Norton Amplifiers (was: Re: Wave Wraper revisted #1)

[Rene Schmitz]

>I do not too. It was also used by ARP for their filters. It seems to be
>interesting for
>low noise designs...
>Is there anybody knowing how to use these "Norton amplifiers ?".

Nortons are current differencing amplifiers, that is they have a current
mirror instead of the differential amp of ordinary opamps.
The input currents flow to the Vee terminal. So both inputs look like a
diode to Vee. There is a high gain amplifier connected to the inverting input.
A simple example of operation:
Think of the inverting amplifier configuration, feedback resistor (R1) from
out to inv-input and input resistor (R2) to inv-input. Lets say that the
Norton is powered from +/- 15V. To make it work with a Norton, we have to
bias the current mirror so that there is zero current difference for 0V
input. So we have to use a resistor equal to R1||R2 from GND to the
non-inv. input.
Usually all these are rather large values, to avoid large currents into the
mirror, which would affect the mirror-gain which we want to be unity. 
This biasing scheme is extremely useful in single supply circuits.

Now, the interesting trick is that when sourcing current into the
noninverting-input the inverting input starts to sink that current! 
In the ARP-filter there is a differential pair working on the current
mirror inputs. This acts as a simple OTA structure. The rest of the
amplifier acts as the buffer-integrator.

For more on the subject of Nortons see Nationals AN-72.

Hope this helps.

René 


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