BD678A

Series:	--
Packaging:	Tube
Part Status:	Active
Transistor Type:	PNP - Darlington
Current - Collector (Ic) (Max):	4A
Voltage - Collector Emitter Breakdown (Max):	60V
Vce Saturation (Max) @ Ib, Ic:	2.8V @ 40mA, 2A
Current - Collector Cutoff (Max):	500µA
DC Current Gain (hFE) (Min) @ Ic, Vce:	750 @ 2A, 3V
Power - Max:	40W
Frequency - Transition:	--
Operating Temperature:	150°C (TJ)
Mounting Type:	Through Hole
Package / Case:	TO-225AA, TO-126-3
Supplier Device Package:	SOT-32-3
Base Part Number:	BD678

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BD678A application field and working principle

The BD678A is a single bipolar junction transistor (BJT) designed specifically for use in power audio amplifiers of up to 200 Watts. It is designed with a very low distortion level and a high amount of power output for its size. The device consists of two NPN transistors combined into one die, connected to an external resistor for the base connection. This allows the gain of each transistor to be accurately matched, reducing distortion and improving overall performance.The BD678A can be used in several different applications, but is most commonly used as an audio power amplifier for high-power stereo systems. The high-gain of the transistor allows for both low- and high-frequency power amplification with low distortion levels. The transistor can also be used in other voltage and current applications, such as switching and amplification circuits.In order to understand the operation of the BD678A, it is important to understand the basic principles of BJT transistors. BJT transistors are composed of three layers of semiconductor material, known as the base, collector, and emitter. The base layer can be thought of as a control layer, providing a path for current to flow between the collector and emitter layers. By controlling the current flowing through the base layer, the current flowing through the collector and emitter layers can be varied.At its simplest level, the BD678A works by providing a path for current to flow between the collector and emitter layers when a forward biasing voltage is applied to the base layer. This forward biasing voltage provides the energy to the electrons in the base layer, allowing them to flow to the collector layer. The amount of current flowing through the collector and emitter layers can be controlled by adjusting the forward biasing voltage applied to the base layer. In the operation of the BD678A, the base current is regulated by an external resistor connected to the base. This resistor has the purpose of controlling the current flow between the collector and emitter layers, ensuring that the transistor does not overload. Additionally, it helps improve the constant current nature of the device, as the base current is always kept in check.The BD678A can be used in both Class A and Class B amplifiers. In a Class A amplifiers, both transistors are always turned on, ensuring that the output voltage is always at peak level. In Class B amplifiers, the transistors are run in complementary operation, with the transistors alternating the output. In addition to amplifiers, the BD678A can also be used in audio power switching applications, such as speakers and subwoofers.The BD678A is a powerful, robust transistor, capable of providing a large amount of power output with low distortion. It is well suited for application in several different types of audio amplifiers and audio power switches, as well as other current and voltage applications. With its high power output, low distortion levels, and high gain, the BD678A is an ideal transistor for the audio enthusiast and circuit designer.

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