Say I need to create a negative supply rail of -5V (+-0.5V) from a DC input of +5V without the use of any ICs whatsoever. The components available to me are BJTs, Mosfets, diodes, resistors and caps. So far my basic understanding is this –
Make an oscillator circuit (maybe astable multivibrator) to convert the positive DC into a 0-5V square wave and then use a voltage doubler to get a DC voltage on the negative side.
The problem is that my output is at around -3.5V. Maybe I should amplify the square wave before passing it to the next stage? Or ditch this method altogether? Suggestions would be appreciated.
Best Answer
Your solution is quite close. However you can take advantage of have 2 low ohmic output phases of your oscillator and build a charge pump. The schematic would be as the following,
simulate this circuit – Schematic created using CircuitLab
Where PhiA and PhiB connected to each phase of your relaxation oscillator.
On the charge phase of either pumping cap you loose 1 diode drop, 0.7 V. On the discharge phase you loose 1 Vcesat plus 1 diode drop of 0.7V.
So for a 1 stage charge pump as you have shown, you will get under no load approximately -(5-0.7-0.7-0.2) ~ -3.4 V.
For the two stage charge pump its approximately double that at -6.8 V.