13MHz Signal/Power Amplifier

[LiquidOrb24]

Hello all,

I'm having trouble amplifying the signal from a VCO running at 13MHz. I need to take the signal and pre-amplify it before it goes into an amplifier but I am stuck in trying to do that.

Right now the signal out of the VCO has an amplitude of 1V with very low power...I would like to have it pre-amped up to about 2-5 Volts with 100mW of power or more. I tried using a 20 db amplifier IC from rfmd.com as a general gain amplifier but it did not work...I tried using a non-inverting op-amp circuit with a high gain to amplify the signal but the power wasn't enough still, and now I'm thinking about using a Darlington array such as the ULN2003 version but I'm afraid that it wont be able to handle the frequency.

Is there any advice you can supply me with so that I can amplify the amplitude/power of the signal coming out of the VCO before it goes into the main amplifier?

 

[crutschow]

What amp did you use from rfmd.com? Are you trying to drive a 50 ohm load?

 

[LiquidOrb24]

Yes I'm trying to drive a 50 ohm load for now but later it will be varying and I used the NBB-500 gain amplifier from rfmd.com...it seemed good and works based off the Darlington transistor but it seems very hard to cascade them together to make multiple gain blocks and they seems to disrupt the integrity of the signal.

 

[Ubergeek63]

https://www.electro-tech-online.com/custompdfs/2008/05/opa684.pdf
https://dkc3.digikey.com/US/images/datasheet.gif

Current feedback amps that are fairly easy to set up and will easily drive a 50 ohm load

 

[LiquidOrb24]

The current feedback op-amp is nice and is something I probably will try but would a Darlington transistor work at these frequencies because I hear they are great at amplifying the power for such uses as LED driver and stepper motor driver.

Also why would the CF op-amp be preferred over the VF op-amp...I tried the VF and for some reason the signal came out worse than what it came in with...do I have to somehow make the input and output see 50 ohms or can the signal just go into the op-amp and come out amplified?

 

[Ubergeek63]

Darlingtons would not work. With a typical 1uS rise and 1us fall time you would be lucky to get 500KHz.

To use an opamp you need resistors to set the input impedance and gain. Using a CFA is basically the same as a VFA accept you can not put reactives in the feedback path and the feedback resister sets the bandwidth instead of the gain setting it, and that is the advantage.

For instance the OPA684 can get a gain of 100 @ 50MHz with out breaking a sweat. A VFA would require a gain bandwidth product of 5GHz to do that, and would be sweating profusely.

 

[crutschow]

Yes I'm trying to drive a 50 ohm load for now but later it will be varying and I used the NBB-500 gain amplifier from rfmd.com...

I can't find that gain amplifier on their website. Is that the correct model number?