Active two-way current mirror
EDN Design Ideas (DI) published a design of mine in May of 2025 for a passive two-way current mirror topology that, in analogy to optical two-way mirrors, can reflect or transmit.
That design comprises just two BJTs and one diode. But while its simplicity is nice, its symmetry might not be. That is to say, not precise enough for some applications.
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Fortunately, as often happens when the precision of an analog circuit falls short, and the required performance can’t suffer compromise, a fix can consist of adding an RRIO op amp. Then, if we substitute two accurately matched current-sensing resistors and a single MOSFET for the BJTs, the result is the active two-way current mirror (ATWCM) as shown in Figure 1.
Figure 1 The active two-way current sink/source mirror. The input current source is mirrored as a sink current when D1 is forward biased, and transmitted as a source current when D1 is reverse biased.
Figure 2 shows how the ATWCM operates when D1 is forward-biased, placing it in mirror mode.
Figure 2 ATWCM in mirror mode, I1 sink current generates Vr, forcing A1 to coax Q1 to mirror I2 = I1.
The operation of the ATWCM in mirror mode couldn’t be more straightforward. Vr = I1R wired to A1’s noninverting input forces it to drive Q1 to conduct I2 such that I2R = I1R.
Therefore, if the resistors are equal, A1’s accuracy-limiting parameters (offset voltage, gain-bandwidth, bias and offset currents, etc.) are adequately small, and Q1 does not saturate, I1 = I2 just as precisely as you like.
Okay, so I lied. Actually, the operation of the ATWCM in transmission mode is even simpler, as Figure 3 shows.
Figure 3 ATWCM in transmission mode. A reverse-biased D1 means I1 has nowhere to go except through the resistors and (saturated and inverted) Q1, where it is transmitted back out as I2.
I1 flowing through the 2R net resistance forces A1 to rail positive, saturating Q1 and providing a path back to the I2 pin. Since Q1 is biased inverted, its body diode will close the circuit from I1 to I2 until A1 takes over. A1 has nothing to do but act as a comparator.
Flip D1 and substitute a PFET for Q1, and of course, a source/sink will result, shown in Figure 4.
Figure 4 Source/sink two-way mirror with a D1 flipped the opposite direction, and Q1 replaced with a PFET.
Figure 5 shows the circuit in Figure 4 running a symmetrical rail-to-rail tri-wave and square-wave output multivibrator.
Figure 5 Accurately symmetrical tri-wave and square-wave result from inherent A1Q2 two-way mirror symmetry.
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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