problems about LCVCO output buffer??

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has anyone seen the LCVCO output buffer? it's function is converting differential output of LCVCO to single one, then the single output is connected to following filter. this buffer must keep the sinewave. My buffer damaged the wave.
could anyone can give me some reference paper about this? or tell me this buffer(or called amplifier) is belonged to which field?

thanks a lot!

 

[shwoo]

Try an emitter follower, or source follower if you prefer a FET.
The purpose of using a buffer is to isolate next stage from the oscillator.
Also try to adjust the bias point and gain of your buffer.
If you want more information, please provide more details, such as
frequency, and the nature of "damage" of the waveform.
S. H.

 

[tiger_ads]

Thanks man!
My VCO's working frequency range is 450MHz to 950MHz, but the buffer seems to be damaged when the frequency is up to 700MHz.
The buffer circuit is attached here. The two input is from the output of VCO, they are differential, and the dc level is VDD/2. The capacitor is composed of two parallel transistor cap. The transistor is PMOS.

pls give me some advise
thanks

 

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I don't know how the load affect the circuit. the output of buffer is connected to pad.

Added after 54 minutes:

because i need to test the signal from the output of the buffer, actually, this output is connected to the input of the offchip filter, I have tried to disconnect it with the filter in the Spectre simulation , and the output signal of the buffer is good , so i get conclusion, the capacitive input(15pF) of filter is affected the performance of the buffer. But I don't know when i test the signal, how much capacitance be added the output? because i am debugging why i can't detect signals in the oscillograph.

If the capacitive input of filter affected the output of buffer, how should I solve the problem? because on the chip the output of buffer is connected to next stage through 50 ohms resistor. during the full circuit simulation including on chip and offchip component, the signal at the output of buffer is bad, especially up to hith frequency.

 

[tiger_ads]

to shwoo

can u reply to this?

thanks!