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Class G circuits using bi polar or mosfets
Class G circuits using bi polar or mosfets
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Old Yesterday, 03:01 PM   #11
nauta is offline nauta  Canada
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Join Date: Sep 2017
Hi Nigel

There are some missing components in your rail switches.

Add a 470uF across each zener as Sajti did.

Add a gate protection zener to each mosfet as this provides a reverse diode path for biasing the floating supply.

Add 100R between the large series gate resistor and the gate.

Add a diode across the high series R - for upper cathode towards gate; for lower cathode towards floating supply.

Size the high series R so that enough mA flows to keep the floating supply charged.

Add a 100nF ceramic across (end-to-end) the floating supply. This is important for eliminating spikes at very low signal levels.

You don't say how much power you are aiming for, since the low rails could be for an amp with 1R load

Generally if there are parallel lower tier devices, there has to be a similar current/heat capability for the upper tier. You can use a single mosfet driver for several BJTs as the rail switches, or use separate mosfets for each BJT in the lower tier. In the latter case it is usual to duplicate the gate circuitry but use a common floating supply.

All of the above works perfectly. No need for opamps, comparators, or hard to get parts.
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Old Yesterday, 03:35 PM   #12
nigelwright7557 is offline nigelwright7557  United Kingdom
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Join Date: Apr 2008
Location: Carlisle, England
Originally Posted by sajti View Post
Here is a working solution, I put together in simulator. You need to change the switching diodes from 1N4148 to something low current switching, which can handle higher voltage (200V or same).
If You need more output devices, just add them parallel with the existing, use individual series resistor (100-220ohms), and reduce R25/26 divided by the number of parallel devices.

Thanks for the working solution.
My circuit had the problem that the voltage across the Vbe multiplier would never get within 12 volts of the rails and so limit badly the output swing by +/- 20volts ! Your circuit doesn't have that problem.
PCBCAD51 download or CDROM 2019 version out now http://www.murtonpikesystems.co.uk
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