Electronics > Projects, Designs, and Technical Stuff
Power supply with arbitrary voltage waveform and variable series resistance
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duak:
Nihial,

You have changed your required bandwidth from a few kHz to 1 - 10 MHz, is this correct?

When you said power supply, I understood that to be 0 to 30 V.  Is this correct or do you mean +/- 30 V?  My scheme can handle bipolar voltages with the appropriate driver stage.

The Instrumentation Amplifier can be replaced by an AD734 multiplier to give a variable output impedance providing its input and output voltages are scaled to operate it within its linear region.  I used an INA to show the idea of variable output resistance as simply as possible.  I should have been more clear in that the gain of the INA sets the output impedance.  Please note that the AD734 will require voltage dividers to reduce the voltage applied to its inputs to avoid damage.

A Howland Current Pump is a more elegant circuit but harder to understand.  Hopefully David could help out here; my math has disappeared.

Somewhere I have a data sheet and application note from a company called Comlinear that made a part called the CLC560 based on current feedback that could have its output impedance set by one or two resistors.  It was such an intriguing concept that I breadboarded one to see if it would work as an output stage for a Direct Digital Synthesizer I was building for myself.  Comlinear was bought by National that in turn was bought by TI and I couldn't find any more information.  I did find that Fairchild and Exar made them as the KH560:
 https://www.digchip.com/datasheets/download_datasheet.php?id=476423&part-number=KH560
 https://www.maxlinear.com/ds/kh560_ds.pdf
I mention this part because the data sheet gives enough information to design an output stage with a variable output resistance.  The active part of the circuit uses a few transistors, some current mirrors and is amenable to MHz bandwidths.

BTW, while looking for this part I ran across the history of Comlinear.  A fellow by the name of David Nelson came up with the CFA while working at hp developing Arbitrary Waveform generators.  He and others formed Comlinear to develop the idea as a product.
David Hess:

--- Quote from: duak on March 15, 2020, 04:45:57 am ---A Howland Current Pump is a more elegant circuit but harder to understand.  Hopefully David could help out here; my math has disappeared.
--- End quote ---

There is no need for me to do the math when someone else has done it better.  :) Today the technique I described is called "active termination" and it does exactly what is needed.  I have used it in the past for exactly the reasons described in the article here:

https://www.eetimes.com/active-termination-reduces-high-speed-interface-loss/

The problem now is that a variable resistance must be synthesized to control the positive feedback but at least the resistance is grounded on one end.
nihial:

--- Quote from: duak on March 15, 2020, 04:45:57 am ---Nihial,

You have changed your required bandwidth from a few kHz to 1 - 10 MHz, is this correct?


--- End quote ---

The more the better 1-10MHz would be much more than I need, in practice 100kHz should be enough.


--- Quote from: duak on March 15, 2020, 04:45:57 am ---When you said power supply, I understood that to be 0 to 30 V.  Is this correct or do you mean +/- 30 V?  My scheme can handle bipolar voltages with the appropriate driver stage.

--- End quote ---

[0 30]V


--- Quote from: duak on March 15, 2020, 04:45:57 am ---The Instrumentation Amplifier can be replaced by an AD734 multiplier to give a variable output impedance providing its input and output voltages are scaled to operate it within its linear region.  I used an INA to show the idea of variable output resistance as simply as possible.  I should have been more clear in that the gain of the INA sets the output impedance.  Please note that the AD734 will require voltage dividers to reduce the voltage applied to its inputs to avoid damage.

--- End quote ---

I was just tired when I read your post it became clearer after a night of sleep


--- Quote from: duak on March 15, 2020, 04:45:57 am ---A Howland Current Pump is a more elegant circuit but harder to understand.  Hopefully David could help out here; my math has disappeared.

Somewhere I have a data sheet and application note from a company called Comlinear that made a part called the CLC560 based on current feedback that could have its output impedance set by one or two resistors.  It was such an intriguing concept that I breadboarded one to see if it would work as an output stage for a Direct Digital Synthesizer I was building for myself.  Comlinear was bought by National that in turn was bought by TI and I couldn't find any more information.  I did find that Fairchild and Exar made them as the KH560:
 https://www.digchip.com/datasheets/download_datasheet.php?id=476423&part-number=KH560
 https://www.maxlinear.com/ds/kh560_ds.pdf
I mention this part because the data sheet gives enough information to design an output stage with a variable output resistance.  The active part of the circuit uses a few transistors, some current mirrors and is amenable to MHz bandwidths.

BTW, while looking for this part I ran across the history of Comlinear.  A fellow by the name of David Nelson came up with the CFA while working at hp developing Arbitrary Waveform generators.  He and others formed Comlinear to develop the idea as a product.

--- End quote ---

Interesting stuff, I see the idea behind it
nihial:

--- Quote from: David Hess on March 17, 2020, 11:19:20 pm ---
--- Quote from: duak on March 15, 2020, 04:45:57 am ---A Howland Current Pump is a more elegant circuit but harder to understand.  Hopefully David could help out here; my math has disappeared.
--- End quote ---

There is no need for me to do the math when someone else has done it better.  :) Today the technique I described is called "active termination" and it does exactly what is needed.  I have used it in the past for exactly the reasons described in the article here:

https://www.eetimes.com/active-termination-reduces-high-speed-interface-loss/

The problem now is that a variable resistance must be synthesized to control the positive feedback but at least the resistance is grounded on one end.

--- End quote ---

That's an interesting circuit, I rarely play with opamps so the maths burns a little bit my eyes but the concept is interesting. I began to work with duak's circuit as a base but I'll look at that circuit when I'm done
nihial:
Once I managed to find a way ton control the amplifier's gain with a voltage I'm done I think
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