Active rectification is the process of converting alternating current (AC) into direct current (DC) with very low distortion in the low-voltage inputs. Input AC levels between 0 and 0.7 volts (V) are a common concern when using silicon or germanium passive rectifiers. The ideal diode will turn “on” with a zero voltage across the anode and the cathode. In real-world circuits, however, there will be a forward voltage drop across the silicon diode of about 0.7 V and about 0.3 V for the germanium diode.
The use of active rectification is for circuits that handle low levels of AC such as for specialized detection of amplitude modulation (AM) signals. In AM, the radio frequency (RF) carrier has an average peak level, or envelope, that carries the modulation or the information transmitted on the radio wave. After recovering the carrier, there is an AM detection circuit in the AM receiver that recovers the envelope. If the envelope is less than 0.7 V and demodulation is required, an active rectification circuit is needed.
In alternators, synchronous rectification instead of active rectification is utilized. Synchronous rectification is made possible by synchronous contacts referred to as commutators. When a rotor winding is progressing in a positive going output, that rotor winding is rapidly connected or commutated into the output terminal. Once its voltage is below a certain positive minimum level, it is disconnected from the output. Several of these windings are utilized for output in turn while the rotor keeps rotating, resulting in DC output without electronic rectifiers.
The active rectification technique may use active devices such as transistors and operational amplifiers. Inverting operational amplifiers with a diode on the feedback loop between the operational amplifiers output and the inverting input exhibit near ideal active rectifier characteristics. The very small forward current needed to operate the mentioned diode will produce a less than 0.01 V distortion in the output signal, and this is a big improvement over passive rectification, which requires at least 0.3 V input. In electrical power conversion, there is little need for active rectification due to the relatively high voltages being converted. Passive rectifiers that cause voltage losses of about 1.4 to 2 volts direct current (VDC) are acceptable since huge voltage margins are available from unregulated voltage sources.
H-bridge is a very useful circuit configuration. When used with diodes, the H-bridge becomes a full-wave rectifier. The DC outputs are taken where the anode-anode and cathode-cathode nodes connect. When used with transistors, the H-bridge is able to provide bidirectional drive to a load such as a motor or a loudspeaker. In active rectification applications, the H-bridge may be used to provide the signal clamping and impedance transformation needed for a more stable circuit operation.
