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Dual and single supply question?

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spbhu

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For dual supply, (e.g. Vdd=-Vss), the output DC level is normall set to zero, for maximum swing purpose. And of course the DC current to the load is zero if no input signal is applied.
However, for single supply case, (Vss=0), the output DC level is normally set to Vdd/2 for maximum swing, it seems the DC current to the load is quite large even no input signal is applied. For example, Vdd/2=1.5v, Rload=50Ω, then Iload=30mA even without signal input ?
What's the problem? Does it mean for drving a low load we always use dual supply?
Anybody can tell me the reason? Thanks very much....
 

In DC applications (or AC with DC component ) while running of single power supply you need to create so called "virtual ground" (1/2Vcc), in AC applications this issue is solved by adding an output coupling capacitor ..
If you don't want or don't like the idea of virtual ground or coupling caps then the only answer is dual power supply ..

Regards,
IanP
 

Do you mean for single supply case, the load is not connected to ground, but to 1/2Vdd? so that the dc current consumption is still zero when no signal input is applied?
 

spbhu, another way, a bit more complicated, you can connect your load between two output stages with 180° phase shift.
At DC you'll have zero voltage at your load, at AC doubled voltage swing
 

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