FAQ
TL;DR: The 2N4123 is an NPN silicon BJT rated 30 V VCEO and 200 mA; "NPN General Purpose Amplifier" per the datasheet. Identify type fast with a multimeter’s diode test. [2N4123 Datasheet by ON Semiconductor]
Why it matters: This FAQ helps students, repair techs, and pedal builders quickly confirm transistor type, specs, and safe substitutions without guesswork.
Quick Facts
- Type and ratings: NPN, VCEO 30 V, IC 200 mA, PD 625 mW, fT ~250 MHz. [2N4123 Datasheet by ON Semiconductor]
- Package: TO‑92. Pin order varies by maker—verify or test before soldering. [Meter Check of a Transistor (BJT)]
- Complementary PNP partner: 2N4125 with similar 30 V/200 mA ratings for push‑pull stages. [2N4125 | General Purpose Transistors | AEM]
- Street price: Approx. $0.15–$0.18 in bulk; through‑hole stock common. [2N4123 Pricing and Availability | Cytech Systems]
Is the 2N4123 NPN or PNP?
It’s an NPN silicon bipolar junction transistor. The datasheet labels it “NPN General Purpose Amplifier,” confirming polarity and intended use. [2N4123 Datasheet by ON Semiconductor]
How do I quickly tell if my 2N4123 is NPN or PNP with a multimeter?
Use diode mode. Find the base pin that conducts to both other pins. If conduction occurs with the red probe on base, it’s NPN; black probe on base indicates PNP. The emitter junction shows a slightly higher forward drop than the collector. [Meter Check of a Transistor (BJT)]
What’s the safest way to identify the base, collector, and emitter (no datasheet)?
With diode mode: identify the base as the pin that forward‑biases to both others. Then compare drops; the higher forward voltage drop corresponds to the emitter. Packaging isn’t standardized, so always test before committing to a PCB. [Meter Check of a Transistor (BJT)]
Give me the key electrical specs I should design around.
Typical design anchors: VCEO 30 V, VCBO 40 V, IEBO/ICBO ~50 nA, IC 200 mA max, PD 625 mW at 25°C (derate 5 mW/°C), VCE(sat) ~0.3 V at 50 mA/5 mA, fT ~250 MHz, hFE ~50–150. [2N4123 Datasheet by ON Semiconductor]
What’s the complementary PNP device to pair with 2N4123?
Use 2N4125 as the PNP complement. It shares similar voltage, current, and power ratings, making it suitable for push‑pull or Sziklai stages. [2N4125 | General Purpose Transistors | AEM]
Can I substitute a 2N3904 or 2N2222 for a 2N4123?
Yes, if the replacement meets or exceeds 30 V VCEO, supports required current, and matches the pinout. Re‑bias if hFE differs. Check VCE(sat) and thermal limits against the 2N4123 baseline before swapping. [2N4123 Datasheet by ON Semiconductor]
What’s the pinout of the 2N4123 in TO‑92 packages?
Manufacturers don’t standardize TO‑92 lead order across all BJTs. Confirm with the datasheet or diode‑mode testing rather than assuming E‑B‑C or C‑B‑E by position. "Packaging, unfortunately, is not standardized." [Meter Check of a Transistor (BJT)]
Is the 2N4123 suitable for guitar pedals or audio stages?
Yes. Builders use it in fuzz and general audio gain stages. Vendors note its popularity in silicon fuzz pedals due to moderate hFE ranges. [Transistor - 2N4123, Silicon, TO-92 case, NPN | Antique Electronic Supply]
What are common failure signs when testing a 2N4123?
Edge cases: shorted device conducts both ways on any junction; an open device shows no conduction; abnormal reverse conduction suggests leakage. Use diode mode to confirm and compare expected forward drops. [Meter Check of a Transistor (BJT)]
What’s the power and thermal limit I must respect?
Keep within 625 mW at 25°C and derate by 5 mW/°C as ambient rises. Observe the 150°C junction temperature limit to avoid thermal runaway or premature failure. [2N4123 Datasheet by ON Semiconductor]
Any quick online way to confirm polarity if I forgot the datasheet?
Yes—search the part number. As one expert put it, you’ll immediately find datasheets stating it’s NPN. [Elektroda, John Harris, post #21665747]
What does VCE(sat) look like in real use?
Expect about 0.3 V at IC = 50 mA with IB = 5 mA. This low saturation voltage helps reduce switching loss and heat in small loads. [2N4123 Datasheet by ON Semiconductor]
How do I test and sort multiple 2N4123s for matched gain?
Measure hFE using your DMM’s transistor socket or a simple jig at the same IC and VCE. Sort parts into bins (e.g., 50–80, 80–120, 120–150) to tighten bias spread. [2N4123 Datasheet by ON Semiconductor]
What’s a quick 3‑step method to identify an unknown BJT (works for 2N4123 too)?
- Set multimeter to diode mode; find the pin that conducts to both others—that’s the base.
- Determine type: red on base = NPN, black on base = PNP.
- Identify emitter: the base‑emitter drop is slightly higher than base‑collector. [Meter Check of a Transistor (BJT)]
Any quote from the datasheet I should remember?
“NPN General Purpose Amplifier.” This reminds you it’s for general amplification or switching up to 200 mA, not power applications. [2N4123 Datasheet by ON Semiconductor]
