1.2) Nonsaturating Precision Half-Wave Rectifier Circuit
The precision rectifier circuit in Fig. 7-2 uses an inverting amplifier configuration.
Diode D1 is reverse biased and D2 is forward biased when the op-amp output terminal is positive.
This occurs when the input signal is negative.
While D2 is forward biased, the circuit output is
During the positive half-cycle of the input, the op-amp output terminal goes negative, causing D2 to reverse biased.
Without D1 in the circuit, the op-amp output would be saturated in a negative direction.
However, the negative voltage at the op-amp output forward biased D1.
So, the output settles at the voltage that keeps the inverting input terminal close to ground level.
In this case, that voltage is the diode forward voltage drop below ground, approximately -0.7V
The sole purpose of D1 is to keep the op-amp output from going into saturation and thus maintain the best possible circuit frequency response.