Consider we have a nonlinear opamp circuit like this.
According to theory this op amp should take log of input.
I wonder whether it is possible to get a response like this :
I tried to plot vOut versus vIn but I didn't get meaningful results.
First try
I replaced diode with a 1N5817 schottky diode and changed resistor value to 200k. This time I got quite interesting result.
To plot transfer function on the horizontal axis I select V(n001). On the vertical axis I select v(vO) trace to plot.
Second try
After reading @Mattman944 's answer I exported Circuit's Lab simulation results as CSV. Since my primary OS is Ubuntu and Excel or LibreOffice Calc skills are quite bad I exported data to MatPlotLib.
import matplotlib.pyplot as plt
import numpy as np
import csv
time=[]
vIn=[]
vOut=[]
with open('devre.csv', 'r') as csvfile:
plots= csv.reader(csvfile, delimiter=',')
for row in plots:
time.append(float(row[0]))
vIn.append(float(row[1]))
vOut.append(float(row[2]))
plt.plot(vIn,vOut, marker='o')
plt.title('Log amplifier')
plt.xlabel('vIn')
plt.ylabel('vOut')
plt.show()
The result looks like this. May be there might be some way to plot data in logarithmic scale.
To be more precise I got these example from the textbook which I study.
In the example they ask \$v_O\$ in terms of \$v_I\$ and \$R\$ and they want us to quick sketch of answer. Also \$ \large i = I_S(e^{\frac{qv}{nkT}} – 1)\$, \$\large \frac{kT}{q} = 26mV\$ and \$ \large n\$ is between 1 and 2. These circuit parameters are given.
I solved the example on paper and sketched the output. Which looks like this.
I wondered whether theory matches practice so I tried to simulate the circuit in LTSpice.
Best Answer
The circuit only works for positive inputs. Add an offset to your input to keep it above zero. Loops in XY plots can be formed by capacitance, but at 5kHz, it should be negligible, I can't explain the loops. Try slower input frequencies, if it was stray capacitance, the loops will change. Finally, the circuit inverts, so the output is negative.
Works in CircuitLab. If CircuitLab can plot XY directly, I don't know how, I have only been using it a few weeks. I exported to Excel and plotted it.
simulate this circuit – Schematic created using CircuitLab
Edit: Based on comments from Kevin White, I re-ran at 5 kHz and a loop appeared. This confirms that stray capacitance can affect the results. Simulate slow and it works good.