|Created||September 22, 2013|
|Last modified||September 30, 2013|
Nearly Linear Voltage ramp-up from 0v to about 22v in one second. Stable output and low noise.
This is a design for a linear voltage ramp up. I'm using it to provide a soft power-up for a phantom power feed to a microphone I've built. The capacitor charges very slowly through the 'Throttle' resistor (RC Time constant of just under 5 seconds) The Transistor Q1 acts like an emitter-follower providing unity gain. The Collector voltage is the feed to my phantom power via a low-pass filter.
The Op Amp feeds the transistor's base and takes its feedback from the collector through a resistor network that ensures the voltage at the collector is about 5 times the voltage seen across the capacitor.
Why? Well I'm only using the first 20% or so of the capacitor's charging cycle. A small portion of the charging curve looks linear, and the total effect is a nearly linear ramp, over about 1 second, from 0v to just under the supply voltage.
I searched around for voltage ramp circuits but because I made this one I understand it and I'm very happy with the result. With the Low Pass filter at the output, and a load draw of about 30mA (which is what the rest of the circuit requires), my oscilloscope measures less than 0.1 mV voltage noise across a 22 V output, and the frequency spectrum of the noise looks quite 'White' so is probably resistor noise which I'm happy enough with.
Hope it's useful. It's probably naive. But it works..
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