New full-wave precision rectifier has versatile current mode output

A classic analog application is the full-wave precision rectifier (absolute value) function. Many variations exist on this theme, each with its own supposed (often rather minor and sometimes downright dubious) advantages compared to competing topologies. This “new” (I haven’t seen it elsewhere) idea for a design shown in Figure 1 joins the mob with the following claims to fame. 

It requires only a single amplifier,
runs from a single supply voltage, and
provides an unusual (and sometimes very useful) current mode output.

Here’s how it works.

Figure 1 Full-wave precision rectifier incorporates complementary transistor pair and current mirror for current mode output.

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The incoming AC waveform (a sinewave is shown) is scaled by R1 and routed by A1 to the Q1/Q2 complementary pair, with the positive half-cycles going to Q1 and the negative to Q2. Q1’s share of the signal passes directly to the output (where it can be converted to a voltage signal by the optional R7 or left as a current—whichever the application requires). Q2’s negative half-cycle component is reflected and inverted by the Q3/Q4 temperature compensated current mirror into its positive image, then summed by simple parallel connection with Q1’s signal to produce the final, full-wave output.

Circuit performance parameters, precision, symmetry, and speed are good, mostly limited by the choice of op-amp and the precision of the R3/R5 resistor pair ratio. Their slightly non-unity ratio is intended to compensate for the slightly less-than-unity current gains of Q2 and the current mirror transistors.

If filtering of the output is required, it can be accomplished with a simple output capacitor of appropriate capacitance.

Should a negative rather than positive output current be preferred, this can easily be accommodated by the circuit modifications shown in Figure 2 including, of course, a negative instead of positive supply voltage.

Figure 2 A bit of rearranging and a change of supply polarity produces a negative output.

 For an example of the utility of a current mode output, see Figure 3. 

Figure 3 Current mode outputs can be easily summed up to produce a new function.

Here an incoming 1 kHz sine is split and phase shifted into two quadrature components by simple RC networks prior to rectification. A +45o phase-lead shift is provided by the Ra, Rb, Ca in the positive-signal path, while a -45o phase lag is introduced into the negative-signal path by Rc, Rd, and Cb. This results in a net 90o quadrature relationship between the two opposite polarity signal pathways.

When the full-wave rectified quadrature signals are summed, the result is an approximate (roughly +/- 3% nonlinearity) triangular waveshape at double the frequency of the sine, with amplitude proportional to the sine’s. 

Adjustment of the quasi-triangle’s amplitude is accommodated with a simple variable resistor.

Stephen Woodward’s relationship with EDN’s DI column goes back quite a long way. Over 100 submissions have been accepted since his first contribution back in 1974.

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