text: Resistive-capacitive (RC) coupled amplifier, using a network of resistors and capacitors By design these amplifiers cannot amplify DC signals as the capacitors block the DC component of the input signal. RC-coupled amplifiers were used very often in circuits with vacuum tubes or discrete transistors. In the days of the integrated circuit a few more transistors on a chip are much cheaper and smaller than a capacitor.
  Inductive-capacitive (LC) coupled amplifier, using a network of inductors and capacitors This kind of amplifier is most often used in selective radio-frequency circuits.
  Transformer coupled amplifier, using a transformer to match impedances or to decouple parts of the circuits Quite often LC-coupled and transformer-coupled amplifiers cannot be distinguished as a transformer is some kind of inductor.
  Direct coupled amplifier, using no impedance and bias matching components This class of amplifier was very uncommon in the vacuum tube days when the anode (output) voltage was at greater than several hundred volts and the grid (input) voltage at a few volts minus. So they were used only if the gain was specified down to DC (e.g., in an oscilloscope). In the context of modern electronics developers are encouraged to use directly coupled amplifiers whenever possible. In FET and CMOS technologies direct coupling is dominant since gates of MOSFETs theoretically pass no current through themselves. Therefore, DC component of the input signals is automatically filtered.
  Frequency range
  Depending on the frequency range and other properties amplifiers are designed according to different principles.