0v to 10v Convert to 4ma to 800mA

It is all Greek. What exactly do you want? What is your Applicaton?

I have designed a circuit with an input 0 to 10 volts. 0 to 40 mA output.Now I want to increase it to 800 mAWith the transistor, which is OK but it did not change linearly with input
Can someone help?

https://www.google.co.uk/search?q=CONVERT+0-10V+TO+4-20MA

His requirement is different. His required out put is as high as 800 mA.

It might be a typographical error given the nature of the post - especially as 4-800mA is not a standard signal transmission range, there is no need to use this level of current for signal transmission, and there is no equipment available off-the-shelf to receive it?

http://lee-dickens.com/ (usual disclaimer).

I have designed a circuit with an input 0 to 10 volts. 0 to 40 mA output.Now I want to increase it to 800 mAWith the transistor, which is OK but it did not change linearly with input
Can someone help?

http://cr4.globalspec.com/thread/81732/0to10v-Convert-to-4ma-to-800mA

I will assume the output transistor is connected with a current sense resistor in the emitter. Lowering the sense resistor will increase the current. There is a limit when the current drive at the base multiplied by the beta of the transistor, is less than 800 mA. You may need to drive more current into the base. Also check that the resistance of the load can handle 800 mA without saturating at the supply voltage.

If you use a MOSFET instead of a bipolar junction transistor (BJT) the error and drive limitation of the input base (now gate) is avoided. This is usually a better design. You need to be careful that the Vgs(th), the gate threshold voltage, is low enough and the Rgs, the channel resistance, is low enough, and then the MOSFET behaves a lot like a BJT.