One traditional set you’ll see in most classic thriller motion pictures is an interrogation room with a “two-way” mirror on one wall. This cool gadget lets the witnesses see the suspects from one other room whereas the suspects can see solely themselves. Which path the two-way mirror works in is set by which aspect has extra mild. The intense aspect is the suspect’s mirror. The dim aspect is the witness’s window.
So, what does that must do with electronics?
This straightforward design thought describes a two-way present mirror that, in doubtful analogy to the optical type, can mirror or transmit in accordance with whether or not the enter or output aspect is extra constructive. It includes simply two BJTs and one diode and is highlighted yellow in Determine 1’s 555 triangle wave VCO.
Determine 1 D1, Q3, and This autumn comprise the two-way present mirror. It passively conducts Q2’s collector present to C1 by way of Q3’s base-collector when OUT versus C1 is destructive, however turns into an lively inverting unity-gain present supply when OUT goes constructive and reverses the polarity.
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Right here’s the way it works.
Let’s name the D1 node the enter, and the C1 node the output. When U1’s OUT pin is low, D1 and the mirror are reversed biased. Now, Q2’s collector present I has nowhere to go besides to forward-bias Q3’s base-collector junction, as proven in Determine 2. This connects Q2 to C1 so present I linearly ramps C1 destructive.
Determine 2 When OUT goes low, This autumn is off whereas Q3 saturates and transmits destructive present I to C1. Preserve Vin < 1 V so Q2 gained’t saturate.
When C1 descends to U1’s set off voltage (1.33 V), OUT transitions constructive. This swaps polarity throughout the mirror, ahead biasing it, pulling the emitters of Q3 and This autumn constructive relative to C1. Q3 and This autumn then assume the function of a traditional lively unity-gain present mirror. They now mirror an inverted constructive model of I to C1, making it ramp constructive, as proven in Determine 3.
Determine 3 With OUT excessive, the mirror goes lively and inverts I right into a constructive present to cost C1.
When C1 prices as much as U1’s threshold (3.67 V), OUT snaps destructive once more and a brand new oscillation cycle begins, ending output of a (theoretically) symmetrical isosceles triangle waveshape. Preserve Vin < 1 V.
Particulars, together with strengths and weaknesses, of the traditional two-transistor present mirror may be present in quite a few electronics design references.
After all, a complementary model of the two-way mirror may be made with NPN transistors. If a present supply replaces Q2’s present sink, it should work equally properly.
You may be questioning what’s D1 for? When OUT goes low, each transistor emitters are reverse-biased, so no present ought to circulate by means of the enter node anyway. Due to this fact, D1 is superfluous, proper?
Effectively, no, it isn’t. The reason being summed up within the time period “reverse beta.” It seems that whenever you reverse-bias a BJT’s base-emitter junction and forward-bias its base-collector, important present circulate is feasible—even seemingly—from collector to emitter. The related present acquire of this upside-down configuration is all the time means decrease than regular beta, nevertheless it’s nonetheless greater than leakage and undoubtedly too important to disregard. We want D1.
That wraps up the whodunit of the two-way present mirror. Thanks for studying. Sorry if it’s outdated information to you. It was new information to me after I considered it, and I hope to point out extra functions for it within the close to future.
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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