Electrical – how to connect two transistors in series

seriestransistors

I'm trying to connect two transistors together so that they share the same base and turn on together. I thought that you could simply connect them like this:

Except for the fact that i'm using only 1 input so it should look more like this:

schematic

^{simulate this circuit – Schematic created using CircuitLab}

where I have an led light bulb at the end that should be on. i tried making this type of circuit on a breadboard but the led doesn't do anything and the current isn't passing through. here is a picture,

the white and black pins are connected to the battery so what could be going wrong here?

Best Answer

Two easy ways to fix this.

Use PNP transistors instead of NPN. NPN won't work ahead of load due to needing it's emitter lower than it's base. Replace the two 2n3904 with their PNP counterparts 2n3906, and move their base to ground.
Move the load around. Place the Led and it's current limiting resistor before the transistor collectors.

schematic

^{simulate this circuit – Schematic created using CircuitLab}

Also, change the resistor values. 100 ohms at 9V for a led is too low. 300Ω or higher for the led resistor, and 1k for the transistors.

But this is pointless. Since you have both bases tied together, the only thing these transistors are doing is wasting power (both VCE and VBE). A single transistor in either setup is better. Frankly no transistor would be better too, since you are just keeping it on.

Best Answer

But this is pointless. Since you have both bases tied together, the only thing these transistors are doing is wasting power (both VCE and VBE). A single transistor in either setup is better. Frankly no transistor would be better too, since you are just keeping it on.

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