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Old 7th September 2011, 12:57 PM   #1
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Default Postive output voltage only

Although the input of this mosfet amp module is DC coupled, I wounder why an amplified pure positive input signal is still positive and negative at the output?
Does this come from the type of bias circuit (please have a quick look at the schematics) and that this is class AB amp? Does perhabs somebody know if there is a easy way to preserve the DC offset? Maybe with class B biasing?

-- I'm new to mosfet amps, and probably that is the typical function of a mosfet amp, so please let me know if it's not possible to amplify a positive only signal with mosfets.

TIA

Oscilloscope image: CH1 (red input) 500mV /1 DC, CH2 (yellow output) 10V /10 DC, time 200us:
Schematics: Cin shorted
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File Type: png amp38.orig.4r4Load.pos-saw.png (11.8 KB, 69 views)

Last edited by okmog; 7th September 2011 at 01:04 PM.
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Old 7th September 2011, 01:23 PM   #2
jaycee is offline jaycee  United Kingdom
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You forgot about the capacitors in the feedback loop. Short these and you will see the desired behaviour - however note you will see DC offset even with the input shorted. To get rid of that you would need to use a trimmer pot to eliminate it.
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Old 7th September 2011, 08:07 PM   #3
AndrewT is offline AndrewT  Scotland
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You have shown an AC coupled amplifier.
If built properly this will not amply DC signals and will not pass an input DC to the output.

If you want/need to amplify DC input signals then you need a DC coupled amplifier.

Then you will have to learn how to keep that amplified DC as an accurate, but scaled, copy of the input signal.
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Old 8th September 2011, 08:59 AM   #4
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Cin is what makes it AC coupled.
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Old 8th September 2011, 09:51 AM   #5
AndrewT is offline AndrewT  Scotland
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Cin + Cia & Cib is what makes it AC coupled.

You need to replace both with a shorting link to get DC coupled.
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