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Why the VCO has high output impedance?

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shankeys2

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Hello everyone,
As we know that the VCO has high output impedance, thats why we need to use buffer at output of VCO. I am designing LC VCO with crossed coupled differential topology.
So my question is why VCO has high output impedance?
 

KlausST

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Hi,

As we know that the VCO has high output impedance

I don´t know.
Please give a link to a related document.

Klaus
 

SLK001

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Usually, a VCO will have a high output impedance to minimize the loading on the active element. Any loading of the active element will pull the frequency, so an output buffer is usually used to take all the loading.
 

dick_freebird

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VCO -needs- a high output impedance in order that its
Q and All Good Things Deriving From It are maximized.
It's not that people design a high Zout because anyone
wants that - they do it because it's the only way a LC
core can ring worth a damn.
 

betwixt

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Everyones answer is correct but the question isn't!

A VCO does NOT necessarily have a high output impedance - it's the following stage that should have a high INPUT impedance to avoid loading it.

In the case of an LC ocillator for example, the output could be from a tap in the tuned circuit just above ground. It's impedance would be very low but shunting it with a capacitive load would still detune it.

Brian.
 

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