Electronic Design

Create A Mixed-Mode Precision Rectifier With Second-Generation Current Conveyors

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:

Iy = 0

Vx = Vy

Iz+ = Ix

Iz- = -Ix

V1 and I1 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 RL is the load resistance, the values of VO and IO for voltage-mode operation (I1 = 0) are:

Vo = (V1R2RL)/(R1R3)

Io = (V1R2)/(R1R3)

In current-mode operation (V1 = 0):

Vo = (I1R2RL)/R3)

Io = (I1R2)/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:

Rx = VT/2IB

where IB is the bias current and VT 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, I1, 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.

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