Electronics > Projects, Designs, and Technical Stuff
Howland Current Pump
T3sl4co1l:
Why do you need such a high impedance at high frequencies?
It can be possible to make such impedances, but it is very often possible to either compensate (e.g., instead of using a current source to measure an impedance, use an impedance divider) or null (say in a narrowband condition).
For example, you're using an op-amp in the first place, can't you tweak it to give a negative capacitance? (Be careful it doesn't oscillate ;D )
Tim
David Hess:
--- Quote from: JoelK on December 10, 2019, 02:47:31 pm ---Do you have an example for such a bootstrapped JFET buffer?
--- End quote ---
Some oscilloscope high impedance input stages used bootstrapping to raise their input voltage range. More often it is used with bipolar input stages to raise low frequency input impedance but the circuits for bipolar transistors still apply to a JFET design to control input capacitance which is done in operational amplifiers. The Tektronix 7A12 and 7A13 are examples but I doubt they are very applicable to what you are trying to do.
I have also seen it done with active probes where it improves the precision of the FET buffer.
Below is an example from National Semicondcutor application note 32. The DC precision could be considerably improved by adding a matched JFET current sink with matched series bipolar base-emitter junction. Alternatively a DC servo loop could be used and I think without adding to the input capacitance.
--- Quote ---Bootstrapping the supply rails is something I already thought about. But my problem is the limitation of a single 5V supply rail. With a signal swing of only +/-1V at the output and a rail-to-rail opamp for bootstrapping it would be possible. So that the feedback opamp is normally supplied by the rails 1V and 4V which rise to 2V and 5V in case of a possitive 1V swing at the current pump output.
But does this make sense or do I even decrease my specs when I use bootstrapping under this limitation?
--- End quote ---
A low supply voltage is going to make things more difficult.
--- Quote ---The current pump is not intended to be used with DC if you mean 0Hz DC. I think the most used frequency will be about 1kHz but the goal is to design a wideband current pump to be used at many different frequencies.
--- End quote ---
What about adding a series output inductor to raise the output impedance at higher frequencies?
JoelK:
--- Quote from: T3sl4co1l on December 10, 2019, 06:58:27 pm ---Why do you need such a high impedance at high frequencies?
It can be possible to make such impedances, but it is very often possible to either compensate (e.g., instead of using a current source to measure an impedance, use an impedance divider) or null (say in a narrowband condition).
For example, you're using an op-amp in the first place, can't you tweak it to give a negative capacitance? (Be careful it doesn't oscillate ;D )
Tim
--- End quote ---
I personally don't need it, but maybe someone else. It's a master thesis so part of it is research and to see whats possible. The output impedance is currently about 300kOhm at 300kHz. So the input impedance is definitely not the only problem. Right now I am trying uncompensated opamps like the LTC6268-10 together with lead-lag compensation. Thus stability is a difficult topic. What do you mean by tweaking the opamp itself? On the current prototyp is a negative impedance converter at the output of each current pump which is not populated (for later).
David Hess:
--- Quote from: JoelK on December 10, 2019, 09:08:33 pm ---Right now I am trying uncompensated opamps like the LTC6268-10 together with lead-lag compensation. Thus stability is a difficult topic.
--- End quote ---
I remember trying that to improve circuit performance before I learned better. It always comes down to raising the noise gain to provide stability which has the same effect as if the input signal was attenuated and closed loop gain increased; the whole exercise is counterproductive. There might be something to gain (haha) if you have access to the nodes required for external compensation but modern operational amplifiers do not provide access to them.
--- Quote ---On the current prototyp is a negative impedance converter at the output of each current pump which is not populated (for later).
--- End quote ---
Doing a transformation to add a negative input capacitance can be done at lower frequencies but I did not propose it because at higher frequencies it is difficult.
JoelK:
--- Quote from: David Hess on December 10, 2019, 08:57:50 pm ---Below is an example from National Semicondcutor application note 32. The DC precision could be considerably improved by adding a matched JFET current sink with matched series bipolar base-emitter junction. Alternatively a DC servo loop could be used and I think without adding to the input capacitance.
--- End quote ---
But dont I get a simulare problem like trying to bootstrap the supply rails of the opamp? The input swing of the discret buffer won't be high neither.
--- Quote from: David Hess on December 10, 2019, 08:57:50 pm ---What about adding a series output inductor to raise the output impedance at higher frequencies?
--- End quote ---
I will simulate this. Thank you!
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