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A bias resistor built-in transistor (BRT), also called a digital transistor, is designed to be used as a switch. A BRT is a bipolar transistor containing a series base resistor (R1) and a base-emitter resistor (R2), making it possible to simplify on-board circuit design.
Typical bipolar transistors can be used as amplifiers, buffers, and switches. Amplifiers and buffers are used in the linear active region of the VCE-IC curves shown in Figure 2. In contrast, BRTs, which are mainly used as switches, operate in the region to the left of the active region and in the region below the active region (called the saturation and cut-off regions respectively). In the saturation region, the collector-emitter voltage drop (VCE) becomes the minimum, causing the maximum collector current to flow. In the cut-off region, the transistor is fully off. Under the cut-off condition, only a tiny collector cut-off current (ICEO) can flow from the collector to the emitter.
Because of the built-in bias resistor, a BRT operates in the saturation region when it is on and in the cut-off region when it is off.
The bias resistors are configured as shown in Figure 1. R1 converts the voltage applied to the B terminal into current to stabilize the BRT operation while R2 acts as a pull-down resistor when the BRT is off, pulling the base voltage to the GND level.
Without R2, the leakage current or the collector cut-off current (ICBO) flowing from the input in the “off” state might cause the BRT to malfunction because of the charge accumulated in the base. R2 helps prevent malfunction by passing leakage current to GND.