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Operational amplifiers for RF

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Newbie level 6
Mar 14, 2011
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I intend to work adding different sin waves at RF badwith (10-100 MHz) but I am not very familiar on how to select an OA for this task.

I see on the internet OAs with 100 MHz bandwith and slew rate of 750 V/us... but I guess these parameters have to be taken as absolute maximuns and thus the OA not working properly for such frequencies.

Can anybody help me on how to select properly an OA for this task?

Thank you in advance.

I guess these parameters have to be taken as absolute maximuns and thus the OA not working properly for such frequencies
Usual OP circuits are based on the assumption of infinite respectively sufficient high loop gain. This assumption most likely won't be true for RF OP circuits operated near the bandwidth limit. The consequence is to calculate the circuit's real transfer function based on the OP data. Non-linear, e.g. slew rate limitations should be considered as well.

You can get OPs with GBW (gain bandwidth product) of several GHz, so you are basically able to design OP circuits that work in a 100 MHZ range. You can refer to the reference circuits suggested in OP data sheets to get a first impression. It should be clearly noted, that suitable circuit layout and particularly supply bypassing are substantial for RF OP circuits. You should consider continuous ground planes a design standard.

Current feedback (CFB) OPs are an interesting variant for RF circuits. You can e.g. refer to TI THS3001 as a work horse of high speed OP circuits or it's above GHz successors.
The only problem with the current-feedback is that I need to give different gain values to the signals I want to add. It is not enough just to amplify the difference or addition of them, but also of them having different gain values.

Is there any way to do this with a current-feedback?

Basically yes. I don't see any restriction for summing amplifiers, it's easy to figure it out yourself. There are only a few circuits, that can't be implemented well with CFB OP, e.g. non-inverting integrators and some filter types.

Using opamps for 10-100 MHz band is now possible but to my opinion quite complex.
I would rather recommend using low-cost and easy to handle "gain block" MMICs. There is a wide selection of such integrated circuits, for ~DC up to > 10 GHz.
I have used many over this frequency range, and can recommend them.
Usually they have inputs and outputs matched to 50 Ohms, require a single DC power supply, from +3 to +6..9 V, current according to a maximum output power. Stability is not a problem. To add two or more signals , you can use a power splitter (e.g. from Mini-Circuits) with a low-loss summing, or, resistive combiners if you can allow 3-6 dB loss.
If you need connectorized devices, see e.g. ZFL-500; if you can handle some soldering, see RAM ans MAR series.
There are many other "gain-block" manufacturers: I used Agilent, RF Micro-Devices, Maxim and Analog Devices models. For "your" band, 10-100 MHz, MAX 4286 is my favorite, as well as MSA0408 by Agilent.
All are stable and much friendlier than opamps. Try some!

I would rather recommend using low-cost and easy to handle "gain block" MMICs.
I completely agree for classical RF applications. OPs would be mainly suggested for wideband (including DC capable) or pulse circuits, also high impedance buffers and similar specialities.

I would steer clear of using "high bandwidth" voltage feedback op amps, as it's often very difficult to get them to behave. I have had better luck with current feedback OA, though they take some experience to get working correctly.

Also using MMIC chips is quite easy and I recommend playing with them if you never have before. As long as you don't need certain things (like high input impedance or very low output impedance, or complex topologies), they are very useful.

For doing a variable gain amp, I would consider the option of using a fixed high gain amp, and a variable attenuator.

Also, for variable gain, there are lots of monolithic VGAs which are quite easy to use. Look up the AD8331, for example.

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