The BJT (Bipolar Junction Transistor) in BiCMOS (Bipolar CMOS) works as a Voltage Controlled Current Source.

Explanation:

BJT is a three-layer semiconductor device that consists of two p-n junctions. It is a current-controlled device, and its output (collector) current is controlled by the input (base) current. BiCMOS technology combines the advantages of both bipolar and CMOS technologies. In BiCMOS, the BJT is used as an output stage to provide high current drive capabilities.

The BJT in BiCMOS operates in the active region, where the base-emitter junction is forward biased, and the collector-base junction is reverse biased. The collector current is proportional to the base-emitter voltage, and it can be expressed by the following equation:

Ic = βIb

where Ic is the collector current, Ib is the base current, and β is the current gain of the transistor.

In BiCMOS, the BJT is connected in a common emitter configuration, where the emitter is grounded, and the collector is connected to a load resistor. The base is connected to a voltage source through a current limiting resistor. The voltage across the base-emitter junction controls the base current, which in turn, controls the collector current.

Therefore, the BJT in BiCMOS works as a Voltage Controlled Current Source (VCCS), where the output current is proportional to the input voltage. The voltage gain of the VCCS can be expressed as:

Av = ΔIc / ΔVbe

where Av is the voltage gain, ΔIc is the change in collector current, and ΔVbe is the change in base-emitter voltage.

In summary, the BJT in BiCMOS works as a Voltage Controlled Current Source, where the output current is proportional to the input voltage. The voltage gain of the VCCS is determined by the current gain of the transistor.