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I came across this circuit which is part of a magnetic levitation system and can't understand a piece of the circuit. What are the roles of Q2 and Q4 ? And the different resistors ? I am used to see typical push-pull circuit with Q10 and Q11 collectors directly connected to Vcc/Gnd. The LM324 comparator inputs are connected to sensors and reference voltages, to produce a bidirectional current in L3 and L4.

Thanks.

enter image description here

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    \$\begingroup\$ Looks to me like Q4 and Q3 are incorrectly drawn. However, I'm not altogether certain about this because sometimes transistors can be operated with E and C swapped to make gains on saturation voltage. The LM324 IS NOT a comparator. \$\endgroup\$
    – Andy aka
    Feb 8, 2022 at 23:13
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    \$\begingroup\$ It looks like a H bridge audio amplifier - i wonder why an audio amp chip wasn’t used vs the discrete solution? \$\endgroup\$
    – Kartman
    Feb 8, 2022 at 23:28

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The LM324 is an op-amp, not a comparator (though it can be used as one.) Each op-amp has a negative feedback path from its corresponding driver pair.

The drivers themselves are just current gain boost, in two stages. They use a Sziklai connection as opposed to a Darlington type. Sziklai pairs are non-inverting. (Q3 and Q4 seem to have their collector and emitter swapped.)

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  • \$\begingroup\$ I am used to see the LM324 as a comparator and thought it was one, thanks. I simulated the circuit with collector and emitter swapped and it works as a Sziklai pair. \$\endgroup\$
    – Ultra67
    Feb 9, 2022 at 8:00
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The odd looking scheme is valid because the output can swing closer to the rails wasting less power and allowing a lower operating voltage .Q3Q4 seem to be a misprint .

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