Active Half Wave Rectifier:

The Active half wave rectifier is shown in fig. 8.

Fig. 8	Fig. 9

If v_in is positive then output of the OPAMP becomes negative (the non inverting terminal is grounded). Thus diode D₂ conducts and provides a negative feedback. Because of the feedback through D₂ a virtual ground exists at the input. Thus diode D₁ acts as open circuit. The output voltage under this condition is given by

v_o = v - = 0.

If v_in goes negative, then output of the OPAMP becomes positive. Thus D₁ is conducting and D₂ is off. Thus, the circuit behaves as an inverting amplifier. The output of the circuit is given by

The resultant output voltage will be positive. If v in is a sinusoid, the circuit per­forms half wave rectification. The transfer characteristic of the half wave active rectifier is shown in fig. 9. The output does not depend upon the diode forward voltage (v_d). Thus, because of the high open loop gain of the OPAMP, the feedback acts to cancel the diode turn-on (forward) voltage. This leads to improved performance since the diode more closely approximates the ideal device.

Axis Shifting of the Half Wave Rrectifier:

The half wave rectified output waveform can be shifted along the v_in axis. This is done by using a reference voltage added to the input voltage of the rectifier as shown in fig. 10. This termed axis shifting. It adds or subtracts a fixed dc voltage to the input signal. This process shifts the diode turn-on voltage point. If a negative reference voltage, V_REF, is applied to the circuit, the diode turns on when the input voltage is still positive. This shifts the v_out/ v_in transfer characteristic to the right. If a positive reference voltage is applied, the v_out/ v_in transfer characteristic shifts to the left. These shifted characteristics are shown in fig. 10.

Fig. 10	Fig. 11

The input-output voltage characteristics can also be shifted up or down. This is termed level shifting and is accomplished by adding a second OPAMP with a reference voltage added to the negative input terminal as shown in fig. 11.

GOTO >> 1 || 2 || 3 || Home