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whats current and volt enter into base npn transistor?

if i enter ac audio signal which has 0.073 volt into base of npn transistor through 2.5 dc bising volt now current entered into base of transistor is 0.073 ac+2.5 dc volt or not and what is the current of base npn transistor

Firstly, the 2.5v is probably the voltage of the base relative to ground, not the base-emitter voltage. I say this because the forward-biassed base-emitter voltage (Vbe) of a silicon transistor is always close to 0.6v.

This means that the emitter of your transistor is not connected to ground; this is normal – a common biassing circuit has a resistor between emitter and ground. But this in turn means that your question cannot be answered without more information. We need to know whether the emitter is decoupled to ground for ac signals by a large capacitor between emitter and ground. This is the case in a common form of common-emitter amplifier, so we could assume that it is.

In this type of amplifier, in order to answer your question, you need to know the small-signal input resistance hie. This is the slope of the Vbe/Ib characteristic at the point where Ib is the static base bias current. It is strongly dependent on the value of the static bias current Ib, so once again, we find that there is not enough information given for us to answer the question. We need to know the complete biassing circuit, and the transistor characteristics in order to be able to estimate hie.

However, you could make an informed guess. If the collector current is a few mA (typical of small-signal amplifiers) hie will be in the range 2-5k Ohms. Taking 2k, the signal current entering the base will be 37uA ac (rms if your 73mV is an rms figure).

The base current will have this as an ac component, together with a dc component which cannot be calculated without more information.