Can You Swap PNP and NPN Transistors?

Semiconductor Discrete Semiconductor Products
,
1

Short Answer

The short answer is no.

You cannot directly swap an NPN and a PNP transistor as the current flows in opposite directions.

For example, the ONSEMI BD139G (NPN) and BD140G (PNP) devices shown in Figure 1 are closely related and have nearly identical specifications. However, they are not interchangeable.

Figure 1 Image of BD139G (NPN) / BD140G (PNP) complementary transistor pair.
Figure 1 Image of BD139G (NPN) / BD140G (PNP) complementary transistor pair.1002×752 64.7 KB

Figure 1 Image of BD139G (NPN) / BD140G (PNP) complementary transistor pair.

Engineer’s Answer

Absolutely, we can swap a NPN for PNP and vice versa provided we are free to redesign the circuit.

Generalized Considerations

There are several common considerations for each transistor circuit including:

  • Driver circuitry
  • Load placement
  • Power supply polarity
  • Transistor type (NPN or PNP)

In theory, an engineer is free to change transistor type, provided the other constraints may also be changed.

In practice there are real-world considerations. For example:

  • Driver constrained: An NPN transistor is easily driven by a microcontroller as they share a common ground. This article shows how a 3.3 VDC microcontroller can directly drive a DBX33 NPN Darlington Pair transistor.

  • Output constrained: A PNP transistor is often preferred for driving industrial loads as described in this article

  • Power supply and general cost constrained: We should endeavor to design simple, reliable, and low-cost circuits. There is usually an obvious preference for either the NPN or the PNP transistor.

Select the transistor for the circuit, not the circuit for the transistor.

However, there are practical and historic reasons to violate this engineering maxim. The following two case studies show exceptions where we deliberately swap the conventional NPN and PNP transistor positions. The Sziklai-Pair is a modern twist on the NPN ↔ PNP substitution while the Quasi-Complementary Pair is a historical workaround to the lack of complementary pairs.

Sziklai-Pair Case Study

The most famous example of an NPN/PNP transistor swap is the Sziklai-Pair as used for audio amplifiers.

  • The PNP and NPN transistors have swapped their conventional positions.

  • In truth, this is not a direct swap as a secondary driver transistor is required to change the apparent transistor type. The drive transistor with feedback makes one transistor look like the other.

  • See Video 1 for an example you could build on a breadboard at your workbench.

  • A complete description of the Sziklai-Pair is beyond the scope of this article. However, for now we recognize that it has the potential to perform better than the traditional circuit in terms of fidelity and thermal stability.

Quasi-Complementary Pair Case Study

There was a time when complementary NPN and PNP transistor pairs were unavailable. You could get one, but not the other. Engineers used a variety of methods to mitigate this problem including transformer coupling as shown in Video 2 or a method called quasi-complementary pair.

What is a quasi-complementary pair workaround?

You could think of a quasi-complementary pair as half of a Sziklai-Pair.

  • Recall that the Sziklai-Pair uses both PNP and NPN transistors with associated drivers to effectively make the NPN look like the PNP and vice versa.

  • The complementary uses an asymmetrical configuration. For example, it may use two NPN transistors. However, a secondary transistor is used to make one of the NPN transistors operates like a PNP.

  • The Brute-70, as described in the February 1967 edition of Popular Electronics, was a popular amplifier. It was based on an RCA application note appearing in the Technical Series SC-12 manual as shown in Figure 2.

Video 1: Author’s breadboard demo of an amplifier output stage featuring a Sziklai Pair.

Video 2: Author’s teardown and analysis of a 1st generation Germanium transistor audio amplifier featuring transistor coupling.

Figure 2: Author’s copy of the RCA SC-12 transistor manual and a partial schematic from the Popular Electronics Brute-70 audio amplifier (Feb 1967).
Figure 2: Author’s copy of the RCA SC-12 transistor manual and a partial schematic from the Popular Electronics Brute-70 audio amplifier (Feb 1967).1017×763 106 KB

Figure 2: Author’s copy of the RCA SC-12 transistor manual and a partial schematic from the Popular Electronics Brute-70 audio amplifier (Feb 1967).

Parting Thoughts

Regarding the faded photocopied schematic in Figure 2, I’ve had this since I was first bitten by the electronics bug in the early 1980s. Since the beginning, I’ve been fascinated by the classic audio amplifiers. That was probably obvious if you watched video 2.

Best wishes,

Aaron

Related Articles by this Author

If you enjoyed this article, you may also find these related articles helpful:

Purchasing Products

All products featured in this article including the BD139, BD140, and NDX33C transistors are available from DigiKey.

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.

1 Like