Ideal diode circuit for positive and negative output

Ideal diode circuit for positive and negative output

Function of the circuit

Commonly used diodes have a forward voltage drop and cannot rectify small signals. When the signal amplitude is large, if the ambient temperature rises, the rectified voltage will change accordingly, making it difficult to form a high-precision circuit.

An ideal diode circuit can obtain a zero-crossing diode characteristic, and this circuit can be implemented using the feedback circuit of an OP amplifier.

How the Circuit Works

OP amplifier A1 is an ideal diode circuit with negative output. Diode D1 is connected in series at the output end, and feedback is performed from the positive electrode of D1. For positive input signals, A1 only acts as a simple inverting amplifier.

When the input is negative, the output of OP amplifier A1 swings to positive, D1 is disconnected, and in order to ensure that it can work in an open loop state and prevent saturation, a diode D2 is connected to the output. The positive output of A1 is clamped by the forward voltage drop of the diode.

OP amplifier A2 is an inverting amplifier with a gain of 1. Its function is to invert the output of A1. If unipolar output is used, A2 can be removed.

The function of the circuit R3 and R6 is to eliminate the offset voltage by using the input bias current IE of the OP amplifier. If a FET input OP amplifier is selected, R3 and R6 can be removed.

Component Selection

In this circuit, the accuracy decreases with the increase of input frequency. This is because the accuracy depends on the open-loop frequency characteristics. The open-loop gain at the highest frequency is extremely critical. When the frequency is less than 10KHZ, a 4558 OP amplifier can be used. When it is below tens of Hz, TL085 or LF353N should be used. When it is greater than 10KHZ, a high-speed OP amplifier must be used. In order to reduce the influence of stray capacitance, the resistance of the feedback resistor should be reduced to 2~5K.

The diode can be a common switching diode for small signals, but it should be noted that some Schottky diodes have poor voltage resistance. The accuracy of the resistor depends on the allowable error of the circuit, and a metal film resistor within plus or minus 1% can be used.

Notes

Small signal diode circuit

The main uses of small signal silicon diodes are rectification and limiting. When used for high-frequency inspection waves, germanium diodes or Schottky barrier diodes are usually used. Their forward voltage drop is lower than that of silicon diodes, especially Schottky diodes. Although their voltage resistance is low, their switching speed is fast and their reverse recovery time is short, making them suitable for high-frequency or high-speed conversion circuits.

In addition to being used as a limiter, the forward voltage drop VF and temperature characteristics of the diode are issues worth considering in the waveform processing of analog signals. Therefore, this circuit puts it in the OP amplifier feedback loop to form a so-called ideal diode circuit. When used at low frequencies, its input and output transfer characteristics are ideal characteristics.

The biggest disadvantage of the ideal diode circuit is that its performance is improved by the feedback effect of the OP amplifier, forming a characteristic that depends on the open-loop performance. Therefore, the performance of the ideal diode circuit can be improved by using circuit design techniques, or selecting an OP amplifier with good frequency characteristics, or using discrete components to form a broadband amplifier with an open-loop gain of 40~60DB.