pwm schematic simulation - Electronic Circuits Projects Diagrams Free

dspcan · 19th December 2007, 03:47 PM

look the pwm schematic in the attatched file ,I made a simple simulation of it,but I found that the lowest voltage of the output is more than 1V. So, if I want reduce it to 0v, what changes should be make to the schematic? thanks for your consideration

j.p.bill · 19th December 2007, 04:11 PM

You may notice that one input of your output op amp is tied to a voltage divider so the op amp will always have an offset voltage. Single-supply op amp circuits have that problem. Either use an output coupling capacitor, or go to a dual supply voltage (much better).

dspcan · 19th December 2007, 04:25 PM

yes,you are right, the schematic is download from this forum. After simulation, I don't know what kind of role the voltage divider and the fouth op play in the circuit, it seems that changing the value of the divide resistor haven't any influence to the output wave, if you want to change the frequency and the output voltage duty, you have to change the capacitor value and the gain of the op, I do think its a 'not good' pwm circuit. anyone has some advice?

dspcan · 19th December 2007, 04:28 PM

and somebody can help me on figuring out the maximum and the minimum frequency the circuit output wave? or provide the figure steps ,thank you!

audioguru · 19th December 2007, 04:43 PM

The minimum output voltage of the TL084 is about 1.2V higher than its negative supply which is ground in this circuit. The output of a TL084 suddenly inverts when an input voltage is about 2V or lower from its negative supply voltage which is ground in this circuit.

Use an old LM324 for low frequencies or an MC34074 for high frequencies since their outputs go to ground and their inputs work fine at 0V.

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19th December 2007, 04:11 PM	(permalink)
j.p.bill	You may notice that one input of your output op amp is tied to a voltage divider so the op amp will always have an offset voltage. Single-supply op amp circuits have that problem. Either use an output coupling capacitor, or go to a dual supply voltage (much better).

19th December 2007, 04:25 PM	(permalink)
dspcan	yes,you are right, the schematic is download from this forum. After simulation, I don't know what kind of role the voltage divider and the fouth op play in the circuit, it seems that changing the value of the divide resistor haven't any influence to the output wave, if you want to change the frequency and the output voltage duty, you have to change the capacitor value and the gain of the op, I do think its a 'not good' pwm circuit. anyone has some advice?

19th December 2007, 04:28 PM	(permalink)
dspcan	and somebody can help me on figuring out the maximum and the minimum frequency the circuit output wave? or provide the figure steps ,thank you!

19th December 2007, 04:43 PM	(permalink)
audioguru	The minimum output voltage of the TL084 is about 1.2V higher than its negative supply which is ground in this circuit. The output of a TL084 suddenly inverts when an input voltage is about 2V or lower from its negative supply voltage which is ground in this circuit. Use an old LM324 for low frequencies or an MC34074 for high frequencies since their outputs go to ground and their inputs work fine at 0V. __________________ Uncle $crooge