Just lately (5/21/25), I revealed a Design Concept (DI) describing a bidirectional present mirror utilizing simply two transistors and a diode, “A two-way mirror—present mirror that’s.”
That circuit works nicely in some functions, just like the one proven within the DI itself, but it surely nonetheless has limitations because of numerous error-introducing results, a few of that are described on this reference.
One of many extra necessary elements of those is the “Early impact”, that kicks in to trigger important present imbalance when the voltage drop throughout the mirror turns into greater than a few volts. Fortunately, a repair for the Early impact was devised again in 1967 by George R. Wilson, an IC design engineer at Tektronix. Wilson’s easy but ingenious repair consisted of including a 3rd transistor to the essential present mirror, Q3 in Determine 1.
Determine 1 The Wilson mirror improves accuracy regardless of Early-effect by including Q3.
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Because of Q3, the distinction in voltage drop throughout the lively mirror transistors Q1 and Q2 by no means differs by multiple Vbe, which reduces Early impact error to lower than 1%. Determine 2 exhibits a variation of the essential two-sided present mirror incorporating Wilson’s accuracy-enhancing additional transistor.
Determine 2 A two-way present mirror with Wilson enhancement.
This benefit is exploited within the Determine 3 triangle-wave oscillator to realize good T-wave symmetry regardless of as a lot as 10v being utilized throughout the mirror.
Determine 3 D1, Q3, This autumn, and Q5 comprise the two-way Wilson present mirror. It passively conducts Q2’s collector present to C1 by way of Q3’s base-collecter when OUT versus Vc1 is detrimental, however turns into an lively inverting unity-gain present supply when OUT goes constructive and reverses the polarity. Preserve variable Vin < 4v so Q2 received’t saturate on detrimental output peaks.
The Wilson mirror is the higher selection for Determine 3, but it surely isn’t at all times the superior possibility. That’s as a result of the additional transistor provides an additional Vbe to the minimal voltage drop when the mirror is lively. That is no downside for Determine 3 due to its operation from 15v, however can be a deal breaker within the earlier DI with its 5v-powered oscillator. Fortunately, for a similar cause, Early already wasn’t a lot of a difficulty there.
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Stephen Woodward’s relationship with EDN’s DI column goes again fairly a good distance. Over 100 submissions have been accepted since his first contribution again in 1974.
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