Check the curves to see the behavior. What happens is that the semiconductor material gets saturated with the base current. You cannot get more conductivity. Increasing the base current will not help. Typically a voltage of 0.2 to 0.3 Volt remains.
A transistor can be in three conditions or states. It can be active (at a voltage higher than the emitter), in saturation or cut off (no current).
output current is zero
This is a particular transistor amplifier configuration. In general, the input signal is applied to the base, the collector is connected to a supply voltage, and the output is taken between the emitter and power supply common. One of the characteristics of the emitter follower is the output voltage "follows" the input, but the output is reduced by the Vbe voltage (the voltage drop between base and emitter, approximately 0.7 V for a silicon bipolar transistor).
When facing the flat side of the transistor, the Emitter - E - is on the left.
The emitter bypass capacitor in a common emitter amplifier will have less resistance as the frequency increases. Since gain in this configuration is collector resistance divided by emitter resistance (within limits of hFe), the gain will thus increase for higher frequencies, making this into a high pass filter.
Collector-emitter saturation voltage refers to the voltage drop across the collector-emitter junction of a transistor when the transistor is in saturation mode. It is the minimum voltage required to keep the transistor in saturation, where the transistor is fully turned on and conducting maximum current.
The emitter resistor in a common emitter configuration provides negative feedback to the transistor, reducing both its voltage gain and distortion.
Common Emitter Configuration has maximum impedance.
Emitter, Collector and Base cutoff region, saturation region, and liner region
DC current gain is collector-emitter current divided by base-emitter current. In linear mode, gain is beta, or hFe. In saturation mode, however, the transistor is over-driven and you can no longer relate collector-emitter current to base-emitter current. The transistor operates like a switch, and collector-emitter current is a function of voltage and load impedance only. (Ignoring the relatively small voltage drop.) To maintain saturation mode, the collector-emitter current must be smaller than the base-emitter current times hFe. Often, it is several times smaller, because hFe can vary from transistor to transistor, and your design must account for this variability.
In a common emitter (CE) configuration, the output characteristics used for switching action are primarily the active region and the saturation region. When the transistor is in the active region, it can amplify signals, while in the saturation region, it acts as a closed switch, allowing maximum current to flow. The transition between these two regions is crucial for effective switching, where the transistor is turned on (saturation) or off (cut-off) to control the output.
A transistor can be in three conditions or states. It can be active (at a voltage higher than the emitter), in saturation or cut off (no current).
Common Emitter - Class A Amplifier.
Forward saturation in a BJT occurs when the ratio of collecter-emitter current and base-emitter current reaches hFe or dc beta. A that point, the BJT is no longer operating in linear mode.
A simple test to determine if a transistor is in cutoff or saturation is to measure the voltage across the collector-emitter (V_CE) terminal. If V_CE is close to the supply voltage, the transistor is likely in cutoff, indicating it is off. Conversely, if V_CE is very low (typically below 0.3V for a silicon transistor), the transistor is in saturation, meaning it is fully on and allowing maximum current to flow.
common emitter using fixed bias
output current is zero