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I'm working on a biomedical project, and need to transfer power from a non-ideal source, with a very high internal impedance (~700Kohm) and low voltage, to a load with a high impedance (~1Mohm). I need to maximize the voltage delivered to the load, and because the source has a high internal impedance, I need to have a buffer between the source and the load. Would an op-amp based voltage follower do the job (non-inverting with gain = 1)?

Also, because of the design limitations, I'm trying to find the right op-amp. The priorities are: 1- It must have a low power consumption, 2- It must have a high input impedance, 3- It has to have a low vcc.

I found this from TI http://www.ti.com/product/lpv821 , which seems to satisfy my constraints.

To summarize, my questions are:

1- Is a voltage follower the right circuit? 2- Is the above op-amp suitable for this purpose? Thanks!

Update:

The input source is a low frequency AC signal (less than 1khz) and no DC. The peak to peak input voltage is less than 2.5V and I'm hoping to transfer the same to the load.

The maximum voltage to power the op amp would be 5V (2.5V for positive supply and -2.5 for negative) but the input wont be more than 1.25 in anplitude.

The circuit is meant to work in room temperature.

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    \$\begingroup\$ define all inputs Vdc , ac and output V for maximum power transfer you must match impedances To Buffer, you must have lower impedance DC \$\endgroup\$ – Sunnyskyguy EE75 Mar 14 '18 at 8:08
  • \$\begingroup\$ Not possible Direct connection would reduce amplitude to 10/17*Vin and with no DC you cant buffer. To create DC would load the signal even more \$\endgroup\$ – Sunnyskyguy EE75 Mar 14 '18 at 8:15
  • \$\begingroup\$ what are you trying to do? Are you trying to get all the power from your 'non-ideal source', or do you have a separate power supply available? \$\endgroup\$ – Neil_UK Mar 14 '18 at 8:30
  • \$\begingroup\$ Ideally, all the power would come from the 'non-ideal source', but I found no way to achieve this without using some active circuit such as op-amp or mosfet which need a separate power supply, but the active circuit has to be very low power. \$\endgroup\$ – vmontazeri Mar 14 '18 at 8:37
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    \$\begingroup\$ Look at the noise PSD, figure 26 in the DS, if you don't need DC you may not want a chopper amplifier. \$\endgroup\$ – sstobbe Mar 14 '18 at 16:23
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The op-amp is suitable but not from a power rail of +/- 2.5 volts. The maximum supply voltage is 4 volts and better to run it from no more than 3.6 volts (or +/- 1.8 volts).

The input source is a low frequency AC signal (less than 1khz) and no DC

Providing the signal is sinusoidal and less than 1 kHz then there should be no problem. The GBW product of the device is only 8 kHz and this means that if the signal were (say) a triangle wave at 1 kHz you would see distortion on the output due to attenuation and irregular phase shifting of harmonics.

In case there is a possibility of exceeding the input rails (either through bench testing or some other unexpected event) I would recommend figure 37 and the inclusion of a series resistor with the +in pin to prevent input currents exceeding 10 mA. A 1 kohm resistor would protect the circuit from a signal of 20 Vp-p.

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  • \$\begingroup\$ Thanks for the answer. Do you know any alternative low power op-amp that's suitable for these specifications? The fact that the bandwidth is limited to 8KHz made me consider looking for alternatives. \$\endgroup\$ – vmontazeri Mar 14 '18 at 17:32
  • \$\begingroup\$ It’s a very specific application and the opamp you found is probably an ideal solution given that current is under a micro amp. If you have reservations about your input bandwidth you need to be clear about what your expectations are. \$\endgroup\$ – Andy aka Mar 14 '18 at 19:51

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