This is our term project and if you tell me some tricks about it, I would be very thankful.
We are asked to design an adjustable voltage regulator . Designed regulator should be able to give 4V-5V and 6V. While the input voltage varies between 10-15V we are given some requirements in for each regulated output individually.
load regulation:%1 at 4,5,6V separately when input is 10V to 15V.
line regulation: %1 at 4,5,6V separately when input is 10V to 15V.
ripple rejection ratio:50 for 4,5,6V separately when input is 10V to 15V.
I succeed in to stabilize load and line regulation in 4V output but in 5 and 6V , regulation is about %4 , it's much more higher than wanted. I used a shunt voltage regulator with zener and added a transistor for feedback.
I posted an attachment about my circuit. If you consider helping me about this, thanks.

 

Download some Manufacturer Data Sheets for IC based Adjustable Regulators and examine the internal schematics for clues to how this type of regulation is done.

I would try reducing the current though the Zener and Q2.

 

looks like your feedback transistor is backwards, the emitter should be connected to the zener, as the reference, and the collector coupled to the base of the series pass transistor.

Also the reference violtage should be as high as possible to the v.out for better regulation.

Sometimes the zener needs a trickle resistor, to bias it.

 

Yes, I realized my feedback transistor is backwards, corrected it. Reducing current, I get more suitable results. Thanks!