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.
It is evident now that the HTML version does not contain the beta term anywhere - neither in the equations nor in the text. I saw beta missing also in this line in the HTML version.
"The transistors’ ideal forward was 200." - in the HTML version above.
"The transistors’ ideal forward beta was 200". - in the actual print in the magazine and proofs forwarded to me.
Thanks once again for pointing out and for the benefit of the subsequent readers.
A. Lahiri -November 02, 2009
The reference given was for some other article. It is my mistake. Kindly refer to the magazine text for the correct equations for the outputs.
In general, both the output voltage and output current in either mode is multiplied by gain factor of beta.
By A. Lahiri
A. Lahiri -November 02, 2009
You are absolutely correct regarding this. Infact, the proofs given to me in the PDF version have all the equations written correctly, i.e all output voltages and currents are multiplied with beta.
I do not understand why this HTML version does not feature the beta in the equations. Nevertheless, it is all correct in the print version in the magazine.
Regarding your other comment: The circuit has been simulated in the ntyp-ptyp corner at 27C. PVT variations are important part of the design, but this is just a concept which could be further extended!
A. Lahiri -November 02, 2009
if z+ & z- are truly current outputs, then V on R2 is Iz+ X beta Q1 X R2 or Iz- X Beta Q2 X R2. Since Beta varies widely and drifts with temperature so will the output. The circuit will work with diodes. The author needs to check his spice model for the CCII.
tkachline -October 26, 2009
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