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How to design an active filter without negative power supply for PLL?

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Hello Everyone;
In some PLLs, active low pass filter is used. How can we design the active filter (that contains op amp) without negative power supply?
 

PLL [hlp]

sometimes we don't need the negative supply , coz u don't need the output to be neagtive

khouly
 

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Re: PLL [hlp]

Hello;
Khouly wrote:
sometimes we don't need the negative supply , coz u don't need the output to be neagtive
Click to expand...
How I can understand wether I need to negative supply or don't need?
 

Re: PLL [hlp]

you are actually feeding the output of the lowpass filter to the VCO in a PLL and so if the VCOs input is always positive then it means the output of the active filter is gonna be positive and hence no negative supply would be needed....
 

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Re: PLL [hlp]

There are special amplifiers called "rail to rail" op amps that can be used with a single supply.
 

PLL [hlp]

u need negative supply if in ur calculations the control voltage of the VCO must go negative , if not , then u don't need negative supply

khouly
 

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