Why base emitter voltage is always constant?

The base-emitter voltage (V_BE) in a bipolar junction transistor (BJT) is typically around 0.7 volts for silicon transistors when in the active region. This voltage remains relatively constant because it is determined by the built-in potential of the semiconductor junction and the thermal voltage, which is influenced by temperature. As current flows through the junction, the V_BE adjusts slightly to maintain the balance of charge carriers, but it generally stays close to this value under normal operating conditions. Consequently, it is considered constant for practical purposes in circuit analysis.