Timeline for LTSpice is frustrating me. Why isn't this Class C amplifier working? I can't get a sinusoudal output
Current License: CC BY-SA 4.0
22 events
| when toggle format | what | by | license | comment | |
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| Jul 11, 2021 at 8:56 | answer | added | qrk | timeline score: 0 | |
| Jul 11, 2021 at 5:00 | comment | added | user290763 | output impedance of the transistor is swamping the tuned tank circuit at the collector. I constructed a circuit per SPICE and it wouldnt tune. | |
| Jun 23, 2021 at 6:12 | answer | added | Sean O'Connor | timeline score: 1 | |
| Aug 12, 2018 at 4:32 | vote | accept | niko20 | ||
| Aug 12, 2018 at 4:32 | vote | accept | niko20 | ||
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| Aug 11, 2018 at 1:21 | vote | accept | niko20 | ||
| Aug 12, 2018 at 4:32 | |||||
| Aug 11, 2018 at 0:03 | answer | added | glen_geek | timeline score: 2 | |
| Aug 10, 2018 at 23:34 | comment | added | glen_geek | A pretty good model of these hard-driven amplifiers is a fast switch that shorts the resonator to ground for a short period (perhaps up to half a cycle). Yes, the collector waveform isn't a pretty sinusoid, and multipole lowpass/bandpass filters are needed to suppress harmonics sufficiently. That approach is often taken. Beware - filters are analyzed with SPICE's AC analysis (which is entirely linear), but the switch model is not-at-all a linear signal source. | |
| Aug 10, 2018 at 23:12 | comment | added | niko20 | @glen_geek I'm definitely trying some of those things, but so far no dice just yet. It seems like Class C may need more "tuning" than any of the sources on the internet seem to indicate. What's weird about it is then why have the LC circuit in here at all, just have it be a square wave and follow the circuit output with a bunch of filters | |
| Aug 10, 2018 at 22:58 | comment | added | Buck8pe | Would killing the Q of the resonator work (adding some resistance in series with L maybe)? | |
| Aug 10, 2018 at 22:47 | comment | added | glen_geek | Your 2N2222 pump is pushing a fairly large pulse of current into the resonator, but the load resistor (R2?) isn't extracting enough energy out of the resonator. If a sine wave is what you desire at the collector, either extract more energy with a smaller R2, or increase supply voltage (the 2N2222 can't take a whole lot more), or decrease the base drive. Or some combination of the three. | |
| Aug 10, 2018 at 22:47 | comment | added | The Photon | I don't think the problem is LTSpice. You have a low impedance source connected to the base of Q1, that means the collector of Q1 will have low impedance whenever you try to pull it below the base. You probably need to decouple your output from this sometimes-low-impedance node before you can recover the full sinusoid. | |
| Aug 10, 2018 at 22:28 | history | edited | niko20 | CC BY-SA 4.0 |
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| Aug 10, 2018 at 22:16 | comment | added | niko20 | @ThePhoton yes , it is expected. How Class C works is the LC network in the collector is supposed to then "ring" and basically reformulate the original sine wave. That's why Class C amplifiers are so efficient, they only stay on a short part of the cycle. Then you have LC networks to "filter" and form the clean sinewave output. This circuit is pretty much textbook of how Class C amplifiers are presented, but yet it doesn't seem to work..I have lots of trouble with LTSpice lately argh! | |
| Aug 10, 2018 at 22:12 | comment | added | The Photon | I am not an expert on class C amplifiers either, but this says that clipping half the cycle of the output is expected behavior. (notice the mention of a "suitable tuned load" toward the bottom of the page) | |
| Aug 10, 2018 at 22:06 | answer | added | niko20 | timeline score: 1 | |
| Aug 10, 2018 at 21:56 | comment | added | niko20 | @Transistor yes, I also "stepped" the capacitor value in the LC circuit and never found a value that gave me a sine wave. | |
| Aug 10, 2018 at 21:47 | comment | added | Transistor | I have a tremendous fund of ignorance about Class-C amplifiers but doesn't 150 pF and 100 µH resonate at 1.3 MHz? | |
| Aug 10, 2018 at 21:45 | answer | added | D.A.S. | timeline score: 0 | |
| Aug 10, 2018 at 21:40 | review | First posts | |||
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| Aug 10, 2018 at 21:39 | history | edited | The Photon | CC BY-SA 4.0 |
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| Aug 10, 2018 at 21:37 | history | asked | niko20 | CC BY-SA 4.0 |