The terms small-signal transistor and large-signal transistor are convenient textbook distinctions. They are not formal standards; instead, they describe the level of analysis:
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Small-signal: The transistor is operating close to a given bias point. This provides relatively low distortion, and the transistor characteristics are linear over the limited signal range. This does not imply low power. As an example, consider an emitter follower operating with a 1A quiescent current while buffering small AC excursions.
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Large-signal: The transistor’s operation deviates significantly from the bias point and enters nonlinear operation. We encounter terms such as cutoff, saturation, breakdown, and Safe Operating Area (SOA) limits.
Addressing Misconceptions
The small vs large distinction applies to all transistors. However, there are a few common misperceptions.
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Rigid adherence to the small vs large distinction is a premature selection criterion that may inadvertently reject many otherwise suitable and low-cost solutions.
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A transistor such as a 2N3904 or BC547 may be evaluated for both small-signal and large-signal characteristics. The distinction also depends on the application such as an audio amplifier for small-signal or a relay driver for large-signal.
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Thermal considerations apply to all transistors. Small-signal analysis is primarily focused on bias drift while large-signal is concerned with overheating. However, this distinction breaks down when we explore power transistors in audio amplifiers where both bias stability and overheating are critical.
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Class A amplifiers stretch the distinction between the small- and large-signal point of view. In this example, we have large hardware (high current power supply, multiple transistors, and heatsinks) yet the signal variance from the bias point is small.
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The list would not be complete without mentioning feedback. This is the primary tool for efficient operation of otherwise nonlinear systems (e.g. Class B output stage). It is also the tool that allows Class D anti-linear on-off switching outputs.
Examples of Small-Signal and Large-Signal Transistors
The distinction between small- and large-signal transistors is convenient textbook archetypes. Here are a few examples, MOSFETs are included as they are also categorized as small and large.
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Small-signal transistor example: 2N3904 (NPN) and 2N3906 (PNP) or the European equivalents BC547 (NPN) and BC548 (PNP)
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Large-signal (power) transistor example: TIP41 (NPN) and TIP42 (PNP) or the European equivalents BD139 (NPN) and BD140 (PNP)
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Small-signal MOSFET example: 2N7000 (N-channel) or BS170 (N-channel), with the commonly used European equivalent BSS138 (N-channel)
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Large-signal (power) MOSFET example: IRF540 (N-channel) or IRLZ44N (logic-level N-channel), with European counterparts such as BUZ11
The canonical examples were not accidentally chosen. Instead, they reflect the go to parts engineers have been using for generations. A closely related logistics topic is Jellybean transistor that identifies multi-source, long-lived, parts with high stock levels at DigiKey.
Figure 1: Image of type 2N3906 TO-92 packaged transistors.
How to Locate Small- and Large-Signal Transistors at DigiKey
The textbook archetypes are often identified using the base part numbers such as 2N3904. This shorthand, using a 75-year-old part number, is convenient but does not reflect the part number changes across multiple suppliers and subtle changes such as surface mount and lead-free RoHS. Today the 2N3904 serves as the base for dozens of related (but not necessarily equivalent) parts.
One effective search method is to use keyword pairs. The first keyword is the truncated base part number. The second keyword is a descriptor. Here are a few examples:
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“3904 BJT” for variants of the classic 2N3904 transistor
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"2N700 MOSFET” for variants of the 2N7000 MOSFET (the truncation captures both 2N7000 and 2N7002)
Note that the search isn’t perfect. Since we have cast a wide net, we occasionally see outliers. If in doubt, consult the datasheet.
Related Articles by this Author
If you enjoyed this article, you may also find these related articles helpful:
- What is the purpose of the emitter bypass capacitor in the Common Emitter (CE) amplifier?
- Can You Swap PNP and NPN Transistors?
- Is the 2N3904 transistor obsolete?
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, completing a decades-long journey that began as a search for capacitors. Read his story here.