Driving a varicap from a higher voltage?

[neazoi]

Hello I have a 1-8v driven varicap. How can I drive it from an input voltage of 15-30v?
A voltage divider wouldn't work I think

 

[D.A.(Tony)Stewart]

Can you use an Op Amp? Using a /2 divider gets you 7.5 to 15 and a unity gain differential amp with 6.5V reference gets you 1~8.5.
Many combinations of gain and reference voltage can lead to the same result.

 

[neazoi]

Can you use an Op Amp? Using a /2 divider gets you 7.5 to 15 and a unity gain differential amp with 6.5V reference gets you 1~8.5.
Many combinations of gain and reference voltage can lead to the same result.

I think this cannot be done in my circuit because the circuit is located remotely (phantom supply through the same coaxial) and it will be varied, so a reference requirement will require a more sophisticated circuit.

A 15-30v input voltage followed by a voltage divider will give about 1-3v roughly. What if I replace one of the resistors on the divider with an LDR pointed by a LED that is light up through a suitable series resistor by the input voltage? This will make an exponentially changing voltage divider I think but I am not sure.

 

[D.A.(Tony)Stewart]

it depends how precise you need. Can you define stability and environment?

Certainly you can put an offset with a zener or a string of LEDs such as 5x white leds @ 1mA then divide by 2

But a simple bandgap regulator for op amp to act as "level shifter" might be most precise.

 

[neazoi]

it depends how precise you need. Can you define stability and environment?
Certainly you can put an offset with a zener or a string of LEDs such as 5x white leds @ 1mA then divide by 2
But a simple bandgap regulator for op amp to act as "level shifter" might be most precise.

Any other way you could think of?