buffer amplifer

please give me some advice on how to design VCO buffer.i desined a 4~8GHz VCO without buffer desined in 2um HBT process.,it works well,but I need a 4~8GHz VCO buffer.is a attenuator needed? i don't think insert a attenuator between VCO and the buffer is a good idea in MMIC,is that wright? why so many VCO papers never discuss VCO buffer? thanks.

it is a wideband VCO , and hence u need a wideband amplifier , try resistive feedback amplifiers

it may help you

khouly

Not all the VCO?s on the market use a buffer at the output.

The buffer improves the output impedance vs frequency of the VCO, improve the load-pulling, and improve the isolation between LO and following stages.

The linearity of the buffer is very important to don?t distort the LO signal.

For your relative high frequency range is recommended an emitter/source follower.

Use an amplifier only if the output level of the VCO is not enough to drive the mixer.

The choice of to add a buffer amp or not depends on two things:
1) If you need higher power (> perhaps 5 dBm), you will need the buffer amp.
2) If the impedance of the load changes, like in a sp2t switch going to two locations, then the VCO will jump in frequency/phase, and if you can not take the jump, then you will need isolation. The amount of isolation will dictate how many buffer amplifier stages and attenuator pads are needed.

thanks!

Obviously, if you are designing a buffer amp, you should try to optimize for |S21|*|S12| to be minimized--at least a -15 dB or smaller.

try to use source follow

source follower , cherry hopper architecture can be used as well , u can also use common source amp

khouly

Source follow is the first stage, it is the best of isolation.

Could you share the buffer topology you selected? any design tips?