Converting PWM to analog

[stafil]

Hi,

Any traps for young players in the circuit below? I am trying to convert a 3.3V PWM signal to a ballpark 4V-12V analog signal. Looks fine in LTSPice, but wanted to double check with somebody experienced.

[Zero999]

It won't give a 4V to 12V analogue signal, but 0 to 13.2V. Connecting a P-channel like that will also result in oscillation, because it will add a considerable amount of gain to the loop. It will need some phase compensation.

R7 isn't needed.

The BSS84 will fry with 500mA of continuous drain current.
https://www.onsemi.com/pub/Collateral/BSS84-D.PDF

[stafil]

It won't give a 4V to 12V analogue signal, but 0 to 13.2V. Connecting a P-channel like that will also result in oscillation, because it will add a considerable amount of gain to the loop. It will need some phase compensation.

R7 isn't needed.

The BSS84 will fry with 500mA of continuous drain current.
https://www.onsemi.com/pub/Collateral/BSS84-D.PDF

Thanks!

Any pointers regarding the oscillation and the phase compensation!

Good catch regarding BSS84! Thanks!

[Jwillis]

Thanks!

Any pointers regarding the oscillation and the phase compensation!

Good catch regarding BSS84! Thanks!

The mosfet is a capacitive load which can cause overshoots, ringing, and even oscillation. By putting a low value resistor in seines with the output . This will  isolate the op-amp output and feedback network from the capacitive load .  Or If you are concerned about gain accuracy you can use a R-C series snubber circuit from output to ground.

[Zero999]

Thanks!

Any pointers regarding the oscillation and the phase compensation!

Good catch regarding BSS84! Thanks!

The mosfet is a capacitive load which can cause overshoots, ringing, and even oscillation. By putting a low value resistor in seines with the output . This will  isolate the op-amp output and feedback network from the capacitive load .  Or If you are concerned about gain accuracy you can use a R-C series snubber circuit from output to ground.

You'll get the same result with a BJT. It's because the transistor increase the voltage gain of the circuit. Adding a resistor in series with the op-amp's inverting input and a capacitor between the inverting-input and the output, forming an integrator, will reduce the high frequency gain of the op-amp and increase the stability.