Postive output voltage only

[okmog]

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

[jaycee]

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.

[AndrewT]

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.

[ingenieus]

Cin is what makes it AC coupled.

[AndrewT]

Cin + Cia & Cib is what makes it AC coupled.

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