Hi Suhas, Happy New Year to you, too!  However regarding your question, I'm afraid there is no solution satisfying your conditions:

>100 Mohm @ 8 GHz results in an input capacitance < 2 * 10^-19 F resp. < 0.2 aF (attoFarad) because C = 1 / (2 pi f Xc), and that's less than any device known to me (or even an empty package) can offer.

That is the main reason why measuring within a microwave circuit is more difficult than measurements at lower frequencies: Even an active probe with only 100 fF input capacitance has an input impedance of about 200 Ohm @ 8 GHz.

Sorry about the negative answer!

Dear Suhas,

I would like to second Joerg Fricke and congratulate him for the Christmas and New Year. If you tell us about the possible applications of the this buffer one may help in revising the required specifications to be feasible for implementation.

Normally the devices in this range of frequencies has a standard input and output impedance of 50 ohms as the testing apparatus and cables have this impedance.

In summary it may be advisable to review the retirements and specifications.

Best wishes and happy new year.

The application of the buffer is for a VCO which has a tuning range from 0.5 GHz to 8 GHz.

When I connect a 50 Ohm port to the output of the VCO, it pulls all the current to the ground resulting in zero output. When a port of 100's of MOhms is connected, the required output is obtained.

The way I thought to get around this problem was to connect a CMOS Opamp buffer, as it has high input impedance and low output impedance. But will it work for such a wide range of frequencies?

Happy New Year wishes in advance to you too sir Abdelhalim Zekry!

Dear Suhas,

Welcome,

What is the highest capacitive load on the voltage controlled oscillator?

From the conceptual point of view you can use source followers. They are very effective buffers. 

This circuit will work well when it is properly designed. 

Best wishes

Hi Suhas,

if even an attenuation of the VCO output signal of about -6 dB were acceptable, and if there is no need for a flat response over the frequency range in your application, you might try the LMH5401 (see link). At 8 GHz it provides a gain of about 0 dB, and adapting it to a 50 Ohm port (by a series resistor) will result in about -6 dB attenuation.

There is also an evaluation board for the LMH5401 available but you cannot use it because it has 50 Ohm inputs. The LMH5401 can be bought from Digi-Key or Mouser, for example.

I hope this helps. Best regards!

http://www.ti.com/lit/ds/symlink/lmh5401.pdf