Dont use the current mirror without emitter resistors; it will likely be inaccurate with discrete devices.
The current mirror as shown uses the operating point of Q2 (which is very controllable and stable) to set the current through Q1. The problem is that in this configuration, Q1's collector current is exponentially sensitive to Vbe. Any mismatch in characteristics between Q1 & Q2 will result in substantial current mismatch between Q1 and Q2. I believe a 26mV mismatch at room temp can produce a 2:1 mismatch in collector current. Circuit wiring will also be a factor because it only takes milliohms to contribute to mismatch. In order to match the currents between Q1 and Q2 independently of device variations, you need to add equal resistance at their emitters. If you are targerting, say 3mA, an emitter resistor of 1K will greatly improve matching. The reason why is that variations in Vbe are much smaller than the voltage drop across the emitter resistor. -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. To unsubscribe from this group and stop receiving emails from it, send an email to neonixie-l+unsubscr...@googlegroups.com. To post to this group, send an email to neonixie-l@googlegroups.com. To view this discussion on the web, visit https://groups.google.com/d/msgid/neonixie-l/04490d97-5cf6-4d2b-ba7f-bd27b3253f39%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
