| Electronics > Beginners |
| AC Coupling and OP Amp Stages |
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| floobydust:
Generally speaking, once you have level-shifted there is no need for blocking capacitors downstream. That circuit looks fine. But... It's trickier for guitar effects because they are usually overdriven so the textbook no longer applies and there are other things to look at. When overloaded, some op-amps will phase-invert and blast out frequency-doubled (2nd harmonic). You can also get a DC-average (sub-harmonic and rectification) that can make one stage upset the bias of following stages, without coupling caps. Example is you overload the input stage and it has a net DC center or operating point move up a volt or two of the 1/2Vcc point. Without blocking caps, this now propagates to the amplification stages downstream. So stage two will now get saturated and cut out. If you have a diode-clipper, it is expecting a symmetrical waveform but will instead be getting a waveform centered up, so it would clip one side only. The art here is to have things sound reasonably like a guitar when overloaded. The "crunch" and "overdrive" sound is a lot of work to do after the basic clean signal path design is done. I'm saying technically you don't need interstage coupling capacitors but they can help a stage stay centered and in balance, instead of just saturating. |
| David Hess:
AC coupling is only required to the extent that DC offsets need to be controlled including the DC level shifting offsets in direct coupled amplifiers which can be quite high. Operational amplifiers typically have such low offset voltages that AC coupling is not required between stages. |
| Zero999:
--- Quote from: eev_carl on November 20, 2018, 07:14:59 pm --- --- Quote from: floobydust on November 20, 2018, 06:47:23 pm ---Your circuit is wrong --- End quote --- Thanks. Here's the circuit with the correct connection to Vcc/2. --- End quote --- The problem with that common emitter configuration is the bias point is highly dependant on the transistor's Hfe. Try changing the transistor model in LTSpice, run the DC operating point simulation and note how VBuffer changes. It will also change depending on the temperature. This is why a proper potential divider is used to bias a BJT circuit. The downside is it lowers the input impedance, but that can be overcome with bootstrapping. |
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