Is that possible to use a voltage follower/buffer circuit as summing amp? In history, what kind of circuit was used for summing amp before people started to use those inverting summing amp circuit?

-ref buffer circuit http://www.learningaboutelectronics.com/Articles/Voltage-follower

Thanks for your help

  • \$\begingroup\$ You don't need an op-amp to make an inverting summing amplifier. \$\endgroup\$ – Andy aka Oct 2 '15 at 13:26

In principle you can, but not well.

The key advantage of the inverting configuration is the virtual earth input. This means that each input signal can have no effect on other inputs, there is no accidental cross-coupling of input signals.

Into a high impedance like your buffer, you can - not sum, but average - several inputs via a resistive network, but if you leave one input open-circuit for example, you'll find that average voltage appears on that input. So it's not wery accurate unless every input is buffered.

So other ways of providing a virtual earth have been used in the past - a common-cathode or common-base amplifier would do.

  • \$\begingroup\$ Brian, why "not sum, but average"? Connecting two equal resistors to the non-inv. input (for feeding in both input voltages) and using a gain-of-two amplifier, you have a perfect non-inv. summing amplifier. \$\endgroup\$ – LvW Oct 2 '15 at 13:40
  • \$\begingroup\$ Right ... Double the average to get the sum of 2 inputs. But the question specified unity gain amplifiers... \$\endgroup\$ – Brian Drummond Oct 2 '15 at 14:20

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