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Q&A

How building a follower satisfying rather extreme requirements.

+2
−1

What is your best suggestion for building a follower that would ideally satisfy the following spec:

  • frequency: from 100 Hz (or lower) to 1 GHz
  • input signal impedance: 1 Mohm
  • input signal amplitude: from 1 mV to 1V at least
  • output load to be driven: 50 Ohm.
  • The follower need not be absolutely flat (gain = 1), but actually, it is allowed to reduce the signal by a factor of 0.5.
  • some signal distortion allowed.
  • also, a small DC offset voltage of few mV can be allowed, as I am interested in the AC component of the signal.

If that is not possible, please give me the suggestion that best approaches these specs. Also, I would be happy if the part names or part numbers are indicated.

EDIT: as pinpointed by Olin Lathrop, the 1 GHz requirement is fundamentally incompatible with the 1 M Ohm input impedance requirement. So, there is no point to go further for the moment.

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5 comments

Have you tried looking for wide bandwidth unity gain stable op-amps? Andy aka‭ 29 days ago

Not with those keywords. Will try and perhaps delete this question. coquelicot‭ 29 days ago

For the moment, I've found the opa695 that is best suited, but it has 70uA input current. Also, the big 50 Ohm load is a problem. coquelicot‭ 29 days ago

This is far from my area of expertise, but have you checked Analog Devices? They have tonnes of more or less exotic amplifiers. https://www.analog.com/en/products/rf-microwave/rf-amplifiers.html. If you for example check their "Wideband Distributed Amplifiers" they have lots of parts explicitly designed for 50 ohm, mostly with matched input/output. Lundin‭ 28 days ago

@Lundin. Thank you for the link. I will have a look. coquelicot‭ 28 days ago

1 answer

+1
−0

I don't have a real answer. This is just a suggestion for how to proceed.

Look for amplifiers intended to drive 50 Ω cable. Sometimes the term line driver is used, even though that also means other things. If you only needed a few 100 MHz, a video line driver might work, but finding something that works up to 1 GHz is going to be difficult.

It might help to find something that meets your specs except for bandwidth, then contact the manufacturer for what they suggest when you need to go to 1 GHz. If anyone is making something like that, they probably know about it.

Added

1 MΩ is absurd for carrying 1 GHz, to the point where you need to go back and seriously question what the real purpose of these specs are. Whoever dreamed up these requirements didn't know what they were doing, so quite likely some of the other specs are silly too.

There is a good reason impedances are much lower than 1 MΩ at high frequencies like 1 GHz. The low pass filter caused by the inevitable stray capacitance will attenuate the signal to oblivion. For example, at 1 MΩ it only takes 0.16 fF (1.6 x 10-16 F) for a 1 GHz rolloff. 100 fF (0.1 pF) against 1 MΩ causes a low pass rolloff at only 1.6 MHz.

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1 comment

@Olin Lathrop. Thank you for your add, a strong argument indeed. In fact, I see no way the stray capacitance could be less than 1pF or so. Even if I keep only 1/20 of the signal, this requires an input impedance of 20 k Ohm at the best. I am doing research and research implies you try to discover something without knowing the characteristics, by trying to guess what could lead to the results. Hence the specs in the question. Obviously, I have to mitigate. Thank you for having pinpointed that. coquelicot‭ 28 days ago

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