what is the difference between...β(dc) and β(a.c)?

Basically β of a transistor is the forward current gain or the hFE which is responsible for the amplification factor in transistors.

A Bipolar Junction Transistor (BJT) when operated with DC inputs, the levels of its collector current Ic and base current Ib are related by the term beta (β), and expressed as:

β(dc) = Ic/Ib = collector current / base current

Practically, transistors may exhibit beta levels anywhere between 50 to 400, depending upon their classification. An increased beta level corresponds to an increased forward current gain of the device, for example a transistor with a beta of 200 may be capable of producing collector currents 200 times more than the applied base current. However, transistors may show considerable difference in their beta levels despite having the same lot and code numbers.

The datasheets showing transistor specifications typically indicate beta as hFE, where the letter h is derived from an equivalent hybrid circuit, while the subscripts F and E are extracted from forward-current amplification and common-emitter configuration respectively.

For alternating potentials beta AC is expressed as:

β(ac) = ∆Ic/∆Ib│V(ce) = constant.

Formally, beta AC (β(ac)) is the factor that decides the common-emitter forward–current amplification of the device. This becomes obvious because the collector current is normally the output current for transistors with common-emitter configurations.

a bit more practical info regarding this is there in the link below.

http://forum.allaboutcircuits.com/showthread.php?t...

Dc Current Gain

dc means direct current while a.c. is alternating current.

Direct current means the flow of electric charges (current) is only from End 1 to End 2 of electrical source.

Alternating current means the current continuously changes, so sometimes the current would flow from End 1 to End 2, then later End 2 to End 1, and so on.

Always when i ask a question, even if it is the easiest one, they cannot provide me a good informed answer here. What happened to people that actually make the effort to answer?

Interesting, I was wondering the same thing myself