Electronics > Beginners

Exponential Amplifier

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eev_carl:

--- Quote ---It could be too late for it - if you've really managed to apply 1V and 10V voltage to the input... .
--- End quote ---

Why?  Isn't the 741 good for +-15V and the PN2222A good for 50V or so?

David Hess:

--- Quote from: eev_carl on July 24, 2018, 03:20:18 pm ---
--- Quote ---It could be too late for it - if you've really managed to apply 1V and 10V voltage to the input... .
--- End quote ---

Why?  Isn't the 741 good for +-15V and the PN2222A good for 50V or so?
--- End quote ---

The transistor's emitter-base breakdown voltage is only about 5 volts which is why the diode is included in the circuit.  Emitter-base breakdown is very damaging to transistors which are intended for use in translinear circuits.

For now why not use just a diode instead of the transistor to get an idea of what should be going on?

And move the capacitor to be in parallel with the feedback resistor.  Where it is now causes the operational amplifier to act as an unstable differentiator.  The circuit as shown even with the transistor and diode used correctly will oscillate.

GigaJoe:
may it help:
https://www.ti.com/ww/en/bobpease/assets/AN-31.pdf

nugglix:
Newer version, revised 2013:
http://www.ti.com/lit/an/snla140b/snla140b.pdf

eev_carl:
Hi,

I built the exponential amp w. a single diode. Here is my schematic and test results.  The results produce a straight line on a semi-log graph which is what I was looking for.

Thanks to all

(in mV)

VinVout200-0.7300-13.6400-109.6500-828600-4590

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