • C1815 or 2SC1815

Read next: Transistor Symbols

What more can you learn about them? We will understand the working of current in a basic transistor circuit.

Look at the figure below. The NPN transistor is shown in a simple circuit.

The NPN transistor in a simple circuit

When we add a low current to the base of the transistor, high current will flow through the load to the collector-emitter leads. This load at the collector lead is often called the load resistor. Sometimes the load can be a speaker too.

When I was a beginner, I had some trouble understanding simple transistors. I had to work very hard and read a lot of text before I started getting the concepts.

The transistor works like a water valve

The transistors are often compared with water valves. Hence we can control the high power of water input to the output with low water.

  • The origin of the water pipe (Input) is like the Collector.
  • The end of the water pipe (output) is like the Emitter.
  • The control (small) pipe is like the Base.

The high water comes to the valve form the input side. Then, the low water comes to the control value and turns on the main valve. In the end, the high water can flow through the pipe to the output.

In contrast, no low water can go to the control valve. It does not turn the valve to control the high water. So no water comes to the output.

Basic principles

In general, we can divide the working range of the transistor into the following limits:

  1. Cut off (transistor stop):

When there is no current but both base current (IB) and collector (IC) will flow through the transistor. There will still be some leakage currents, but they will be very low.

  1. Saturated range:

There is electricity flowing through the transistor fully until it saturated. After that, the current will not increase anymore. This can limit the current through the connection of the resistors.

  1. The active range:

This is the period where the transistor operates or conducts current. It works by driving the collector current (IC) making it proportional to the base current (IB).

So when the transistor is used as an audio amplifier, the circuit works in the active phase.

Do you understand the concept now?

Experiment; a current transistor:

I understood transistors systematically through experimentation. You may also find it better to understand through experiments. Let’s move on towards the experiment.

Look at the diagram below:

simple current transistor circuit

A simple current transistor circuit is shown.

It is a very simple circuit. This circuit is used to test the current flowing through the transistor. In this type of circuit, we use the red LEDs of size 0.5 mm and an NPN transistor with low power (such as BC108, BC182, or BC548).

Here is a step by step a process of the working of the transistor circuit.

The low base current controls the high collector current.

The S1 shows the closed switch. The current flows from R1 and LED1 to the base of a transistor.

This is called the base current while LED1 will become dim.

collector current path of transistor

The transistor in this circuit amplifies the low current and the current flows through the collector (C) to the emitter (E). This collector current is high enough to make LED C very bright.

collector current path of transistors

When the S1-switch is opened, there will no base current that flows. So the transistor will cut-off the collector current making both LEDs go out.

The transistor is often used to amplify the current and switches.

The arrangement shows emitter (E) in the base current and also in the collector current. We can call this the emitter common mode. The working of a transistor circuit in such a way is used quite often and very extensively. So we must learn about it first.

Working model and structure of NPN transistor:

There is no easy way to explain to you the internal structure of the NPN transistor which makes me very frustrated. However, I will try to compare it with the diode and variable resistor which will help you understand the concept more easily.

Look at the figure below.

Here is the step by step process:

  • The base-emitter joint is like a diode.
  • The base current IB flows only when the VBE voltage between the base-emitter is 0.7V or more.
  • The tiny base current (IB) controls the high collector currents.
  • IC = hFE × IB (unless full active and saturated transistor)

  • The hFE is the current amplifier gain (In DC current gain). The normal value for hFE is 100 (there is no unit because it is the ratio).
  • The resistance between the collector-emitter (RCE) is controlled by the base current (IB) by:
    • IB = 0 RCE = infinite value. Transistor (off)
    • Less IB, RCE lower, the transistor turns on only partially
    • IB added. RCE = 0. The transistor runs (on) fully (saturated)

Additional Notes:

  • It is necessary to connect a series resistor to the base so that the base current can be limited to prevent damage to the transistor.
  • The transistor has the highest collector current rate of IC.
  • The hFE current gain can have different values even though it has the same type.
  • The transistor that is fully connected (on) (when RCE = 0) is called saturated.
  • When the transistor is saturated the emitter-collector voltage VCE is reduced to 0V.
  • When the transistor is saturated, the collector current IC is determined by the voltage, the supply, and the external resistance in the collector circuit.

It is not related to the transistor’s current gain.

For this reason, the ratio of IC / IB for saturated transistors is less than the hFE current gain.

  • The emitter current is IE = IC + IB, but the IC is much larger than IB.

Darlington Transistor pair

Two transistors are connected in a circuit as shown in the above diagram.

It causes the current to be amplified with the first transistor and then it is amplified with the second transistor.

The current gain is equal to the gain of each of the values multiplied together:

The current gain of the Darlington pair hFE = hFE1 × hFE2
(hFE1 and hFE2 are the gain of each transistor).

For this reason, the Darlington pair has a very high current gain, such as 10000. Therefore, we use only a small base current to allow the Darlington pair to switch.

The Darlington pair of a transistor contains three legs (B, C and E),   instead of a single transistor with a very high current gain. These three legs are equivalent to the legs of a single transistor.

be of darlington pair

We can use the Darlington pair as shown as it works well.

By putting the voltage 0.7V between the base-emitter (VBE) of both transistors in series internally they will require the voltage of 1.4V to turn on.

The experiment of the touch switch circuit

The transistor circuit that works as the Darlington pair is quite sensitive to the small currents that flow through our skin. Thus if you want to be able to use a touch switch circuit you must do so as shown in the diagram.

For this circuit, we use two low-power general-purpose transistors.

When we touch it, the LED is illuminated.

touch switch circuit

Here the 100K resistor is used to limit the base current.

Using the transistor as a switch:

When we use the transistor as a switch, it will either turn off (OFF) or it will turn on (ON).

In the (ON) voltage, the VCE across the transistor is almost zero volts and we call it a saturated transistor because it cannot have more collector current (IC).

Look at the simple switching transistor circuit working as shown below.

The output device that is switched by this transistor is called the load.

The power generated in the switch transistor is very low:

  • In the OFF state: power = IC × VCE, but IC = 0, so power is zero.
  • In the ON state: power = IC × VCE, but VCE = 0 (most), so the power is very low.

This shows that the transistor used in this way will not become hot so you must not consider the maximum power rate.

The important point in the switching circuit is the maximum collector current IC (max) and the minimum current gain hFE (min).

The voltage of the transistor must not be taken into account except when it is used with a power supply higher than 15V.

The Protected Diode

If the load is a Motor, Relay or Solenoid (or other devices that is a coil) it must be connected to a diode across the load. That way it will protect the transistor circuit works (and IC) from damaging during the cut offload.

Look at the circuit diagram below.

protected diode for transistor circuit

The diagram shows a connected reverse biased diode which normally does not conduct currents.

It will conduct current only when the load is cut off. During that time the current that collects energy in the coil will try to flow through the coil. Since the transistor will be cut-off, the current flows through the diode instead.

If there was no diode, the current will not flow. This coil will produce a high spike voltage. It is dangerous when it tries to flow.

When should we use the relay;

A transistor cannot be used to switch AC voltage or any high voltage (such as AC main). It is also not suitable to switch too high current (> 5A). Hence we need to use the relay.

Here we also need to use a low power transistor to drive the current to the coil of the relay.

DC 12V Coil 5 Pins Mini SPST Power Relay PCB

Benefits of using relays:

  • The relay can switch AC and DC power both while the transistor can switch only DC power.
  • It can switch on the high-voltage power, the transistor cannot do that.
  • Relays are a better option for circuits that need switching to high-currents (> 5A).
  • The relay can switch multiple contacts at the same time.


Disadvantages of using relays:

  • A relay is too big compared to the transistor so it can’t fit in the small current switch.
  • The relay cannot switch with increasing speed; the transistor can switch with increasing speed many times per second.
  • Relays require more power. Observe the current that flows through the coil.
  • Relays require more current than the IC can drive. So for using them, we need to use a low-power transistor to switch the relay coil current.

Cr: Photo DC 12V Coil 5 Pins Mini SPST Power Relay PCB

Credit: https://electronicsclub.info Thanks a lot. This content makes everything easier to understand.

Connecting the transistor with the output from the IC

Most IC outputs cannot supply high current hence it becomes necessary to use a transistor. The transistor switches currents that are too high for the output devices. For example, light bulbs, motors, and relays, etc. Except for the timer 555 IC; this device can normally supply a current of up to 200mA.

This timer is high enough for the output devices that require a small current for example a small lamp, buzzer, or relay. So it doesn’t require the help of a transistor.

Look at the basic circuit in the diagram below. The transistor is connected to the IC’s output.

Resistor R1 is attached so that it limits the current that flows into the base of the transistor. It also prevents any damage.

However, R1 must be low enough to ensure that the transistor is saturated, hence preventing overheating. This measure is important if the switch transistor has a high current (> 100mA). The safest way to choose for this circuit would be if the base current (IB) is 5 times higher than the current that makes the transistor saturated.

Do you understand? Read more and you will feel more confident.

Choosing the right NPN transistor

The circuit diagram below shows the connection of the NPN transistor. This circuit will switch to load when the output from IC is high (+ V).

On the other hand, if you want to continue loading when the output from IC is low (0V), you can take help from the circuit for the PNP transistor shown below.

The steps described below explain how to select a suitable switching transistor:

  • The maximum collector current (IC max) of the transistor must be more than the load current.

We can find the load current (LC) = Voltage supply (VS) / the resistance of the load.

For example, we can use the light bulb 12V 3W. It uses the current
= 1W / 12V = 0.083A. So IC max we use should be more than 0.1A or 100mA.

  • The minimum gain current, hFE (min) of the transistor must be at least 5 times the load current IC divided by the maximum output current of the IC (chip).
  • The calculation for the approximate value for the base resistor:
    R1 = 0.2 × RL × hFE or
    R1 = (Vs × hFE) (5 × IC)

Choosing the right PNP transistor

Connecting PNP transistor with output of IC

Look at the circuit diagram above showing the connection of PNP transistor.

This circuit will switch towards the load when the IC output is low (0V).

The procedure for choosing a suitable PNP transistor is similar to choosing an NPN transistor described above.


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