This post contains a set of Frequently Asked Questions (FAQs) for the Bipolar Junction Transistor. Don’t see an answer to your question? Add it to the comments section.
What is a Bipolar Junction Transistor (BJT) and how does it work?
The transistor is classically defined as a three-terminal semiconductor device that operates as a linear amplifier, oscillator, or digital switch. The transistor is designed so that a small base current controls a larger current flowing collector to emitter.
Transistors are classified as either NPN or PNP types. The designation describes the semiconductor layers that are used to construct the transistor. In practical terms, the designation describes the flow of current through the device.
The transistor’s fundamental operating principle is described in terms of current flow where a small current applied to the transistor’s base controls a larger current flowing from collector to emitter. This small to large current control allows the transistor to operate as an amplifier.
Expanding the definition of transistor
Today, the word “transistor” is often used in place of “semiconductor.” Consequently, the term may identify any number of semiconductor components such as BJTs, JFETs, MOSFETs or IGBTs. For clarity, the classical transistor should be described as a BJT.
Tech Tip: A good way to remember transistor schematics is to recognize the NPN transistor as Not Pointing In.
How to learn about BJTs: The importance of experimentation
Electronics is not a spectator sport. Instead, deep learning about a complicated topic such as transistors requires hands-on experimentation. To intuitively understand transistors, you will need to build a circuit. This will likely be an uncomfortable process as you push yourself to the limits.
Consider one of my first transistor experiments where I wanted to control the intensity of a 12 VDC car headlamp. The classic 2N2055 appeared to be a good candidate. I learned many lessons about power dissipation and linearity. Through this and hundreds of related experiments, I developed an intuitive understanding that I apply to all circuitry. Know that this is a lifelong learning exercise, and I still make mistakes.
This article about deep learning is applicable to students of all ages.
One final suggestion is to obtain The Art of Electronics By Paul Horowitz and Winfield Hill. Start by building the beginner transistor circuits. Then challenge yourself by modifying the circuit and then building circuits of your own design.
Can I replace a transistor with any other transistor?
The short answer is no.
A replacement transistor must be carefully chosen to match the original circuit. This is important as the circuit is dependent on transistor parameters such as:
-
transistor type such as NPN, PNP, Darlington
-
voltage capabilities such as the emitter to collector voltage
-
current capabilities typically expresses as collector current
-
physical packaging
-
gain
Tech Tip: An incorrect replacement transistor could result in immediate destruction of the new transistor. It could also destroy related circuitry, including delicate Printed Circuit Board (PCB) traces.
How to locate a replacement transistor?
Whenever possible, a direct replacement transistor should be used. This is a recognition that the original design team built the circuit around the characteristics of the transistor. Deviation from these expectations can result in unstable operation, reduced equipment lifespan, or even immediate destruction of the circuit.
Obsolescence makes locating a direct replacement difficult. For instance, the driver transistors for a classic 1970s audio amplifier may no longer be produced. This is especially problematic for power transistors where we occasionally find discontinues packages. In such cases, it is necessary to modify the equipment or seek expensive options such as purchasing donor equipment.
Use DigiKey to locate replacement transistors
There was once a thriving business to provide transistor substitution for the repair technician. Companies such as NTE (Philips ECG) provided cross reference books along with a set of replacement semiconductors. This was ideal for the technician, as many transistors could be represented by a small number of physical components. For example, the classic NTE 123 could be used to replace hundreds of transistors.
Unfortunately, NTE closed its doors in 2024. However, the valuable ECG and NTE cross reference document may still be located on the internet. You can still use this technique to locate a DigiKey sourced solution:
-
Locate an ECG / NTE cross reference online
-
Determine the ECG / NTE equivalent transistor for the transistor you are replacing
-
Search for the ECG / NTE part number at https://forum.digikey.com/ to find a recommended substitute.
You can also request a DigiKey recommendation by posting a request to https://forum.digikey.com/. It is important to provide as much information as possible. This allows the DigiKey specialists the best chance to locate the replacement.
What is the replacement for the 2N2222 transistor?
The Jellybean 2N2222 has been with us for about 60 years. The original devices were packaged in the iconic TO-18 metal can as shown in Figure 1 tracing back to John (Jack) Haenichen’s famous patent 3,226,61 dated 28 Dec 1965 (filed 04 Nov 1963).
The 2N2222 is considered a jellybean component as it is widely available part that has stood the test of time. DigiKey offers many variants of the classic 2N2222. They can be located by searching the terms “2222 BJT” in the DigiKey search bar.
Today, DigiKey offers the classic 2N2222. This is ideal for restoration or reproduction of classic equipment. DigiKey also offers a repackaged 2N2222 in a TO-92 case. The onsemi PN2222ABU is representative of this low cost modern 2N2222 variant.
Figure 1: Sketch of a TO-18 packaged transistor featured in Jack Haenichen’s patent 3,226,614 dated 28 Dec 1965 (filed 04 Nov 1963).
What replaces 547 transistor?
The Jellybean BC547 is a small signal transistor approximately equivalent to a the 2N2222 or the 2N3904.
The BC547 is the go-to transistor for European hobby projects. Once you recognize the iconic part number, you will see it everywhere. However, this does not imply that the BC547 is a toy. Instead, this is a recognition of an extremely popular line of semiconductors.
The Jellybean 2N3904 and 2N2222 may generally be used to replace the BC547. However, we need to be careful with the transistor’s full part number especially the gain suffix. For example, the BC547C is a high-gain component with performance unmatched by the previously mentioned transistors.
For reference, here is the part breakdown for the BC547 and related family members:
-
First letter: The letter “B” indicates the semiconductor material with a band gap between 1.0 to 1.3 eV (silicon).
-
Second letter: The letter “C” identifies the transistor designed primarily for audio frequency applications.
-
Series Number: Three figures are used to identify the individual semiconductor.
-
Gain suffix: Close inspection of the BC107 datasheet reveals that a gain designator is appended to the series number. The BCXXXA has a typical DC current gain 90, the BCXXXB 150, and the BCXXXC is 270.
Related information
Please follow these links to related and useful information:
About this author
Aaron Dahlen, LCDR USCG (Ret.), serves as an application engineer at DigiKey. He has a unique electronics and automation foundation built over a 27-year military career as a technician and engineer which was further enhanced by 12 years of teaching (interwoven). With an MSEE degree from Minnesota State University, Mankato, Dahlen has taught in an ABET-accredited EE program, served as the program coordinator for an EET program, and taught component-level repair to military electronics technicians. Dahlen has returned to his Northern Minnesota home and thoroughly enjoys researching and writing articles such as this.