Papabravo said:Would you care to offer a precise definition of what it means to "linearize" an exponential waveform? It sounds like a really silly idea. Why would you want to do such a thing?
ronsimpson said:An RC causes the exponential decay.
Use a current source and capacitor.
k7elp60 said:As I remember if you take and op amp in the inverting mode, put a capacitor from the - input to ground and connect a resistor to a + supply the output from the op amp will be a linear increasing rampas the capacitor charges. If once the capacitor is charged and then the resistor lead that was connected to the + supply is grounded the output of the op amp would be a linear ramp going down. Perhaps some modifications of this circuit would solve the problem.
Papabravo said:Would a logarithmic amplifier do the job?
Papabravo said:Yes. The output of a logarithmic amplifier is the logarithm of the input. So, says me, if you feed an exponential signal in you'll get the exponent out which is a linear function of the time t. N'est pas?
Roff said:DigiTan, he has already made it clear that his goal is not to generate a linear ramp per se. His goal is to linearize the exponential discharge of a capacitor.
I agree that a log amp is the way to do it.
DigiTan said:If you take a 555 timer and inject a constant current source into its its threshold and discharge pins, you will see linear ramp appear on that pin. The straightness of the ramp depends on the flatness of the current source's I-V characteristic.
I used the attached system in a ramp ADC running close to 44kHz. The ramp will appear on the collector of Q1. The trigger can be the output of another 555 if you want. I'd also adjust C1 and C2 to get right time constant.
The main limit is that it will only drive high-impedance loads. Our load was about 100kOhms. When we lowered it below 50k, the ramp seemed to destabilize.
JimB said:Am I missing the point here?
I understand that the idea is to create a linear ramp from an exponential decay.
But why? What is so magical about the original exponential decay?
If you need a linear ramp, then make one using the appropriate techniques.
This makes as much sense as saying:
"I have a steam engine and want to convert it into a diesel engine"
Just my two Ringits worth.
JimB
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