zero is the frequency of a dc voltage.
what is the frequency of a dc voltage?
zero
the term "dc" is used to refer to power systems that use only one polarity of voltage or current, and to refer to the constant, zero-frequency, or slowly varying local mean value of a voltage or current. for example, the voltage across a dc voltage source is constant as is the current through a dc current source. the dc solution of an electric circuit is the solution where all voltages and currents are constant. it can be shown that any stationary voltage or current waveform can be decomposed into a sum of a dc component and a zero-mean time-varying component; the dc component is defined to be the expected value, or the average value of the voltage or current over all time.
direct current
for voltage amplification, the range of allowed output voltage swing in this amplifier is tied to voltage gain when a resistor load "r" is employed, as in figure 1. that is, large voltage gain requires large "r", and that in turn implies a large dc voltage drop across "r". for a given supply voltage, the larger this drop, the smaller the transistor "v" and the less output swing is allowed before saturation of the transistor occurs, with resultant distortion of the output signal. to avoid this situation, an active load can be used, for example, a current mirror. if this choice is made, the value of "r" in the table above is replaced by the small-signal output resistance of the active load, which is generally at least as large as the "r" of the active transistor in figure 1. on the other hand, the dc voltage drop across the active load has a fixed low value (the compliance voltage of the active load), much less than the dc voltage drop incurred for comparable gain using a resistor "r". that is, an active load imposes less restriction on the output voltage swing. notice that active load or not, large ac gain still is coupled to large ac output resistance, which leads to poor voltage division at the output except for large loads "r" ≫ "r".
common base