# ac & dc in circuits

Discussion in 'General Electronics Chat' started by ronn, Dec 20, 2007.

1. ### ronn Thread Starter Member

Nov 1, 2007
16
0
hi everyone,
what is happening if both ac & dc is present in a circuit like an amplifier?
how both of these affect the workings of the circuit?
thanks,
ron

2. ### Salgat Active Member

Dec 23, 2006
215
1
At the most basic level, the AC rides the DC which acts as an offset. The negative part of the signal would reduce the voltage while the positive part of the AC signal would increase the voltage. Sometimes in amplifiers you'll have a DC voltage present at the base of a transistor, and the AC signal will ride that DC offset in order to avoid cutoff (which would remove half the signal at the output of the transistor).

3. ### David Bridgen Senior Member

Feb 10, 2005
278
0
Both a.c. and d.c. are always present in an amplifier.

Without a signal there are various point in the circuit which will have a standing d.c. The base of a transistor, for example, as stated by Salgat, and at its collector.

Upon the application of a signal there is an a.c. component added to these d.c. ones. Think of it as the d.c. being modulated by the signal.

Any d.c. component at the output of the amplfier, in the case of a single-supply, for example, will be blocked from the load with a capacitor and only the a.c. component passed to it.

4. ### Dave Retired Moderator

Nov 17, 2003
6,960
171
Further to the above, if you were to take the Fourier Transform (essentially maps the time-domain to frequency-domain) of the signal in question, you would see a component at f = 0, i.e. DC, along side a series of harmonics at various frequencies. Therefore, as has been stated above, AC and DC invariably can exist together in a signal (though not always).

Dave