The **D548C** is a general-purpose, high-gain NPN silicon epitaxial planar transistor. It is widely used in electronic circuits for low-noise amplification and switching applications. Below is a detailed breakdown of its specifications, pinout, and typical use cases. --- ### 1. Technical Specifications The D548C is essentially part of the **BC548** family. The "C" suffix denotes the highest current gain (hFE) classification for this specific model. | Parameter | Symbol | Value | | :--- | :--- | :--- | | **Type** | Polar | NPN | | **Collector-Emitter Voltage** | $V_{CEO}$ | 30V | | **Collector-Base Voltage** | $V_{CBO}$ | 30V | | **Emitter-Base Voltage** | $V_{EBO}$ | 5V | | **Collector Current (Continuous)** | $I_C$ | 100mA | | **Total Power Dissipation** | $P_D$ | 500mW | | **DC Current Gain** | $h_{FE}$ | 420 to 800 (Class C) | | **Transition Frequency** | $f_T$ | 150 MHz | | **Package Type** | Case | TO-92 | --- ### 2. Pin Configuration (TO-92 Package) When looking at the flat side of the transistor with the leads pointing downwards, the pinout is as follows: 1. **Pin 1 (Collector):** Current enters the transistor here. 2. **Pin 2 (Base):** The trigger that controls the flow between Collector and Emitter. 3. **Pin 3 (Emitter):** Current exits to the ground or negative side of the circuit. --- ### 3. Key Operational Features * **High Gain:** The "C" variant provides the highest gain in its series, making it excellent for amplifying very weak signals (like those from a microphone or sensor). * **Low Noise:** It is designed to minimize electronic noise, ensuring signal integrity in audio pre-amplifiers. * **Switching Speed:** With a transition frequency of 150 MHz, it can handle fast switching in digital logic circuits. --- ### 4. Common Applications * **Audio Preamplifiers:** Boosting small audio signals before they reach a power amplifier. * **Sensor Circuits:** Acting as a switch for light-dependent resistors (LDRs) or touch sensors. * **Signal Processing:** Shaping and filtering signals in radio or communication devices. * **Relay Drivers:** Using a small microcontroller current to flip a larger electromagnetic relay. --- ### 5. Equivalent/Replacement Parts If the D548C is unavailable, these are common alternatives: * **BC548C** (Direct equivalent) * **BC547** (Higher voltage rating) * **2N3904** (Common NPN, though pinout may differ) * **BC549** (Ultra-low noise version)