# Voltage level shifter

#### akis02

Joined Apr 30, 2011
57
Hi

I need to feed the output of one stage to the input of the following, but the DC voltage levels do not match ( sometimes they do match coincidentally and all is good ). Of course a capacitor solves the problem nicely, but it introduces phase shifts and it is not down to DC.

I have tried to devise my own voltage level shifter but at best it requires extremely accurate biasing or my simulation software refuses to deal with it. I tried to use two transistors in series (cascade ?).

Would anyone be able to suggest a simple circuit I could use as a starting point for learning purposes?

The parameters are:

Input:
DC and AC at audio range (ie no capacitors)
Voltage range from 0V to X Volts, where X < Vs (Vs is supply voltage)
Source impedance - relatively high, eg 10K, 20K, 50K

Thanks

#### R!f@@

Joined Apr 2, 2009
9,747
Welcome to the forum.

Why don't you attach ur schema ?

#### BillO

Joined Nov 24, 2008
990
A schematic would definitely help. Include the output stage of the source voltage and the input of the following stage.

As long as Vs is less than 36v, something like an AD820 FET input op-amp might be the ticket.

#### akis02

Joined Apr 30, 2011
57
OK here is the circuit I devised and was playing with yesterday on the simulator. I need to actually make it on a breadboard and try it out.

The idea is this: T2 acts as a constant current source so that the current is held the same across the whole circuit, therefore the voltage drop across R1 determines the "shift" we want to introduce. Of course the voltage drop across T2 cannot be negative, so there are limits to what it can do.

The choice of required voltage drop across R1, the hfe of the transistors plus the (constant) current value determine the input impedance of the circuit.

I am working it out now in Excel so I can play with different input parameters: Vs, V in, voltage shift, Ic etc

However for some reason the simulator just stops...

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#### akis02

Joined Apr 30, 2011
57
This is a proper circuit (somewhat contrived).

Vs=10V
Vin is 7V
Vout is 5V

Rin is 220K and Rout is ~ 2.2K

As far as I can see the limitation is that it is very dependent on Vs and I would like it to be independent as much as possible so for example I could use it in an op-amp

Any comments/help on how to make it Vs independent or propose another circuit would be greatly appreciated.

Akis

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