RECTIFIERS PERFORM AN IMPORTANT signal-processing function in many analog circuits. But conventional half-wave, full-wave, or bridge rectifiers employing diodes can’t be used with low-amplitude signals. The circuit described below is a mixed-mode precision rectifier that can handle both low-amplitude voltage and current signals.

Many previously described precision rectifiers use voltage op amps, but conventional voltage op amps suffer from a finite and dependent gain-bandwidth product and a low skew rate. Secondgeneration current conveyors (CCIIs) overcome these disadvantages and also provide a highoutput- impedance current terminal, which should somewhat reduce on-off transition problems (zerocrossing distortions).

The new precision rectifier circuit uses dual-output CCIIs and bipolar junction transistors (BJTs) and is a good fit for mixed-mode (both voltage and current) operation (*Fig. 1*). The CCIIs can be created from commercially available current-feedback amplifiers, like the Analog Devices AD844.^{1} The circuit is suitable for monolithic integration.

Details regarding the internal construction of CCIIs, which are active building blocks, are readily available.^{2,3} The block’s characterizing equations are:

I_{y} = 0

V_{x} = V_{y}

I_{z}+ = I_{x}

I_{z}- = -I_{x}

V_{1} and I_{1} are the input voltage and current to be rectified. The BJTs provide an inherent amplification. Considering that the npn transistors are matched and have the same current gain, , and that R_{L} is the load resistance, the values of V_{O} and I_{O} for voltage-mode operation (I_{1} = 0) are:

V_{o} = (V_{1}R2R_{L})/(R1R3)

I_{o} = (V1R2)/(R1R3)

In current-mode operation (V_{1} = 0):

V_{o} = (I_{1}R2R_{L})/R3)

I_{o} = (I_{1}R2)/R3

The amplification, not provided by conventional diode-based halfwave or full-wave rectifiers, is desirable since precision rectifiers handle low-amplitude signals.

This circuit can be modified to perform voltage control and current control. For voltage control, replace R1 and/or R3 by nonlinear, cancelled MOSFETs working in the triode region.^{4} For current tunability, current conveyors 1 and 3 can be replaced by current-controlled current conveyors (CCCIIs).5,6 These devices have parasitic resistances at terminal x that are tunable by the bias current:

R_{x} = V_{T}/2I_{B}

where I_{B} is the bias current and V_{T} is the thermal voltage.

PSpice simulation verified the circuit’s operation. For the simulation, V1 was 0 (current-mode operation), all external resistors were 1000 , and the input current, I_{1}, was a 1-kHz sinusoid with a 20 A p-p amplitude. The transistors’ ideal forward was 200.

The results showed a little distortion at the zero crossing, due mainly to the on-off switching of the transistors (*Fig. 2*). Also, the CCIIs’ voltage transfer gain from y to x and from z to w, as well as the current-transfer gain from x to z, all differed slightly from their ideal values of unity by very small voltage/ current tracking errors. This caused a slight deviation in the results.

References:

1. *Linear Products Data Book*, Analog Devices Inc., Norwood, Mass. (1990).

2. S.S. Rajput and S.S. Jamuar, “Advanced Applications of Current Conveyors: A Tutorial,” *Journal of Active and Passive Electronic Devices*, Vol. 2, p. 143-164, 2007.

3. E. Brun and O.H. Olesen, “Conveyors Implementations of Generic Current-Mode Circuits,” *International Journal of Electronics*, Vol. 73., No. 1, p. 129-140, 1992.

4. R. Senani, “Realization of Linear Voltage-Controlled Resistance in Floating Form,” Electronics Letters, Vol. 30, No. 23, p. 1909-1911, 1994.

5. A. Lahiri, “Oscillator Uses Dual-Output Current- Controlled Conveyors,” *EDN*, p. 62, Nov. 13, 2008; www.edn.com/article/CA6611645.html.

6. A. Lahiri, “Sinusoid Generator Uses Dual-Output Current-Controlled Conveyors,” *EDN*, p. 54-56, Jan. 22, 2009.