A733 (2SA733) — -50V -150mA General-Purpose PNP Audio/Switching BJT (TO-92)

Overview

The A733 (officially 2SA733 or KSA733) is a general-purpose PNP bipolar junction transistor originally developed by NEC and Toshiba, and manufactured globally by onsemi (Fairchild) and UTC. Housed in a through-hole TO-92 plastic package, it is the official PNP complement to the ubiquitous C945 (2SC945) transistor.

Featuring a collector-emitter breakdown voltage ( $V_{CEO}$ ) of $-50\text{V}$ , continuous collector current of $-150\text{ mA}$ , a transition frequency ( $f_T$ ) of $180\text{ MHz}$ , and ultra-low noise figure ( $NF \approx 0.5\text{ dB}$ ), the A733 is universally found in Asian consumer electronics, imported DIY kits, audio preamplifiers, discrete multivibrators, and power supply control stages.

Quick reference


Transistor Type	General-Purpose Audio / High-Frequency Switching PNP BJT
Package	TO-92 (3-pin through-hole) / SOT-23
Collector-Emitter Breakdown ( $V_{CEO}$ )	$-50\text{ V}$ max
Collector-Base Breakdown ( $V_{CBO}$ )	$-60\text{ V}$ max
Continuous Collector Current ( $I_C$ )	$-150\text{ mA}$ max
DC Current Gain ( $h_{FE}$ )	$70$ to $700$ (Classified by gain rank: R, Q, P, K)
Transition Frequency ( $f_T$ )	$180\text{ MHz}$ typ ( $100\text{ MHz}$ min)
Noise Figure ( $NF$ )	$0.5\text{ dB}$ typ ( $3.0\text{ dB}$ max at $1\text{kHz}$ )
Total Power Dissipation ( $P_D$ )	$250\text{ mW} \dots 400\text{ mW}$
Complementary NPN Pair	C945 (2SC945)
Pinout Standard (Japanese JIS)	`E - C - B` (Pin 1: Emitter, Pin 2: Collector, Pin 3: Base)

Pinout (TO-92 Package - Japanese JIS Standard)

Looking at the flat front face with leads pointing downward:

text

        ┌─────────┐
        │  TO-92  │
        │  A733   │
        └─┬───┬───┬─┘
          1   2   3
          E   C   B

Pin	Name	Type	Description
1	`EMITTER (E)`	Emitter Terminal	Emitter (Connect to positive supply rail or emitter degeneration resistor)
2	`COLLECTOR (C)`	Collector Terminal	Collector (Center lead; connected to load or negative rail)
3	`BASE (B)`	Base Terminal	Base control input (Right lead; driven with base current)

(Note: Standard Japanese 2SA733 parts follow E-C-B. However, certain Western variants like Fairchild's KSA733C with a "C" suffix use E-B-C. Always check with a component tester or datasheet).

Specifications

Parameter	Symbol	Min	Typ	Max	Unit	Conditions
Collector-Emitter Breakdown	$V_{(BR)CEO}$	-50	—	—	V	$I_C = -1.0\text{mA}, I_B = 0$
Collector-Base Breakdown	$V_{(BR)CBO}$	-60	—	—	V	$I_C = -100\ \mu\text{A}, I_E = 0$
Emitter-Base Breakdown	$V_{(BR)EBO}$	-5.0	—	—	V	$I_E = -10\ \mu\text{A}, I_C = 0$
Collector Cutoff Current	$I_{CBO}$	—	—	-100	nA	$V_{CB} = -60\text{V}, I_E = 0$
DC Current Gain ( $h_{FE}$ )	$h_{FE}$	70	200	700	—	$I_C = -1.0\text{mA}, V_{CE} = -6.0\text{V}$
Collector Saturation Voltage	$V_{CE(sat)}$	—	-0.10	-0.30	V	$I_C = -100\text{mA}, I_B = -10\text{mA}$
Transition Frequency	$f_T$	100	180	—	MHz	$I_C = -10\text{mA}, V_{CE} = -6.0\text{V}$
Noise Figure	$NF$	—	0.5	3.0	dB	$I_C = -0.1\text{mA}, V_{CE} = -6\text{V}, f = 1\text{kHz}$

$h_{FE}$ Classification Ranks

Rank	$h_{FE}$ Range	Application Suitability
R	$40 \dots 80$	General digital switching
Q	$70 \dots 140$	High-speed logic switching
P	$120 \dots 240$	Standard audio driver (Most Common)
K	$200 \dots 400$	High-gain audio preamplifiers

Common mistakes

Swapping with 2N3906 or BC557 without checking pinout:
- A733 (2SA733 JIS): Emitter - Collector - Base (Pin 1: E, Pin 2: C, Pin 3: B)
- 2N3906 (USA): Emitter - Base - Collector (Pin 1: E, Pin 2: B, Pin 3: C)
- BC557 (Europe): Collector - Base - Emitter (Pin 1: C, Pin 2: B, Pin 3: E) Always verify pin ordering before replacing transistors in imported circuits.

Notes

Naming Convention: 2SA733 is the full Japanese Industrial Standard (JIS) part number; A733 is the standard top-mark abbreviation printed on the physical TO-92 body; KSA733 is the onsemi/Fairchild part number.