I have been reading about Norton Operational Amplifiers, I get a good idea of how they work, even at the transistor level, I understand that difference current in = voltage output. What I have been having trouble is finding about the advantages of these opamps VS the traditional voltage ones, is it speed?, bandwidth?, what?

Where should one use a Norton Op-amp instead of a regular voltage Op-amp?, I ask this because if its a matter of applications in which the output is a current (like in a DAC) one can still use regular voltage opamps, for example, an inverting transimpedance amplifier with its input as a virtual ground.

Two example data sheets are:

LM3900 LM359

The following video explains fairly well how a Norton (Current Differencing Amp) works. Look at 19:30 on the vid

Norton Amp

However again, I need to know in which applications this amp is best, and what should drive me to use a Norton amp instead of a voltage amp.

  • \$\begingroup\$ Can you link to a datasheet for an example of the kind of part you're asking about? \$\endgroup\$ – The Photon Sep 17 '19 at 21:48
  • \$\begingroup\$ Just added 2 datasheets at the end of my question. Thanks \$\endgroup\$ – S.s. Sep 17 '19 at 22:11

A norton opamp works with currents, you use one if you want to find the difference between two currents, then use a Norton operation amplifier. Even the Norton opamp outputs a voltage.

Most applications and sensors don't use currents as a signal carrier. Using current usually means burning up power. One problem is many sensors are resistive and this also creates problems if you wanted to subtract currents to find the difference between them. I can't think of a reason to use a Norton op amp and subtract two currents (which is probably why you don't see them all that often), all of the examples in the link below show the Norton opamps being used to convert the Norton opamp to a voltage opamp.

The LM3900 is a 14-pin DIP containing four identical op-amps, each with inverting and noninverting inputs and an output. However, these op-amps are very different from the usual op-amp, and must be used in a completely different way. The usual op-amp responds to a differential voltage at its inputs, but the LM3900 responds to a differential current. Instead of a differential amplifier, the input stage is the current-differencing circuit shown at the right. The current mirror at the noninverting input subtracts the current at that input, I+, from the current at the inverting input, I-, to form the difference current I- - I+ that is furnished to the amplifier with overall gain of 70 dB. If I- is greater than I+, the output saturates low, and if I- is less, the output saturates high. Feedback from the output to the inverting input acts to reduce the difference current, which in normal operation is very small. This is just like the usual op-amp, except with current instead of voltage. The output, however, is a voltage as with the usual op-amp. The label "Norton" refers to the Norton equivalent circuit for a current source.

Source: https://mysite.du.edu/~etuttle/electron/elect21.htm

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  • \$\begingroup\$ "all of the examples in the link below show the Norton opamps being used to convert the Norton opamp to a voltage opamp.", Yes thats why I dont understand what are the advantages of such amp. \$\endgroup\$ – S.s. Sep 17 '19 at 22:34
  • \$\begingroup\$ That's because there are no advantages to the modern opamp. In 1984 there may have been \$\endgroup\$ – Voltage Spike Sep 17 '19 at 22:36
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    \$\begingroup\$ The big advantage of a Norton OP is its high bandwidth. This is due to the current subtraction right in the input stage, even before the first transistor. Only the current difference ever gets amplified. See both the internal schematic of the LM359 and the LM3900. \$\endgroup\$ – Janka Sep 17 '19 at 23:15
  • \$\begingroup\$ @Janka Thanks!! If I could up your comment twice, I'd do it. That was the first thing I was looking to see in Spike's answer and missed seeing. \$\endgroup\$ – jonk Sep 18 '19 at 4:34
  • \$\begingroup\$ High bandwidth as in unity gain around 10MHz? There are opamps that do 100MHz now \$\endgroup\$ – Voltage Spike Sep 18 '19 at 5:15

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