What’s active rectification?

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Active rectification converts AC to DC with low distortion at low voltages. It is used for circuits handling low levels of AC, such as AM sensing. Synchronous rectification is used in alternators. Active devices like transistors and op amps can be used for active rectification. H-bridge can be used for full-wave rectification and bi-directional drive.

Active rectification is the process of converting alternating current (AC) to direct current (DC) with very low distortion at low voltage inputs. AC input levels between 0 and 0.7 volts (V) are a common problem when using silicon or germanium passive rectifiers. The ideal diode will turn on with zero voltage across the anode and cathode. In real circuits, however, there will be a forward voltage drop across the silicon diode of about 0.7V and about 0.3V for the germanium diode.

The use of active rectification is for circuits that handle low levels of alternating current, such as for specialized sensing of amplitude modulation (AM) signals. In AM, the radio frequency (RF) carrier has an average peak level, or envelope, which carries the modulation or information transmitted over the radio wave. After recovering the carrier, there is an AM sense 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 required.

In alternators, synchronous rectification is used instead of active rectification. Synchronous rectification is made possible by synchronous contacts called commutators. When a rotor winding is progressing to a positive going output, that rotor winding is rapidly connected or switched into the output terminal. When its voltage is below a certain minimum positive level, it is disconnected from the output. Several of these windings are used in turn for the output as the rotor continues to rotate, resulting in DC output without electronic rectifiers.

The active rectification technique can use active devices such as transistors and operational amplifiers. Inverting op amps with a diode in the feedback loop between the op amps output and the inverting input exhibit nearly ideal active rectifier characteristics. The very small forward current required to operate the mentioned diode will produce less than 0.01V distortion in the output signal, and this is a vast improvement over passive rectification, which requires at least 0.3V at the input. In converting electrical energy, 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 can provide a bi-directional drive to a load such as a motor or speaker. In active rectification applications, the H-bridge can be used to provide signal locking and impedance transformation needed for more stable circuit operation.

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