Cheapest analog way to Vout = exp(Vin)

[king.oslo]

Hello there,

I have been trying to satisfy:



This may also be acceptable:



What are my analog options which roughly does what I want? I found something called exponential  opamp circuit which does:



But I cannot do this because of constrains I cannot easily overcome, I need the correct sign.

What are my options?

Thank you for your time.

Kind regards,
Marius

[IanB]

Probably you can make use of the fact that if:

y = e^x

then

dy/dx = e^x = y

Therefore if you set up an op amp in an integrating configuration and make it integrate its output, you will get exponential growth.

Most likely you will need suitable gain in the circuit to obtain the right time scaling.

[robrenz]

Use a second unity gain op amp as an inverter to change the sign of the output or maybe the input.

[IanB]

Actually, I see this is not quite what you are asking for. If you want a static exponential function of an input voltage, maybe you can make use of the diode characteristic equation in some way?

http://en.wikipedia.org/wiki/Diode_modelling

This inherently has an exponential characteristic.

[tszaboo]

If I remember correctly from the university "useless circuit collection" you have to put a transistor in the feedback of an opamp to get log, and on the non-inverting input for exp.
After searching for 5 mins, here is some stuff:
http://www.electronics.dit.ie/staff/ypanarin/Lecture%20Notes/DT021-4/6LogAntiLogAmplifiers.pdf
I dont expect them to work awesomely.

[dannyf]

Using a diode + resistor + opamp.

[king.oslo]

Actually, I see this is not quite what you are asking for. If you want a static exponential function of an input voltage, maybe you can make use of the diode characteristic equation in some way?

http://en.wikipedia.org/wiki/Diode_modelling

This inherently has an exponential characteristic.

Hello Ian,

I attempted that, but I failed. Perhaps you have some suggestions?

What I did was to put a diode in series with a current limiting resistor, and then measured the voltage across it as a function of V_IN using a difference amp.

As you can imagine, the resulting V_OUT is almost linear as dictated by the resistor. Do you have any suggestions?

Thank you for your time.

Kind regards,
Marius

[dannyf]

Using a diode (+ opamp) to convert the input voltage to current. Use a resistor to convert that current signal back to output voltage.

Use two diodes if you want it to be bi-directional (+ and - voltage).

[SArepairman]

what are these kinds of circuits even used for now adays? the only thing I can think of is a voltage controlled gain signal in order to replace a logorithmic pot...