Slew Rate 200 V/µsec Freq=27MHz ==>T=37n
See

 

ok, after checking AD9850 ebay module output amplitude voltage when attached to the rest of the circuit, I saw in scope that the DDS output signal amplitude is only about 400mVpp. So I need a gain of about 7.5 to get about 3Vpp.

So redoing math:

Av = 3 / 0.4 = 7.5
Rseq = 50Ω // 1kΩ = 47.6Ω
Rf = 7.5 * 47.6Ω = 357Ω * 10 = 3570Ω
Rg = 357 / (7.5 - 1) = 55Ω * 10 = 550Ω
But with these values I'm only achieving an amplification of about 2.4 / 0.4 = 6.

Mr. @Bordodynov I know you did some changes to be able to measure the gain wrt to input signal amplitude. I don't know how to perform that measurement so I haven't performed it. Anyway, the gain is short to what I was expecting!

 

See

 

@Bordodynov
Post #23.
Missing BFR/BFT92 models, have searched through your posted LTS+ version, not found.
E

 

You will have to install my additional library.
https://forum.allaboutcircuits.com/...nents-models-of-ltspice-free-download.133690/
or
https://yadi.sk/d/7ZiMW2kg3UWYk3
or minimum
https://yadi.sk/d/WCKrh8a33WWP3h
My site can help you.
http://bordodynov.ltwiki.org/
But keep in mind that I update the files listed in the links above in the first place. On my website, I do this later.
In the complete collection there are files HELP.txt and HelpRus.txt.
My collection of models for LTspice is great. I added the operational amplifier in question to it. When appearing in consideration of electronic circuits with models that are not in the collection, I immediately add new models to my collection. I have a goal to be able to use any other people's schemes in LTspice. I used to do a complete project - the schema + models + the symbols of the elements, but it's tedious. Moreover, I noticed that PsySc0rpi0n uses my collection of models. Therefore, I advise you to use my experience.

 

hi Jose,
I have used @Bordodynov 's post #25 to make a simplified extract, it will make it easier/quicker for you to try.
Unzip the zip file and run the RF....asc file.

EDIT:
What load impedance will the Vout be driving into, the circuit performance has a lower limit of around 1000R, if you need to maintain the Gain= 7.5
E
Thanks again bordodynov for your help.

 

Hello.

But why these circuits suggested by datasheets never work as they should?
Why this things can ever be simple?
At this moment I have no freakin' idea what the change you made are supposed to do and why there were needed in the first place!
The circuit is completely different and I have not enough knowledge to be able to analyse it by my own!

If it is possible to give a brief explanation for the changes, I would appreciate!
Thanks to all that have been helping and specially to @ericgibbs and @Bordodynov .

 

hi J,
The LMH is not able to give the voltage swing of 3.2Vppk at the Vout at that 27MHz frequency.
So his circuit uses the two left side transistors as diodes, the voltage across the diodes/100R's varies in sync with current drawn from the supply line.
This varying voltage drives the two right side transistors configured as emitter followers, this increases the voltage swing at the Vout to the required level.
Its a way of adding an active pull up, pull down drive at the signal output.
E

 

And isn't there any other OpAmp or any other component that is able to amplify the voltage at that frequency without all that additional components?

And all 4 transistors are the same? I can only see the reference of 2 of them!

And another question:
I was not using Mr. @Bordodynov libraries for default so I have downloaded the contents of the 2nd link from post #25. But even after I copy the whole contents inside LTSpiceXVII (examples, sym and lib) into my local LTSpic einstallation folder, I still can't find those bfr92.

Edited;
I visited this link:
http://ltwiki.org/files/LTspiceIV/Vendor List/Infineon/RF-Transistors/index.html

but the bfr92.zip contains a folder SPRA and SPICE that I don't know how to use. SPICE folder has 3 files that I'm also not familiar with because they are .txt files. And are 2 of them. Usually models are a single .sub file, as far as my knowledge tells me. I don't know how to use these 2 files.

Thanks

 

hi,
If you unzip my file in post #26, you will find the bfr and bft files there.
E

 

I managed to figure it out. But somehow I would like to be able to use or at least to have Mr. @Bordodynov files. Anyway I found that the model and symbol are already available from within LTSpice itself. I don't even need to make any ".includes".

If you can tell me another way of making this more simple, like some other OpAmp or transistor that can amplify the signal to the level I want/need without this mcuh additional electronics, I would appreciate!

Thanks

 

hi J,
What load impedance will the Vout be driving into.?
Please post more details of the project.
E

 

See

I advise you to go to my site and install my libraries. Follow the instructions.
But if you want to transfer your schema to other people who do not have my library installed,
then you should follow the following instructions:
1. Create a project folder
2. Place in the folder a schema, non-standard libraries and symbols of the elements that are used in the project.
3. Pack the folder (make a ZIP file).
This ZIP file is unpacked by the recipient and can immediately analyze your project.

 

See

 

hi B'.
Tried your sim with a 50R load, its an impressive OPA.
E
Reasonable price ~£4.0

 

hi J,
What load impedance will the Vout be driving into.?
Please post more details of the project.
E

The load impedance may vary because its a resistive bridge where one of the resistors of one of the bridge arms will simulate a load that can be any value. But in a controlled environment that resistor will have 3 or 4 known values of short circuit, 25Ω, 50Ω, 75Ω, 100Ω and open circuit.

 

Mr. @Bordodynov I couldn't find a matching symbol for OPA691 so I created my own and used the spice model from here:
http://www.ti.com/lit/zip/sbom151

I think I got pretty much the same results!
I just would like an explanation about why if we remove the factor of 10x from the feedback resistors, the gain is higher?

 

I added a symbol and a model later, and then I drew the last schema. If you have correctly added my libraries, then this is done simply. Start typing the element and write the name of the element in the empty line.
See

 

Ok, thanks.

And what about the gain to be higher with smaller feedback resistors than with bigger ones?

 

hi Jose,
I don't see that, using Bordo's OPA model, a smaller Rfb = lower gain.
Check your model.
Eric

EDIT:
Added a zip with Boro's models, run the AB3 asc