Thanks Jan, that's it exactly 
50mv per diode doesn't sound like much but it is enough to change the current in the emitter. This is a simplified version based on Dougs circuit. The only thing that has changed between left and right is the resistor biasing the diodes. You can see the resulting change in the forward voltage dropped across each diode and yet the forward biased B-E junction shows a remarkably small change.
(I just tried a 2N3055 for curiosity, and that throws up a slightly unexpected result)
50mv per diode doesn't sound like much but it is enough to change the current in the emitter. This is a simplified version based on Dougs circuit. The only thing that has changed between left and right is the resistor biasing the diodes. You can see the resulting change in the forward voltage dropped across each diode and yet the forward biased B-E junction shows a remarkably small change.
(I just tried a 2N3055 for curiosity, and that throws up a slightly unexpected result
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