Talk:Circuit Idea/Op-amp Inverting Current-to-Voltage Converter

The text below is the comment that I wrote a few monts ago to the Bob Pease's article What's All This Transimpedance Amplifier Stuff, Anyhow? published in EDN.

-- Although the article is great, it does not show the basic idea behind the circuit in the introductory part; as a result, the readers "cannot see the forest for the trees". In order to really understand the circuit, they need to know what the op-amp really does there.

Actually, the idea is extremely simple and well-known from our human routine (maybe, because of that I needed years more to reveal it:). We frequently use this technique to "neutralize" some negative quantities by equivalent positive quantities and v.v. In the circuit of a transimpedance amplifier, the op-amp does exactly the same - it compensates the "harmful" voltage drop VRf across the resistor by an equivalent "antivoltage" Vout = -VRf; as a result, a virtual ground appears. For this purpose, it adds so much voltage in the circuit as it loses across the resistor R; in this way, it actually "helps" the input excitation voltage source, which creates the input current (note that the two voltage sources are connected in series, in one and the same direction - "- +, - +" - so that their voltages are added).

Finally, we use the compensating (op-amp) voltage as a "mirror" output (just like in life when we estimate indirectly some positive quantity by an equivalent negative "antiquantity" and v.v.). The advantages: first, the load is connected to the common ground; second, it consumes energy from the "helping" voltage source instead from the input source (so, it may be low-resistive enough). Maybe, some disadvantage is the inverted output.

All the circuits with parallel negative feedback (i.e., all the inverting op-amp circuits) exploit this idea. By the same way, the op-amp compensates the voltage drop across the capacitor in a charge amplifier and in an op-amp integrator, across a diode - in an op-amp logarithmic converter, etc.

If you want to know more about this topic and the basic ideas behind circuits, visit my site of www.circuit-fantasia.com and the Wikipedia pages about transimpedance amplifier, voltage-to-current converter, virtual ground and negative resistance.

Cyril

Circuit-fantasist (discuss • contribs) 16:00, 6 August 2013 (UTC)

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