# Common base bias divider resistances

I have been trying to grok the common base amplifier, and put together a few examples, but there is something that I do not get at all. I have the setup below. Things are working more or less as expected, but I measured the resistance of the bias divider, and even though I have a $$\50k\$$ and a $$\100k\$$ resistor, I measure $$\\sim 33k\$$ across both parts of the bias divider. Why does this happen?

I suspect that $$\R_2\$$, $$\R_4\$$, and the BE junction make up one part of a divider somehow and $$\R_1\$$, $$\R_3\$$, and the BC junction make up the other. Am I on the right track? How can I calculate these results reliably?

I should mention (since it's probably nearing finals time for students) that I am not a student.

simulate this circuit – Schematic created using CircuitLab

• You are going to have a lot of trouble if you don't follow why the $50\:\text{k}\Omega$ resistor and the $100\:\text{k}\Omega$ resistors set between two power rails make up $50\:\text{k}\Omega\mid\mid 100\:\text{k}\Omega\approx 33.3\:\text{k}\Omega$. That's a basic element you need to follow right away. Next is that the common base arrangement should, if oriented well for reading, look a lot like a CE arrangement except that the signal is removed from the input capacitor with that end of the capacitor then grounded, instead. – jonk Apr 23 at 21:22
• Evidently it's been a long day. Clearly 50k || 100k = 33k. No clue what I was thinking there. – TrivialCase Apr 23 at 21:25
• So, are you interested how the common base functions? You could examine this answer I wrote, earlier. I've oriented the schematic better, so it might make more sense. – jonk Apr 23 at 21:26
• Well, yes. But here I thought something had gone mysteriously wrong because I was dumb and not thinking of the equiv value of the resistance in the divider for some reason. It's like if I had forgot how to add numbers while learning how to solve PDEs... – TrivialCase Apr 23 at 21:28
• Well, read the above link I provided and see if it helps. – jonk Apr 23 at 21:29