Electrical – op amplifier with both negative and positive feedback

circuit analysisoperational-amplifier

For must of the circuits, I know how to analyze them but I am having difficulty with this one, and with the understanding of its purpose.


the switch changes his state every 0.5 msec.
VZ is 5.6V

the problem I am having with this circuit, which is where I think I am doing my mistakes, is that I can't decide if this is a negative feedback or positive feedback.
Also because of my fail to analyze the circuit correctly, I can't understand what is it for? I think it supposed to be astable multivibrator but I can't confirm it.

edit :
the output of the circuit is the output of the opamp.
image link : img

edit2 :
assume general ideal op-amp, I used the TL because it was the 1st op-amp I saw.

edit 3:
I am trying to draw the timing diagram and this is what I get and I can't understand why I am wrong here.
sorry for the poor handwriting, no ruler was around.
Vs = Vswitch. Vx= Voltage point between R1 and R2.


Best Answer

This circuit produces a linear ramp output. The zener diode and R1 produce a constant voltage across R2 (since the op-amp output tracks the capacitor voltage as it increases). The output slope is \$+\frac{V_Z}{R_2 C_2}\$ (once the switch opens, before that it sits near 0V, depending on the op-amp and supplies).

The output ramp will deviate from linearity when either the op-amp output saturates or the zener is no longer able to maintain regulation. The op-amp should have bipolar supplies for this to work well, or use a 'single supply' op-amp that has an output that can get close to the negative rail.

For better results you can replace the zener with a shunt reference such as the LM4040.

Edit: Here, in the below simulation, you can see the action- the output voltage vs. time is very linear for the first millisecond or so. The switch opens at t=0+, allowing the current to flow into the capacitor.


simulate this circuit – Schematic created using CircuitLab

enter image description here