Precision Rectifiers:

Recall from basic circuit principles that a rectifier circuits can be implemented with a diode/diodes (half wave rectifier or full wave rectifier). The major limitations of these Precision Rectifiers circuits is that they cannot rectify voltages below V_D(ON) = 0.7 V, the cut-in voltage of the diode. In these circuits V_i has to rise to a threshold of the order of V_D(ON) before any appreciable change can be seen at the output.

Therefore, above this threshold we have V_o = V_i – V_D(ON) ≈ V_i – 0.7 V and below threshold V_o = 0V i.e.

Due to this, output of the conventional rectifier is distorted.

To achieve Precision Rectifiers we need a circuit that keeps V_o equal to V_i for V_i>0V. This can be achieved by using op-amp along with the diodes and these circuits are called Precision Rectifiers. These are used to precisely rectify voltages having amplitudes less than 0.7V.

Precision Half Wave Rectifiers:

There are two types of precision half wave rectifiers available,

1. Noninverting half wave rectifier
2. Inverting half wave rectifier

Noninverting Half Wave Rectifier:

Fig. 2.56 shows precision half wave rectifier. It consists of noninverting amplifier with diode D₁ in the feedback loop of an op-amp.

The analysis of this circuit can be done considering two distinct case V_i > 0 V and V_i < 0 V.

CASE 1 : V_i > 0 V : Recall that for closed loop op-amp V_p = V_n, due to virtual ground.

When V_i > 0, op-amp tries to keep V_o = V_n = V_p = V_i and it does this because forward biasing diode provides closed loop feedback path. The voltage drop across forward bias diode V_D(ON) = 0.7 V.

To accommodate the voltage drop across diode the op-amp swings about 0.7V higher than V_o, as shown in the Fig. 2.57.

CASE 2 : V_i < 0V : When V_i  < 0V, diode D₁ is reverse biased and the op-amp is working in the open loop, as shown in the Fig. 2.58.

Therefore, op-amp is no longer capable of keeping V_n = V_p.

With no current through resistance R we have V_o = 0.

the op-amp output V_oA is saturated at V_sat-.

The input and output waveforms are shown in Fig. 2.59.

Inverting Half Wave Rectifier:

Fig. 2.60 shows another popular circuit, inverting half-wave rectifier. It consists of two diodes and two resistors and op-amp is connected in the inverting configuration.

CASE 1: V_i > 0 : Recalling virtual ground concept we can say that V_p = V_n = 0V. For V_i > 0, V_i is positive with respect to V_n and hence current through R₁ flows from left to right. Only one path for this current to flow is through diode D₁. Hence diode D₁ is forward biased and diode D₂ is reverse biased. As current flow through R₂ is zero, V_o = V_n = 0V.

CASE 2 : V_i < 0 : For V_i < 0, V_i is negative with respect to V_n and hence current through R₁ flows from right to left. Only one path for this current to flow is through diode D₂ and resistor R₂, indicating that V_oA > V_n. Hence diode D₁ is OFF, and diode D₂ is ON. With these diode states, circuit acts like an inverting amplifier and output voltage is given as

If R₁ and R₂ are made equal, then we can write V_o = -V_i

The input and output waveforms are shown in Fig. 2.61.