In the following circuit i want to find the output voltage Vo related to the input Voltage Vi. Diodes D1 and D2 are the same and have a 0,7V drop when they conduct. Op-amp is ideal too. For the zener diode we know that: $$Vzener=8,6V$$
Well, when our input is positive (Vi>0) , the output Vo is negative(Vo<0), so Zener diode conducts but both D1 and D2 are reversed biased,so the circuit works as an inverting operational amplifer with Gain=-Rf/Rin=-10K/1K=-10. So Vo=-10Vi.
I am a bit confused with the other case about the negative input(Vi<0). Then the output will be positive(Vi>0). Will this make the zener diode conduct in the opposite direction and D1,D2 forward biased as well, so that we have another path from the output to the inverting input?
Best Answer
When the input is negative (and small) the diodes and zener can be ignored.
As the input signal gets bigger negatively you will reach a point when the output becomes +(8.6 V + 2*0.7V = 10 volts). Prior to this you have a gain of -10k/1k and at (or above) this point you have full negative feedback and the circuit has zero gain hence, the output limits at 10 volts. It can do nothing else theoretically.