Electronic – Op-amps circuit with diodes

diodesoperational-amplifiertransfer function

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I am a beginner at electronics and I am trying to solve my first problems with diodes and opamps. I have to find and draw the output voltage uο related to the input voltage ui knowing that the voltage drop on the diodes is 0,7V when they conduct. It is also known that opamps are ideal so i guess that no currents flow in or out of the inputs and the difference between the inverting and non-inverting terminals can be considered to be zero. However, i am bit confused with the diodes. How do i start solving a problem like this?. Do i have to assume first that ui is positive and then that ui is negative? Any help is appreciated!Thanks in advance.

Best Answer

You can analyse it in two conditions of input ui.

  1. When ui > 0

The op-amp output is positive and hence D1 will be forward biased and D2 will be reverse biased. Assuming ideal diodes and op-amps, we can draw the circuit like this.

enter image description here

Op-amps are ideal and have negative feed back. So \$V_B = V_A = ui \$

At the node B, apply KCL/nodal analysis for three branches.

Input currents to op-amps are zero. So:

$$\frac{(V_B - uo)}{R} + 0 + 0 = 0$$ $$\implies V_B = uo$$ $$\therefore uo = ui$$

  1. When ui <0

The op-amp output is negative and hence D1 will be reverse biased and D2 will be forward biased. Assuming ideal diodes and op-amps, we can draw the circuit like this. enter image description here

A little more neatly.

enter image description here

It is simply a buffer driving an inverting amplifier. The input to the inverting amplifier is ui itself. I can therefore write the traditional input output relation of inverting amplifier like:

$$uo = -ui(\frac{R}{R})$$ $$\therefore uo = -ui$$

Now you can plot a sine waveform for ui as \$V_{in}\$ and u0 as \$V_{out} \$ to see that it would just act as a full wave rectifier. enter image description here

--EDIT--

Even if diode drops are considered, the same output is obtained at the end. This is an example of Full Wave Precision Rectifier. It acts as a rectifier with ideal diodes.

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