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Transistor Design As A Switch

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Palbert

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I am designing a transistor system where I have a BJT NPN Transistor with Base, Collector, and Emitter resistors. No feedback, just set up to have the input come into the base through a base resistor, my emitter grounded, and a Vcc coming in through Rc. I know the input voltage is 5Vdc and my βdc is 109. I want a Vc of 2.1V and an Ic of 13.5mA. I calculated the Ib I would need (123.9uA) to get my 13.5mA and then I calculated the Rb value (40.4KΩ) to get the Ib I need. I am trying to figure out what Rc and Re values to choose to get my Vc of 2.1V and still have an Ic of 13.5mA. Any help would be very much appreciated.

Does the equation Vc = Vcc - IcRc apply in this situation?
 
If you want the collector voltage to be 2.1VDC then the transistor is not a saturated switch. It is a linear amplifier and will also be a thermometer if it does not have some negative feedback (usually an emitter resistor).

Usually a circuit is designed to use any amount of beta that a certain transistor might have that is listed as a range in its datasheet. Then the base is fed from a voltage divider not just a single resistor to a supply voltage.
 
I am designing a transistor system where I have a BJT NPN Transistor with Base, Collector, and Emitter resistors. No feedback, just set up to have the input come into the base through a base resistor, my emitter grounded, and a Vcc coming in through Rc. I know the input voltage is 5Vdc and my βdc is 109. I want a Vc of 2.1V and an Ic of 13.5mA. I calculated the Ib I would need (123.9uA) to get my 13.5mA and then I calculated the Rb value (40.4KΩ) to get the Ib I need. I am trying to figure out what Rc and Re values to choose to get my Vc of 2.1V and still have an Ic of 13.5mA. Any help would be very much appreciated.

Does the equation Vc = Vcc - IcRc apply in this situation?

You say in your second sentence that you have the emitter grounded (therefore no "Re" resistor) and in your last sentence you ask what the values should be for Rc and Re. To answer your question above, your equation is correct if you are assumming no "Re" and your defintion of "Vc" is the voltage taken across the collector to emitter; in which case the proper term to use is "Vce". And as audioguru said this holds true only within the limits of the range of Beta.

With a "Re" resistor in place the equation is:
Vcc=Vc+Vce+Ve

Using the NPN transistor as a switch:

Vce ideally is 0 volts (transistor saturation and Beta << 109) and Vce = Vcc, meaning if you want Vc = IcRc = 2.1 volts, then your Vcc must be 2.1 Volts plus a small saturation Vce voltage; easily measured.
 
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For further clarification, the overall point of this was to drive an LED by taking about 4.5mA and getting the 13.5mA we needed. I originally started with the traditional voltage divider setup and had my calculations correct but at the end of our lab she told us to take away R1 (top resistor in voltage divider) and have the signal come straight in through R2. I'm just going to go back to the voltage divider setup because otherwise I don't think it is going to amplify the current. Thanks for the help though.

(In my last post, I meant my emitter was grounded through a resistor Re, my apologies)
 
With a single base resistor, the emitter grounded and no negative feedback then it is impossible to have a collector current of exactly 13.5mA.
The transistor will either slam on hard with Vce of only 0.1V or it will barely turn on. If it barely turns on then it will slowly turn on more and more as it heats up.

Does your teacher want you to add an emitter resistor to give an emitter voltage of 2.0V and a saturated collector voltage of 2.1V?
Then the base current must be 1/10th the collector current to ensure good saturation as explained in the transistor's datasheet.
 
She basically said that we could do anything we wanted with a transistor setup to get the output of 2.1V and 13.5mA but she suggested the voltage divider setup. She had us change it at the end because the voltage divider was keeping the LED on at times when it should be off. We had an initial AND gate setup with two switches for the inputs and the output was what we were amplifying to the 2.1V and 13.5mA. I think I'll just get some help tomorrow. I was just making sure there wasn't something I was missing because I didn't think the signal coming in through a base resistor would work. Thanks though.
 
You forgot to say what type of AND gate was turning the transistor on and off and forgot to tell us the supply voltage for the gate.
When the output of the gate goes low then the transistor should turn off and the LED should also turn off.
Post the schematic with part numbers, resistor values and supply voltages.
 
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