Positive exponentials (against time)in electronics anywhere?Thought "experiment" - Page 1
Forum section: electronics / general

author: Infraviolet
author_rank: Super Contributor
timestamp: March 20, 2024, 12:02:07 am
content: Just some idle ponderings but...Can anyone think of an example of a positive exponential voltage (between any two points) or current (with respect to time in whichever case) occuring anywhere in electronics?There are plenty of negative expontentials "e^(-const*t)", and of "const - e^(-const*t)", such as in the discharging and charging of capacitors, but what about positive expontentials which run away with themselves, until some practical limit is reached and the behaviour is stopped at a steady state (including by the circuit being ruined in the process).The closest thing I can think of is where the resistance inside a MOSFET rises with the MOSFET's temperature, in which case the temperature can go up expontentially until heat losses reach equilibrium or the mosfet is destroyed, but that's a temperature rising exponentially, rather than a strictly electrical parameter, like a voltage or current.

author: IanB
author_rank: Super Contributor
timestamp: March 20, 2024, 12:10:02 am
content: It's going to happen anywhere you have positive feedback. Practical examples would be with a comparator, or a Schmitt trigger.If you plot the change in state over a short enough time scale on an oscilloscope you should be able to see the exponential rise of the signal.

author: Benta
author_rank: Super Contributor
timestamp: March 20, 2024, 12:16:17 am
content: Current through an NTC resistor at constant voltage?Voltage over a PTC resistor at constant current?Both have to be self-heating, of course.

author: twospoons
author_rank: Frequent Contributor
timestamp: March 20, 2024, 12:17:55 am
content: Thyristor turn-on?

author: tom66
author_rank: Super Contributor
timestamp: March 20, 2024, 12:19:53 am
content: Avalanche breakdown (in diodes, zeners > 5.6V) has a positive exponential term in it, if I remember correctly.

author: SiliconWizard
author_rank: Super Contributor
timestamp: March 20, 2024, 12:32:56 am
content: An exponential is a reasonable approximation of a diode I-V characteristic (Shockley model). Generate a ramp across a silicon diode in series with a resistor, look at the voltage across the resistor and you'll have roughly an exponential as long as you stay away from breakdown of course. The breakdown part is a different story which I'm not covering here.

author: ebastler
author_rank: Super Contributor
timestamp: March 20, 2024, 07:36:50 am
content: Quote from: SiliconWizard on March 20, 2024, 12:32:56 amThat approximation is good enough to be widely used in analog computers to calculate log and exponential functions.Including the use in multipliers which calculate a*b = exp (log(a)+log(b)). Typically using a transistor in diode connection, rather than a plain diode, for improved dynamic range to my knowledge.Edit: Oops, only now have I registered the "against time" in the thread title. So log or exponential amplifiers don't count... As others have pointed out, positive exponential growth can be generated via electronic components or circuits, but I can't think of a common technical application.

author: SiliconWizard
author_rank: Super Contributor
timestamp: March 20, 2024, 07:39:38 am
content: Yes. The relatively "common" application is the log amplifier:https://en.wikipedia.org/wiki/Log_amplifier

author: Circlotron
author_rank: Super Contributor
timestamp: March 20, 2024, 09:15:29 am
content: Take the output of an opamp integrator, put it through a X1 inverting amplifier then feed the output back to the integrator input.