What does "turn on" the transistor? The current applied to the base of the transistor or the voltage?

Also what happens if you apply constant current through an led with the voltage less then the forward drop voltage?

 

You need both, the current and the voltage.

Also what happens if you apply constant current through an led with the voltage less then the forward drop voltage

The current source will go into saturation, and the current will drop.

 

1. In the case of a BJT you can't really separate the two. Cannot get current without voltage and neither can you get voltage without current. BUT that said BJTs are basically current driven devices.

 

Both the LED and the transistor base are diodes, and they will not conduct until the required voltage is reached. This is ~3V for a typical LED and ~0.6V for a silicon diode, including the base of a transistor. Below the threshold voltage, there is only a tiny leakage of current, orders of magnitude less than when conduction starts.

 

how do you propose to put constant current through an led while dropping the voltage? voltage across a junction determines current through the junction. just like a resistor, if you have a 100 ohm resistor with 1 amp of current through it, you have 100 volts across the resistor. if you reduce the voltage, you will have less current. there is no way to hold the curent constant when varying the voltage.

 

1. In the case of a BJT you can't really separate the two. Cannot get current without voltage and neither can you get voltage without current. BUT that said BJTs are basically current driven devices.

Oh,oh. You've said the trigger words. There have been (nearly endless) discussions on these forums about the nature of a BJT's operation. Solid-state theory basically states that they are voltage operated with the base current being incidental to that, due to the low input impedance of the base-emitter junction. And they are usually considered as voltage operated, low-input-impedance device with transconductance (Gm) gain when used as AC amplifiers. But when operated as switches or calculating the DC bias point of AC amplifiers, it's usually easier to consider them current operated and use the beta or hFE current gain to do the calculations.

 

Both the LED and the transistor base are diodes, and they will not conduct until the required voltage is reached. This is ~3V for a typical LED and ~0.6V for a silicon diode, including the base of a transistor. Below the threshold voltage, there is only a tiny leakage of current, orders of magnitude less than when conduction starts.

More exactly, solid-state diodes (BJT emitter-base, LEDs, etc.) have a logarithmic relationship between forward voltage and current . If you look at their voltage versus current plot on a semi-log graph you will see a nearly straight line down to a low voltage (see this for example). For most applications, that current is not considered significant until it reaches a practical operating level (typically in the μA to mA region) at which point the voltage is considered to have reached a characteristic (threshold) voltage as determined by the diode material. That's the voltage wayneh is referring too.

 

1. In the case of a BJT you can't really separate the two. Cannot get current without voltage and neither can you get voltage without current. BUT that said BJTs are basically current driven devices.

In general this is true, since modern devices are optimised to work in this fashion, but actually you can. You can even operate the transistor with the base disconnected!

http://www.youtube.com/watch?v=NoeuVxYoNio

If you watch the above video you will see some footage of the original transistors and a simulation of the discovery of the transistor effect. The original effect was light activated.