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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.

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ 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. \$\endgroup\$ – jonk Apr 23 at 21:22
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    \$\begingroup\$ Evidently it's been a long day. Clearly 50k || 100k = 33k. No clue what I was thinking there. \$\endgroup\$ – TrivialCase Apr 23 at 21:25
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    \$\begingroup\$ 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. \$\endgroup\$ – jonk Apr 23 at 21:26
  • \$\begingroup\$ 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... \$\endgroup\$ – TrivialCase Apr 23 at 21:28
  • \$\begingroup\$ Well, read the above link I provided and see if it helps. \$\endgroup\$ – jonk Apr 23 at 21:29
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schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Thanks, I'll be accepting this to close the question. I know how dividers work, for some reason (lack of sleep?) I didn't recognize that the value should be 33k here... Sorry for the dumb question \$\endgroup\$ – TrivialCase Apr 23 at 21:27

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