Some years ago I built such amps.
This one had a digitally controlled gain (feedback variaton) of 1.000 to 15.000, others are possible.
I can send you an schematic by email, if you want.
Just now I develop another precision amp with a similar remote gain control.
tiki
If you are trying to develop a circuit that does this there are any number of approaches, all varying in complexity. One of the simplest and most elegant ways to do this is to take your garden variety op amp and stick a Digital to Analog converter in the feedback loop. The converter you would use in this particular instance would have to be a basic R/2R ladder type. (Information can be found at
the Analog Devices website.) This would give you a linear gain response as your binary inputs advance through the binary count. You would calculate the gain range by setting your minimum and maximum gain at a binary input of all zeroes then with all ones. Substitute these resistances into the basic op-amp gain formula with the suitable input resistor to set your input impedance where you want it and you have the basic setup for what you want. You can change the frequency response characteristics by adding capacitors in the standard ways, such as one across the DAC to roll off the upper frequency response and one in series with the input resistor to limit the low-frequency response, or you can take care of that in an earlier gain stage.
I can send you some formulas to calculate the gain of op amps if you send me an e-mail at elektron10@comcast.net I can recommend a number of manufacturers (National Semiconductor, Analog Devices, Maxim and others.)
If you can find it, one of the most excellent books on the planet ever written on the subject of op amps and how to use them was done by Burr-Brown many years ago. The book is a hardback book with just about every possible formula you can think of. That book is quite rare and hard to find because those who have it will go to the grave clutching it in their fingers rather than lose it. (And to think that they used to GIVE them away like candy!)
I hope I have been of some assistance to you. Please feel free to e-mail me anytime and if I can help I will be happy to do so as soon as I can.
this book, written by BB, seems to be interesting. Do you have any more information about it (title, year of publishing)? I know the BB linear applications handbook (1994) only.
Thanks!
tiki
