Once virtually destroyed the ohmic resistance of a movement
https://www.researchgate.net/post/How_do_we_improve_the_real_ammeter_How_do_we_create_an_almost_ideal_ammeter_What_does_the_op-amp_really_do_in_the_circuit_of_an_op-amp_ammeter2
and a resistor
https://www.researchgate.net/post/Is_there_any_connection_between_the_humble_resistor_and_the_transimpedance_amplifier_What_does_the_op-amp_really_do_in_this_electronic_circuit,
why not try to virtually destroy the nonlinear diode resistance? Thus we will obtain a virtual “ideal diode” with zero forward voltage drop Vf.
The arrangement is the same – we add a variable voltage source in series with the diode so that its voltage adds to the voltage of the exciting voltage source and make it equal to the forward voltage drop Vf across the diode. As a result, the voltage drop as though disappears and the new “diode” looks as a “piece of wire” to the input source. What a magic – we increase the current through the “diode” but the voltage across it stays zero!
Finally, to obtain a true “diode”, we just replace the “helping” voltage source with a properly supplied op-amp and make it “observe” the difference between the voltage drop across the diode and its output voltage... and change the latter so that to keep the virtual ground. Practically, this means to connect the diode between the op-amp output and its inverting input and to inject the input current into the node at the inverting input (I will place a picture immediately below the question). As a result, its output voltage will always be equal to the voltage drop and the voltage across the whole network (seen by the input source) will be almost zero. Thus we have used a real diode to create a “virtual diode” with zero threshold voltage.
If you like this idea, visit the page below to see how my students applied it at the laboratory in 2008 (one of them is now an assistant professor in the same Computer Systems department)
http://en.wikibooks.org/wiki/Talk:Circuit_Idea/How_to_Make_Perfect_Components
Here is the op-amp implementation of an "ideal diode" - the so called "parallel diode limiter". It is interesting that here the virtual ground serves as an output while the op-amp output is not used. If we use it as an output, we will obtain another ubiquitous circuit. Can you guess what this circuit is?
We can create ideal rectifiers without using a diode but a MOSFET, an opamp and a voltage comparator instead. This is the idea behind, e.g., the AD633 rectifier. A year ago, I pose a question concerning this idea to my students in an exam. This very simple example can be found here https://cv3.sim.ucm.es/access/content/group/portal-uatducma-43/Webs/examenes/PDF/2012_09_04.pdf at page 11. Sorry, written in Spanish!! However, graphs and formulae are universal and easy to understand.The trick is simple... If the input is positive, the NMOS is a short circuit to ground. If negative, an open circuit. Thus, the input is rectified. The main advantage of this solution is the speed.
Hi Francisco! I can't stop wondering how wonderful analog electronics is... how intuitive it is ... how close it is to the human way of thinking ... if this whole world was collected into it!
Yes Francisco, the language could not be a barrier to understanding circuits... And to prove this, let me try to guess what you want to say in each figure...
Figura 1: I see the ubiquitous current mirror with a current setting resistor R connected to its input and a +5 V voltage stabilizer connected to its output. The current mirror should draw a current Vcc/R from the stabilizer. Here is my viewpoint at this circuit:
http://en.wikibooks.org/wiki/Circuit_Idea/How_to_Reverse_Current_Direction
Figura 2 shows an op-amp current source (sink) buffered by a transistor. It sinks a current Vin/R from the +5 V voltage stabilizer. The op-amp eliminates Vbe0 and the current is exactly Vin/R. See my Flash interpretation of this idea (click on the transistor in the library on the left):
http://www.circuit-fantasia.com/collections/tools/strange-things.html
Figura 4 shows an NMOS common source amplifying stage.
Figura 5 - an op-amp differential amplifier at a common-mode input signal.... and maybe something else?
And the figure at the end of the material represents a little exotic rectifier. The trick is to pass the positive half-wave and to invert the negative one. Am I right? As far as I can remember, the more traditional circuit solution was implemented with two op-amps and diodes and it was even more exotic - it passed the positive half-wave but subtracted the negative half-wave from the doubled positive half-wave (or something similar to this idea)...
Regards, Cyril
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