You need to match three main parameters:
: An NPN transistor must be replaced with another NPN ; PNP with PNP. all transistor equivalent book
Many books group transistors into families (e.g., "Small Signal Low Noise," "Switching," "Power Darlington"). Never substitute a power transistor (e.g., 2N3055) for a small-signal device (e.g., 2N3904) even if the book lists them under the same number. Read the category heading. You need to match three main parameters: :
Measure the voltages at the collector and emitter. If the collector is at Vcc (cutoff) or 0V (saturation) when it should be mid-supply, your substitute has the wrong gain or leakage. Read the category heading
No equivalent book is perfect. Manufacturers discontinued parts, changed specifications without notice, or used "house numbers" that were custom-ordered with unique gain curves. In audio amplifiers, mismatched hFE can cause distortion. In switch-mode supplies, a slightly slower ft can cause overheating and failure.