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:

I_y = 0

V_x = V_y

I_z+ = I_x

I_z- = -I_x

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 R_L is the load resistance, the values of V_O and I_O for voltage-mode operation (I₁ = 0) are:

V_o = (V₁R2R_L)/(R1R3)

I_o = (V1R2)/(R1R3)

In current-mode operation (V₁ = 0):

V_o = (I₁R2R_L)/R3)

I_o = (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:

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, 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.