Electronics > Beginners
Rail splitter circuit (looking for approval :D)
MrAl:
--- Quote from: Marulo on May 05, 2019, 05:24:11 pm ---Also I tried to make a simulation with some "parasitic" properties, still looks pretty good.
--- End quote ---
Hi,
That's nice but you may want to add small value resistors in series with each mosfet source.
Check the current from the power supply with no load and with load too. It should be low if this is done right.
iMo:
The best rail splitter is none rail splitter :)
exe:
--- Quote from: Marulo on May 05, 2019, 05:13:25 pm ---Hello! I've done a few spice simulations and it looks pretty good.
--- End quote ---
I don't see component values for opamps and fets. Without them ltspice uses unrealistic models, which are not representative for real circuits.
Zero999:
--- Quote from: MrAl on May 05, 2019, 09:47:51 pm ---
--- Quote from: Zero999 on May 05, 2019, 04:15:35 pm ---Yes, if the MOSFETs were connected the other way round, they would form another inverting stage and the circuit would just latch up and sit with its output saturated at either the positive or negative rail.
I don't see how using more than one op-amp would solve the problem of crossover distortion. It's not something I've seen in an amplifier circuit before.
--- End quote ---
Hi,
I came up with the configuration back in the 1980's when i wanted to create a near crossover distortion free amplifier for audio. Back then mosfets were just becoming popular and i realized that they offered a big advantage over bipolars when driven as voltage followers. Namely, the control was smoother to begin with.
Then i realized that i needed to drive the output both positively and negatively in order to keep the impedance low for each section of a test sine wave. So i got the idea to use two amplifiers in parallel, one driving high and one driving low. Then it was just a matter on how to adjust the quiescent current because they both have to be on at all times at least a little bit.
That works out pretty well because each op amp controls it's own mosfet.
As you can see the post that follows your shows such a configuration although that is a bit more basic then we actually have to use. We have to be able to drive each mosfet with gate voltage enough even with higher amplitude output (for audio work). For a rail splitter it may not matter as much though.
The basic circuit should have some small value resistors in series with each source. That helps to mitigate the op amp offset voltages. We also have to make sure we have both turned on slightly with no load.
So a few little adjustments and it will be great.
--- End quote ---
I did have a brief play with LTSpice afterwards and the result was as I predicted: crossover distortion. The two op-amps still have to swing two gate thresholds, when the output current changes direction. Using two op-amps in that configuration doesn't solve anything. Something to ajust the quiecent current, to bias it into class AB is required.
I'll post the .asc file when I get time, if no one beats me to it.
MrAl:
--- Quote from: Zero999 on May 05, 2019, 10:59:55 pm ---
--- Quote from: MrAl on May 05, 2019, 09:47:51 pm ---
--- Quote from: Zero999 on May 05, 2019, 04:15:35 pm ---Yes, if the MOSFETs were connected the other way round, they would form another inverting stage and the circuit would just latch up and sit with its output saturated at either the positive or negative rail.
I don't see how using more than one op-amp would solve the problem of crossover distortion. It's not something I've seen in an amplifier circuit before.
--- End quote ---
Hi,
I came up with the configuration back in the 1980's when i wanted to create a near crossover distortion free amplifier for audio. Back then mosfets were just becoming popular and i realized that they offered a big advantage over bipolars when driven as voltage followers. Namely, the control was smoother to begin with.
Then i realized that i needed to drive the output both positively and negatively in order to keep the impedance low for each section of a test sine wave. So i got the idea to use two amplifiers in parallel, one driving high and one driving low. Then it was just a matter on how to adjust the quiescent current because they both have to be on at all times at least a little bit.
That works out pretty well because each op amp controls it's own mosfet.
As you can see the post that follows your shows such a configuration although that is a bit more basic then we actually have to use. We have to be able to drive each mosfet with gate voltage enough even with higher amplitude output (for audio work). For a rail splitter it may not matter as much though.
The basic circuit should have some small value resistors in series with each source. That helps to mitigate the op amp offset voltages. We also have to make sure we have both turned on slightly with no load.
So a few little adjustments and it will be great.
--- End quote ---
I did have a brief play with LTSpice afterwards and the result was as I predicted: crossover distortion. The two op-amps still have to swing two gate thresholds, when the output current changes direction. Using two op-amps in that configuration doesn't solve anything. Something to ajust the quiecent current, to bias it into class AB is required.
I'll post the .asc file when I get time, if no one beats me to it.
--- End quote ---
Hi,
Well then something isnt being done right. I'll take another look too when i get a chance.
Remember each mosfet must stay on all the time at least a little or there will be crossover distortion. That means each op amp stays in the linear mode and never saturates in either direction.
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