| Electronics > Projects, Designs, and Technical Stuff |
| bare bones charge pump |
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| John B:
Firstly, is this actually an audio circuit? If there's only a few op amp stages, a half rail solution can work, but they do get annoying when you want to add in more stages and volume control. 150mA is achievable with a charge pump, but the issue is the output impedance. I made a synchronous charge pump out of 2x 555 where the second is 180 degrees with the master timer. It reduces the ripple and output impedance. I think I drew a max of 200mA from it, but can't remember the negative voltage (it was being fed from +12V). You will get a lot of voltage drop maybe leaving you with only -4V or so. 100mA maybe quite workable, but then don't forget the voltage drop from the linear regulator. Also the output of a charge pump really needs an LC filter. |
| ledtester:
FWIW, this video covers charge pumps circuits to produce both negative voltages and voltage doublers/triplers: https://youtu.be/T47TYuLDtrs |
| OM222O:
Yes, it is somewhat an audio circuit :P I want to use my PC/Phone as a signal generator in a pinch (there are quite a lot of tone/signal generator apps available for that). I have already created the circuit to manipulate the signal which consist of multiple amplifier stages: First is a non inverting amplifier with adjustable gain(or a attenuator with unity gain follower) for the "volume" control. Then it is fed to a instrumentation amplifier (INA122 with a fixed gain of 5). Then there is a summing amplifier which is used to add a DC offset, followed by another inverting stage to get rid of the inversion in the summing stage. I know it's a bit too complicated but that signal is then used to test a few different circuits so I need control over all aspects of the waveform. The -12V rail will be used for the DUT as well, not just the op amps and the actual max current is about 50-60mA. I just want some margin and if I can use multiple oscillators and charge pumps to achieve 100mA, I think it's worth doing so, just in case I need to change the circuits which might consume more current. Edit: I tried simulating a push pull (class B amplifier) but the issue is that the switching voltage is only as high as the op amp voltage and as I mentioned, the MCP6002 can only be powered from 5V. I tried making a "Not gate" from an NPN and a 10K resistor which worked fine and boosted the PWM voltage to 0 to 12 instead of 0 to 5.I then followed that with a class B amplifier again and the output of that was correct too! I then tried connecting the inverting charge pump and LTspice crashed! I then tried putting a 1K load on the charge pump which only resulted in massive noise spikes and another crash! is there something I'm doing wrong with the simulations? How can I boost the output voltage without creating so much noise? using the resistor approach, I couldn't spot much noise, so I'm not sure why it exists? maybe that resistor was slowing the charge and discharge time of the capacitors and reducing the noise? |
| MagicSmoker:
--- Quote from: OM222O on July 15, 2019, 04:10:32 am ---...Then it is fed to a instrumentation amplifier (INA122 with a fixed gain of 5). --- End quote --- You go with an IA when you have to amplify a small differential signal in the face of high common mode voltage (a load cell in a Whetstone bridge, a high-side current shunt, etc); neither apply when a phone's headphone jack is your signal source: the signal level is high and there is no common mode voltage. --- Quote from: OM222O on July 15, 2019, 04:10:32 am ---...I then tried connecting the inverting charge pump and LTspice crashed! --- End quote --- Hmm, LTSpice has never crashed on me, though I have often brought it to its proverbial knees with Defcon 1 warnings and time steps dropping down into the picosecond range... You're probably going to have to post your LTSpice .asc file if you want others to help you on this, then. |
| OM222O:
--- Quote from: MagicSmoker on July 15, 2019, 09:26:00 am --- --- Quote from: OM222O on July 15, 2019, 04:10:32 am ---...Then it is fed to a instrumentation amplifier (INA122 with a fixed gain of 5). --- End quote --- You go with an IA when you have to amplify a small differential signal in the face of high common mode voltage (a load cell in a Whetstone bridge, a high-side current shunt, etc); neither apply when a phone's headphone jack is your signal source: the signal level is high and there is no common mode voltage. --- Quote from: OM222O on July 15, 2019, 04:10:32 am ---...I then tried connecting the inverting charge pump and LTspice crashed! --- End quote --- Hmm, LTSpice has never crashed on me, though I have often brought it to its proverbial knees with Defcon 1 warnings and time steps dropping down into the picosecond range... You're probably going to have to post your LTSpice .asc file if you want others to help you on this, then. --- End quote --- the INA122 is the lowest noise and offset op amp that I have on hand. it's probably overkill, but hey, I don't have many options right now.just the chips that I have in my SMD box. it works quite well, so no complaints there! after the LTspice crashed, I went back and this time increased the base resistor values and dropped the capacitor values, added a second inverting stage and a few other minor tweaks and this time it works fine. I initally assumed the 13V is due to high current load (100mA constant load) but after dropping it to 10mA the voltage was still only 13v! I checked the PWM signals and after the push pull transistors, the signals are about 8 to 8.5V, instead of the expected 12V. is there something loading down the transistors? maybe fets would be better for this application? also on a side note: I tried a very basic inverting charge pump with the op amp oscillator. the input voltage was 4.73 and the output was about -5.2v without any load. I then tried adding a 10k resistor and the voltage dropped to -4.38. I then tried adding a class B amplifier to the output of the oscillator but the voltage dropped instead of increasing! I first tried it with TIP122 and TIP127 (output was about -3.2V) and with IRFZ44N + IRF4905 (output was -2.2V). I suspect the result of the fets is due to the fact that they are not logic level fets and won't fully turn on / off with 5V Vgs but I'm not sure why the BJTs also did worse! any comments on that? I'm also going to attach the .asc file for the two stage inverting simulation. feel free to play around with it. |
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