Electronic – Level shifter using BJT schematic

Your circuit is a half decent regulator and would be good for powering most things that need a small current. The problem, as you quite rightly point out, is the 0.7V lost between input and output.

Low drop-out regulators are really quite common-place and some work really well down to a few milli volts between input and output BUT, they are far more complex than what you think and cannot be made from a couple of transistors, a handful of resistors and a zener diode.

However, in pursuant with your wishes, the best idea I can come up with is to construct a small single transistor colpitts oscillator that runs from the incoming power supply and, via two capacitors and two diodes can create a voltage that is bigger than the incoming voltage - this can then feed the zener diode via a resistor ensuring that the BJT is always biased on at the zener voltage even when the supply is quite close to the zener voltage.

Probably the best you could hope for is a volt-drop across the transistor of 0.1 to 0.2V - but that is better than 0.7V but not as good as some commercially available devices from the usual vendors.

Short summary,

• A diode can be forward or reversed biased. This concept only makes sense in the presence of external voltage bias.
• The intrinsic potential difference of the p-n junction is -0.7V. The negative sign is by convention as it goes against turning on/forward biasing the diode.
• By applying an external bias of +0.7V we counteract the intrinsic potential difference. The positive sign of the bias is by convention as it helps turning on the diode.

In detail,

• p-n junction diodes are one of the most important Silicon building blocks, so they come with their own set of conventions:

When a diode is externally biased by applying a more POSITIVE potential to the p end of the junction than to the n end of the junction, we say that it is FORWARD biased and we define that bias voltage with a POSITIVE sign.

When a diode is externally biased by applying a more NEGATIVE potential to the p end of the junction than to the n end of the junction, we say that it is REVERSED biased and we define that bias voltage with a NEGATIVE sign.

Following the previous, we say than the forward current is POSITIVE and the reverse current is NEGATIVE. A positive current is defined as the current in which the holes (h+) move in the same sense and/or when the electrons (e-) move in the opposite sence.

• You can think/visualize the p-n junction diode as having an internal or intrinsic potential difference of -0.7V (referenced from p to n). This will help you when analyzing any circuit!
• Without external biasing, you can think/visualize the p-n junction as being "intrinsecally reversed".
• Now, if you overcome the intrinsic potential difference with an external positive voltage bias higher that 0.7V, you will have reduced to zero or reversed the intrinsic potential difference, effectively turning on the diode and making it forward biased.

Note that, in electronics, the way voltage and current signs are defined is simply a convention,

• It's key to internalize these conventions as soon as possible during your learning process, as most people in the field, textbooks and articles adhere to these conventions.
• It is a sign of deeper interest/curiosity/knowledge trying to understand the rationale behing some of these conventions. It'll surely will help you to step on more advanced concepts in the future. However, note that some of these conventions have no deep rationale behind, instead are a mixture of historical reasons or, in some cases, were defined arbitrarily.
• Communication is a process inherent to human beings so, unless you want to avoid communication with others I suggest you to follow these sort of pre-established "rules".