Technology changes over time. The single-packaged BJT was the pinnacle of 20th century technology. There were many specialized transistors for a wide variety of applications including signal amplification, high frequency radio, digital logic, and power. As technology and manufacturing processes improved, many of these applications transitioned to advanced MOSFETs, IGBTs, and highly specialized integrated circuits. The BJT remains relevant for general purpose work and for niche application such as audio amplification.
One unfortunate result is that BJT replacements can be difficult to locate. For example, suppose you are attempting to repair a classic Marantz 2270 receiver. It used complementary Motorola output transistors type SJ2517 (NPN) and SJ2518 (PNP). It’s been 50 years since the receiver was manufactured, and those transistor types are no longer available. Consequently, you must locate suitable replacements using tools such as DigiKey’s parametric search engine. In this particular instance, the MJ21194G (NPN) and MJ21193G (PNP) are considered suitable replacements.
Must-know facts about bipolar transistors
There are a few key BJT facts to consider:
No substitution is complete until the transistor is tested in the final equipment
Selecting a suitable replacement transistor is one of DigiKey customer’s most frequently asked questions. It’s not a trivial question as it takes time to carefully compare the original specification to a potential candidate. In rank order, we must verify
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package,
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pinout
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design maximum parameters (VCE, IC, PD)
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subtle parameters such as gain or saturation voltage
The process is further complicated as the end product may be dependent on the specific characteristic of the older and perhaps less capable transistor. Consequently, no substitution is complete without testing in the finished product. In the end, the user is responsible for selecting the proper transistor for their equipment.
Use of complementary transistor pairs
Many transistors are designed in complementary pairs with matching properties for the NPN and PNP devices. A classic example is the audio output section of an amplifier such as the previously mentioned the MJ21194G (NPN) and MJ21193G (PNP). However, the complementary devices are available for small signal devices such as the KSC1845FTA (NPN) and KSA992FBTA (PNP).
The heatsink is a critical component for power transistors
No conversation about transistors is complete without exploring the concept of a heatsink. I remember my first experiments with a 2N3055 transistor where I attempted to dim a 55 W automotive headlamp. While the transistor is more than capable of handling the current, it got very hot. A large aluminum plate took care of the situation. Today, I would recommend a switching power supply. However, we all need to start somewhere.
Use of Darlington transistors
A student’s first encounter with a transistor is typically associated with a microcontroller. Here, the transistor is used as a buffer between the microcontroller and a load such as a relay or motor. There is more to this “simple” application than most people realize as the transistor must be fully turned on with a base current about 1/10 of the collector current (forced beta condition). At the same time, we need to be careful not to overload the microcontroller’s I/O pin. The Darlington transistor can improve the situation as the high gain device is easy to drive. Instead of the microcontroller supplying a 1/10 base, the Darlington transistor requires 1/100 of the collector current.
Tips for transistor circuit design
Transistor-based audio amplifiers remain popular and can be a rewarding hobby. To learn more, please review the works of Douglas Self including “Audio Power Amplifier Design Handbook” and “Small Signal Audio Design.” I learn something new every time I read the material. Be sure to visit Elliott Sound Products and PassDiy. Feel free to share other resources in the comments section.
Looking forward to continuing the conversation on this forum.
Best wishes,
APDahlen
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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.