In the case of the one-quadrant amplifier, the motor can only be driven in one direction (e.g. braking by the amplifier is not possible) whereas with the 4-quadrant amplifier it can be driven and braked in both directions. All amplifiers include a speed control device with cascade current control which guarantees that speed and torque are constant according to the adjusted set values at varying loads. A speed range of up to 1:10,000 is possible for some types of amplifiers, however, the standard range is 1:3,000. In general, the amplifiers are suitable for tacho regulation, but some types can also be used with EMF- or IxR regulation.
Linear amplifiers: their final stage is constructed in such a manner that the voltage required to maintain a certain speed is permanently available at the motor. This linear application requires large heat sinks to prevent the transistors, which are permanently on line, from overheating. Thus, these amplifiers are very large and have a very restricted capacity range. The advantage of these appliances is that they do not cause electrical or acoustic interference. Linear amplifiers are therefore recommended for use in laboratories and medical appliances.
Unlike linear amplifiers, pulse-width-modulated amplifiers work on the principle of the final stage transistors being either switched through or being totally inhibited. The desired output voltage is achieved by changing the pulse width that determines the "on" and "off"-time of the final stage transistors. The clock frequency is between 8.5 and 19kHz, depending on the type of amplifier, the advantage being that the heat sinks are much smaller than those of linear amplifiers with the same capacity and that the capacity range is higher. Moreover, the p.w.m. amplifier is available at a lower price than a linear amplifier with the same capacity. The high clock frequency does, however, involve slight electrical and acoustic interference, such as the familiar "whistling", but these interferences are nowadays minimal and can be shielded.
Pulse-width-modulated amplifiers for brushless (EC-) motors: generally, the same applies as for the p.w.m. amplifiers. However, these amplifiers are those with the most complicated electronics. This is due to the fact that, unlike conventional amplifiers, this type requires three current regulators and a rotor position encoder on account of the special construction of the motor. The p.w.m.-amplifiers and the suitable motors must be connected with the system earth by a cable with a diameter of at least 2.5 mm2. For each amplifier, a galvanically separated power supply is needed. In addition to these protective measures also follow the regulations of the EMC.
Pulse-width-modulated amplifiers for brushless motors with resolver: for these amplifiers, the same applies as to the amplifiers for brushless motors with rotor encoder. However, in this case the motor is supplied continually in the form of a sine wave - as compared to the step function of the above mentioned amplifier -. The amplifier can also output encoder signals which are created from the resolver signals.