# Problem with Op amp and zener diode circuit

I have the following problem.

Consider the circuit below:

The resistors have the resistance $$\R=1000 \Omega\$$.

The zener-diode has the breakdown voltage $$\V_{break}=2\mbox{V}\$$

When $$\V_{in}=-3 \mbox{V}\$$ what is $$\V_{o}\$$?

Okay, so here is my thought process:

Since $$\V_{in}\$$ is negative, the zener-diode cannot be in forward bias mode. If $$\V_o\$$ is less than $$\\mbox{2V}\$$, then there is no feedback loop, thanks to the zener-diode's breakdown voltage.

When $$\V_{o}=2\mbox{V}\$$ the breakdown voltage is reached, and a feedback loop is in place. And since this is negative feedback, the system will stabilize.

So my answer is that $$\V_{o}=2\mbox{V}\$$.

Now, this solution required me to "think a little", which is not a bad thing. However, in an exam situation I maybe won't be as calm or relaxed, to allow me to think like this.

So my question is: Is my solution even correct (if not then where did I go wrong?) and secondly, is there a solution that I can do in "auto-pilot" so to speak. Like node-voltage analysis or another way that doesn't need clever thinking.

I hope someone can help me with this.

• Isn't it fun to think about circuits in "clever" way? I always try to think in "clever" way when I find these kinds of circuits instead of going through all those equations involving solutions (if any). – G-aura-V Mar 13 '20 at 16:42
• Assuming ideal components your answer is fine, but your reasoning is not. Even if the diode is not in breakdown mode, there is a feedback loop with the two resistors. Maybe think along the lines that the voltage difference between the inputs of the opamp is zero (that's what it's trying to achieve) – Arsenal Mar 13 '20 at 16:43
• Hmm, now I'm a little confused. If there is a feedback loop even before the breakdown voltage is reached, then doesn't the system stabilize at a lower output voltage, before $V_{o}$ reaches the breakdown voltage? – Carl Mar 13 '20 at 16:51
• You should be able to calculate the voltage at the output if the diode wouldn't be there. – Arsenal Mar 13 '20 at 16:58
• The system stabilizes only when the inverting and non-inverting pins are at equal potential. We do our calculations assuming that the system is stable whenever there is a negative feedback. – G-aura-V Mar 13 '20 at 16:58