PWM amplifier

[gsg]

Hi,
I have a circuit that gives a PWM signal at 20Khz with maximum average voltage at 1.6v. I need to amplify and filter this signal to a variable DC voltage ranging from 0-5v or 0-10v.
I tried using a 741 op-amp and RC filter. It simulates perfectly in Multisim but practically gives a constant VCC at output all the time.

Can someone please help???Or suggest an alternate circuit for the same...
Thanks

 

[NickS]

Perhaps I misunderstood what it was you are trying to do because that setup does not work at all like I understood you to want.

It sounded to me like you want to put a square wave in at variable duty cycle and get a nice DC level out the other side that raises as the duty cycle goes up. If that sounds right then you might be interested in my circuit which is a precision rectifier followed by a two pole salen key filter. The response is slightly under-damped as you can see from the plot.

The plot shows the output for 10%, 30% and 50% duty cycle. Also note if your clock is AC coupled then you will always see the same level because the rectifier is a full wave rectifier(I attached a plot using a sine wave centered at zero volts to illustrate this).

Also note that you can change the gain of the circuit by adjusting values for R7 and R8

Good Luck
-Nick-

 

[gsg]

Thanks for the circuit.
But i need an amplifier, not a rectifier. The RC circuit is doing all the filtering part properly. Please tell me how i can amplify the square wave with variable duty cycle...

 

[NickS]

I think we are not understanding each other because I simulated the circuit you posted and it behaves nothing like what I interpreted you as wanting. Your filter knocked off the square corners of the input signal but it is still very much an AC output not DC as you said.

As for your last request, amplifying a square wave is no different than amplifying a sine wave. The duty cycle has nothing to do with it. The circuit you posted uses R1 and R2 to amplify the square wave. So I am quite confused. Perhaps you are using the wrong terminology to describe what you really want.

***EDIT***
Try just using the last stage of the schematic I posted.
so your PWM input is to R3 and your output is net 12. Drop everything before and after that. Now what you have is a 2pole salen key filter with a very low cut off frequency(I think it was about 50Hz) and you can play with the values of R7 and R8 to adjust the level of gain you desire(that is the same thing as amplification). The equation for calculating gain is Vout = Vin(1+ (R7/R8))