Why do we need amplifier in PLL block?

[tran]

Hi all,
In basic PLL block, there are an amplifier but I don't known Why we need an amplifier. If we ignore the amplifier, is there any error in function of PLL?
Can anyone explain for me the function of the amplifier and Why we need it?
Thank you very much!

 

[DDavid]

Re: PLL block

Hi,

Can you attach this circuit!

David

 

[dinone]

Re: PLL block

Tran

probably the amplifier is put on the PLL output to improve the signal to drive a mixer. Probably it is used as buffer with a gain also.

By

 

[safwatonline]

PLL block

no need , PLL cam work without it but it intoduce an extra degree of freedom to control the loop gain ,on the other handit may limit the BW. but in general basic PLL doesnt contain an amplifier.
basic pll should have a PD + LF +VCO.
the amplifier may also be used to make a shift of the o/p of the PD "shift and may be compression too" to be compatible with the VCO range.

 

[biff44]

Re: PLL block

If by "amplifier" you mean an op amp circuit, there are three reasons:

1) an op amp allows a bigger tuning voltage range than a charge pump output phase detector can give you by itself

2) an op amp set up as an integrator can give you infinite (or close to it) DC gain, so the phase error approaches zero. Without the high DC gain of an op amp, when the loop locks there will be a larger static phase error. (And in most digital phase detectors, a large static phase error manifests itself as significant pulses at the clock reference frequency on the VCO tuning line which then cause large RF spurious output sidebands--not desireable).

3) it is easier to design a more complex loop filter, with numerous poles and zeros and DC summing junctions, if there is an op amp. Without an op amp, you are pretty much limited to two poles and one zero in your control loop filter design.

 

[DDavid]

Re: PLL block

Hi,

Probably the amplifier is put on the PLL output to improve the signal to drive a mixer. Probably it is used as buffer with a gain also.

Sometimes VCO built with dicrete components: first stage vco (resonator) second stage go to amp and third stage feedback to pll usualy is buffer!!!!

David D.