Is this a voltage reference or a power supply (etc...)? (determines how efficient/accurate the solution must be). For example, you could do it with a resistor and a zener diode if this is a simple reference and accuracy and efficiency are not important. Please give more detail on your specification/requirements.
if you use a resistance capable of dropping such a voltage then the lower cut off of 6v can not be maintained at the op and also take into consideration the resistance of the load as well in calculating the resistor
connect your ip in series with the calculated resistor and then your load the net op will be around your 6 - 13.9 V region but never have the same voltage 6v at op for an ip of 6v
if you use a resistance capable of dropping such a voltage then the lower cut off of 6v can not be maintained at the op and also take into consideration the resistance of the load as well in calculating the resistor
connect your ip in series with the calculated resistor and then your load the net op will be around your 6 - 13.9 V region but never have the same voltage 6v at op for an ip of 6v
I think I know what you're trying to do. The easiest way is probably to use a low drop out linear regulator similar to the LM317, with the output programmed to 13.9V. The output will (more-or less) track the input at low voltages, until it gets to the programmed voltage and then it will stick at 13.9V as the input rises above* 13.9V. For an example, see the 5V VOUT vs. VIN characteristic on page 6 of the LM117 / 317 data sheet:
But the output will always be below the input by an amount equal to the dropout voltage (usually 1 to 2 volts). If you want the output to be exactly equal to the input, you will need to use a switch mode power supply design which behaves the same way. This will also give you greater efficiency but will cost you more in terms of design effort, component count, circuit complexity etc. Hope this helps.
* Footnote: the input will have to be (13.9V + dropout voltage) before the output stays at 13.9V.
You can use a series regulator and can limit the out put maximum voltage to 13.9 volts. How you will change the input voltage? Will it be supplied gradually changed, manually or auto? If manually you can use a variable supply regulator like LM317 or any LDO series regulator.
There are some circuits for your reference: https://www.radio-electronics.com/i...rent_limiter/power_supply_current_limiter.php