In the context of a transistor operating in the active mode, the concept of voltage drop can be explained using Ohm's Law. Ohm's Law states that the voltage (V) across a resistor is equal to the product of the current (I) flowing through it and the resistance (R) of the resistor. Mathematically, Ohm's Law is represented as:
V = I * R
Now, let's apply this concept to a transistor operating in active mode. In active mode, a bipolar junction transistor (BJT) or a field-effect transistor (FET) is used as an amplifying device. For simplicity, we'll focus on a common-emitter configuration for a BJT transistor, which is widely used for amplification purposes.
In a common-emitter BJT configuration, we have a base-emitter junction (BE) and a collector-emitter junction (CE). The emitter is connected to the ground, and the transistor is connected in a circuit with a power supply (Vcc) and a load resistor (RL) in the collector circuit. The input signal is applied to the base-emitter junction, and the amplified output signal is taken from the collector-emitter junction.
The transistor operates in active mode when the base-emitter junction is forward-biased, allowing a current (IB) to flow from the base to the emitter. This current is responsible for controlling the much larger collector current (IC) that flows from the collector to the emitter.
Now, let's consider the voltage drop across the transistor when it is operating in active mode:
Voltage drop across the base-emitter junction (VBE):
When the base-emitter junction is forward-biased, it behaves like a diode, and a voltage drop occurs across it. The voltage drop across the base-emitter junction can be approximated to a constant value, typically around 0.6 to 0.7 volts for silicon-based transistors.
Voltage drop across the collector-emitter junction (VCE):
The collector-emitter junction behaves like a resistor when the transistor is in active mode. The voltage drop across this junction depends on the collector current (IC) flowing through the transistor and the resistance of the collector-emitter junction when it is forward-biased.
Using Ohm's Law, we can express the voltage drop across the collector-emitter junction as:
VCE = IC * RC
where RC is the effective resistance of the collector-emitter junction when it is forward-biased.
It's essential to note that in active mode, the transistor operates in a region where it is acting as an amplifier, and the collector-emitter junction is not saturated (fully on) or cut-off (fully off).
In summary, the concept of voltage drop across a transistor in active mode involves the voltage drop across the base-emitter junction (VBE) and the voltage drop across the collector-emitter junction (VCE), both of which can be analyzed using Ohm's Law in their respective regions of operation.