That's the approximate voltage for silicon transistors.
That's what I get...

Can you see how the output voltage would vary with load current?

I can see how the output voltage would vary with load current. When the transistor is in the active region it, higher load current would vary the voltage across the load which would in turn change the voltage at the base emitter junction.

 

How can that be if the base to emitter voltage is always 0.7V and the base voltage is always 5.7V? How can the base emitter voltage vary? How much will it vary, if say, the load current goes from 100 mA to 200 mA?

(I'm not saying that your are wrong, it actually will vary, but the question is by how much)

Edit: To make the question simpler, make the current go from 2mA to 10mA and use the datasheet posted in #6.

Bob

 

I can see how the output voltage would vary with load current. When the transistor is in the active region it, higher load current would vary the voltage across the load which would in turn change the voltage at the base emitter junction.

Yes, but remember that is a regulator circuit.

Changing the input voltage would probably have more affect on the output because the current thru the zener would change and they are very sensitive to thermal affects.

 

There is nothing wrong with that circuit, it's a simple Zener regulator with a series pass transistor, wouldn't get any hotter than any other type of linear regulator.

(the 30 volt input is a bit high }

I know what the circuit is. I did not say there was anything wrong with the circuit. I said that I was not in favor of using the circuit, and I did give reasons why I would not use that circuit under the stated conditions of high Vce drop and substantial current because there are superior methods of performing the function. Criticize me for what I said, not what you think I said, or keep your own counsel.

 

Of course you would know what the circuit is, and that should have been the first clue I wasn't speaking to you.

I wanted to point out the fact that the circuit wouldn't produce any more heat than any other linear regulator, simply for the OP's information.

 

How can that be if the base to emitter voltage is always 0.7V and the base voltage is always 5.7V? How can the base emitter voltage vary? How much will it vary, if say, the load current goes from 100 mA to 200 mA?

(I'm not saying that your are wrong, it actually will vary, but the question is by how much)

Edit: To make the question simpler, make the current go from 2mA to 10mA and use the datasheet posted in #6.

Bob

The increase in load current from 2mA to 10mA would typically cause an increase of .05V from the base to the emitter. If load current increases, the pn junction at the emitter would become more saturated and the current from base to emitter would also increase?

 

Yes, exactly. But the voltage change would be very small compared to the current change. That is what nakes it a voltage regulator circuit.

Bob

 

Below is the LTspice simulation of the circuit with some typical component values.
It shows the change in voltages for a input change of 15V to 30 (0 to 10ms), and an output load change from 10mA to 2mA (10ms to 20ms).

The transistor base-emitter voltage is shown by the purple trace.

Note that Vout is affected by the change in input voltage due to the change in Zener voltage from the change in its bias current.
Using a programable accurate reference voltage in place of the Zener, such as the TL431, would eliminate this variation.

The output voltage is only slightly changed by the change in load from 10mA to 2mA, due to the change in the transistor's Vbe voltage.

The output voltage is not quite 5V since standard Zeners are 5.6V, not 5.7V.