Timeline for Why my RC-oscillator design does not produce any output?
Current License: CC BY-SA 4.0
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| Jul 16 at 7:52 | comment | added | periblepsis | @ScienceDiscoverer The DC bias resistor pair current should be about 10 times the base current. Since the base current is about a factor of 150 to 350 less than the collector current, this means the bias current should be about 15 to 35 times less than the collector current. \$100\:\mu\text{A}\$ is in that ballpark. This stuff can be done in one's head almost as fast as one can type. It's not complex -- if and only if you are reasonably familiar with bipolars. But others may make other choices. The nice thing is that there is a range of practicality. So back of envelope stuff is good enough. | |
| Jul 16 at 7:49 | comment | added | periblepsis | @ScienceDiscoverer Most small signal bipolars sport \$\beta\$ in the range of about 150 to about 350. Something like that. \$r_\pi\$ is \$\beta\$ times the thermal voltage divided by the collector current. And it is only a part of the input resistance. However, in the case here I made sure it was the most important part -- the rest doesn't matter much. So it dominates because I made it dominate. Don't count on it, elsewhere. Just here. As the collector current is around \$3\:\text{mA}\$ and I know the thermal voltage is a factor of 10 larger, I know \$r_\pi\$ is about 10 times \$\beta\$. | |
| Jul 15 at 17:04 | comment | added | ScienceDiscoverer | I will try to build your rendition of the circuit as soon as I'll get some 2N2222s. But for now I have a few questions about your answer: 1. r[pi] is input resistance, right? 2. How do you know that for 100 <= beta <= 300 it must be 1k-3k Ohm? 3. How do you know that DC bias current must be >= 100 uA? | |
| Jul 14 at 13:22 | vote | accept | ScienceDiscoverer | ||
| Jul 14 at 8:18 | history | edited | periblepsis | CC BY-SA 4.0 |
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| Jul 14 at 3:48 | history | answered | periblepsis | CC BY-SA 4.0 |