One basic 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 route 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 should do with electronics?
This straightforward design thought describes a two-way present mirror that, in doubtful analogy to the optical form, can mirror or transmit based on whether or not the enter or output aspect is extra optimistic. It contains simply two BJTs and one diode and is highlighted yellow in Determine 1’s 555 triangle wave VCO.
Determine 1 D1, Q3, and This fall comprise the two-way present mirror. It passively conducts Q2’s collector present to C1 through Q3’s base-collector when OUT versus C1 is detrimental, however turns into an lively inverting unity-gain present supply when OUT goes optimistic 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 detrimental.
Determine 2 When OUT goes low, This fall is off whereas Q3 saturates and transmits detrimental present I to C1. Maintain Vin < 1 V so Q2 received’t saturate.
When C1 descends to U1’s set off voltage (1.33 V), OUT transitions optimistic. This swaps polarity throughout the mirror, ahead biasing it, pulling the emitters of Q3 and This fall optimistic relative to C1. Q3 and This fall then assume the position of a traditional lively unity-gain present mirror. They now mirror an inverted optimistic model of I to C1, making it ramp optimistic, as proven in Determine 3.
Determine 3 With OUT excessive, the mirror goes lively and inverts I right into a optimistic present to cost C1.
When C1 costs as much as U1’s threshold (3.67 V), OUT snaps detrimental once more and a brand new oscillation cycle begins, ending output of a (theoretically) symmetrical isosceles triangle waveshape. Maintain Vin < 1 V.
Particulars, together with strengths and weaknesses, of the basic two-transistor present mirror might be present in quite a few electronics design references.
After all, a complementary model of the two-way mirror is also made with NPN transistors. If a present supply replaces Q2’s present sink, it’ll work equally effectively.
You could be questioning what’s D1 for? When OUT goes low, each transistor emitters are reverse-biased, so no present ought to stream by way of the enter node anyway. Subsequently, D1 is superfluous, proper?
Properly, 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 stream is feasible—even probably—from collector to emitter. The related present acquire of this upside-down configuration is all the time manner decrease than regular beta, but it surely’s nonetheless greater than leakage and positively too important to disregard. We’d like 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 indicate 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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