D1 and D2 are normal diodes in forward direction[1], in series they drop 2⋅0,7V = 1.4V. R2 just pulls them down. So the cathode of D2 will sit 1.4V below supply voltage.
Q1 in this schematic is acting as a current source: If conducting a reasonable amount of current, a transistor needs a base-emitter voltage of Ube = 0.6V. This is a PNP transistor[2], so base needs to be 0.6V below emitter, hence the voltage over R1 will be U(d1d2) - Ube = U(R1) = 0.8V. The current then is of course set (to first order, ignoring the base-current contributing) to be U(R1)/R1.
Why such a "complicated" circuit and not only use R1 to bias D3? Because a current source has a much higher (theoretically: infinite) impedance: Z=dU/dI, but dI=0 (stays constant) whatever the change in voltage (dU > 0) might be.
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1) forward direction: A diode that is operated, like a LED supposed to light, will have a certain voltage drop. For a standard silicon diode, this is 0.7V, a red LED would have 1.6V or so.
2) PNP transistor: A transistor has three terminals, base is the cross-bar, emitter is the diagonal line with the arrow, collector is the other one. a NPN transistor runs on a base current (technical direction + to -) into the base terminal and conducts current from emitter to collector, a PNP transistor runs the reverse.
Edit: emitter/collector mixup
Q1 in this schematic is acting as a current source: If conducting a reasonable amount of current, a transistor needs a base-emitter voltage of Ube = 0.6V. This is a PNP transistor[2], so base needs to be 0.6V below emitter, hence the voltage over R1 will be U(d1d2) - Ube = U(R1) = 0.8V. The current then is of course set (to first order, ignoring the base-current contributing) to be U(R1)/R1.
Why such a "complicated" circuit and not only use R1 to bias D3? Because a current source has a much higher (theoretically: infinite) impedance: Z=dU/dI, but dI=0 (stays constant) whatever the change in voltage (dU > 0) might be.
-------------------
1) forward direction: A diode that is operated, like a LED supposed to light, will have a certain voltage drop. For a standard silicon diode, this is 0.7V, a red LED would have 1.6V or so.
2) PNP transistor: A transistor has three terminals, base is the cross-bar, emitter is the diagonal line with the arrow, collector is the other one. a NPN transistor runs on a base current (technical direction + to -) into the base terminal and conducts current from emitter to collector, a PNP transistor runs the reverse.
Edit: emitter/collector mixup