Timeline for Multistage common-emitter 2N4401 amplifier
Current License: CC BY-SA 4.0
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| Oct 25, 2023 at 19:57 | comment | added | periblepsis | @user350400 Thanks so much for taking a moment to let me know it mattered. :) | |
| Oct 25, 2023 at 19:54 | history | edited | periblepsis | CC BY-SA 4.0 |
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| Oct 25, 2023 at 18:17 | comment | added | user350400 | Thanks Periblepsis, that was very helpful and advances my knowledge of the design of a two stace CE amplifier when using global negative feedback. | |
| Oct 16, 2023 at 10:15 | history | edited | periblepsis | CC BY-SA 4.0 |
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| Oct 16, 2023 at 10:00 | history | edited | periblepsis | CC BY-SA 4.0 |
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| Oct 16, 2023 at 9:09 | comment | added | periblepsis | @Carl Thanks for the kind words. One of the benefits of BJTs over FETs is that they have significantly higher gm. Another is that their 'gate voltage' is a lot lower, so they can work well with lower Vcc. But of course they also have their problems -- notably their recombination base currents. | |
| Oct 16, 2023 at 9:03 | comment | added | Carl | Thanks for the details periblepsis! | |
| Oct 16, 2023 at 8:51 | comment | added | periblepsis | @Carl One last detail. The open loop gain is, itself, temperature-dependent. Especially that of the 2nd stage! So the 1875 varies with temperature. But if you plug in varying values here for the Avclosed = Avopen/(1+Avopen*NFB) equation, you'll find that it is remarkably stable over variations of Avopen. So the overall system is gain-stable over temperature, despite the fact that the open loop gain itself varies somewhat with temperature. Global NFB is a very, very powerful tool. | |
| Oct 16, 2023 at 8:37 | comment | added | periblepsis | @Carl This isn't the better topology, though. The only reason i selected if is because of the questioner being more familiar with these CE structures. (And my desire to copy the DC biasing twice.) Otherwise, I'd use the structures found in opamps, which provide far far greater open loop gain and far far better performance. | |
| Oct 16, 2023 at 8:29 | comment | added | periblepsis | @Carl You want and need as much open loop gain as you can possibly manage. Did you notice that i almost managed to get 2000? It's important! The reason for lower gain in the first stage is that i needed some finite and temperature independent resistance at AC against which to apply the global NFB. If I had also fully bypassed the first stage then I could not have managed temperature independence. So the first stage had to be handled this way. I could increase its gain somewhat but I'd pay for that in making the closed loop gain more dependent on temp. Trade-offs. | |
| Oct 16, 2023 at 8:13 | comment | added | Carl | What a great walkthrough - especially for those of us who haven't had the time (or courage) to work with bipolar junction transistors yet. One question: Why is it even necessary to bypass the emitter resistors in the first place? What would happen if you removed \$C_1\$ and \$C_2\$ from the circuit? | |
| Oct 16, 2023 at 3:26 | history | edited | periblepsis | CC BY-SA 4.0 |
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| Oct 15, 2023 at 20:45 | history | edited | periblepsis | CC BY-SA 4.0 |
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| Oct 15, 2023 at 20:03 | history | answered | periblepsis | CC BY-SA 4.0 |