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Every TO-92 BJT I've ever seen reads collector-base-emitter when read left-to-right facing the flat side. Then I found this lovely part, BC517. It reads emitter-base-collector.
Have I only been exposed to a strangely homogeneous subset of TO-92 transistors, or is this one really an exception? If it's really a weird transistor, what possible technical reason is there for defying what I understood to be a universal standard?
The most important thing to recognize in this circuit, is that the current through R1 and R2 is not equal and that everything can be solved algebraically. Let's just write down everything we know and see where we land. In this notation, \$I_x\$ means the current through x
This system of equations has too many unknowns: \$I_{R_2}, I_B, I_C, V_{BE}\$ and \$V_{CE}\$. We need to either find a fourth independent equation or guess one of them. I assume that you are being expected to assume that \$V_{BE}\$ is constant, as per your statement that it is 0.7V. This allows you to solve this system of equations:
The base emitter (diode) junction is forward biased just like the two diodes you added and so, in effect, you get 3 lots of variations in junction voltage (for a given current) with temperature. The answer is no, I'm afraid not. Here's how a typical diode alters its voltage, for a given current, against temperature AND, the same is true of the base-emitter junction: -
Three different operating currents produce surprisingly similar slopes that tell you that a diode's forward voltage drop largely reduces at 2mV for a one degC rise in temperature. Resistors don't do this of course and you also have to consider that a diode has got a very low dynamic resistance once biased at some arbitrary operating point of a few hundred micro-amps upwards. This dynamic resistance is lower than a typical emitter resistor: -
Look at the right hand portion of the diagram - I've drawn two horizontal lines at 10mA and 15mA with the corresponding forward voltage drops. The difference (deltas) allows you to calculate the dynamic resistance = 0.1 volts / 5mA = 20 ohms i.e. probably less than the emitter resistor you might choose BUT, it changes with current so you get more gain than you bargained for (gain harder to define) and high signal non-linearity (distortion).
Setting the operating point of the collector at about half the supply voltage is useful to be able to obtain the maximum swing of signal (in terms of Vp-p) at the collector i.e. one side of the output signal doesn't clip much earlier than the other. There are subtleties here but that's the basic rule to maximize output amplitude and no, neither does this affect temperature stability.
Either use negative feedback (with care) or use an emitter resistor to lower the gain of the common emitter amplifier.
Best Answer
There are historical reasons for the E-B-C configuration. Early junction transistors had Emitter and Collector pellets diffused onto either side of a Germanium Base, so the wires naturally came out of the case in that order.
Silicon planar transistors have the Collector connected to the substrate, with Base and Emitter connections on the top side. In a TO92 case it makes more sense to have the Collector in the middle with Base and Emitter leads either side of the substrate.
American and European manufacturers carried over the E-B-C order from diffused to planar, but the Japanese changed to B-C-E. That is why many 'BC' and '2N' series TO-92 transistors are E-B-C, but '2S' series are B-C-E.