Transistor circuit help ?

[tron87]

How do i calculate the IB and resistance values of r1,r2,r3,r4 required to do the following to the collector current when each pushbutton is pressed in turn:

1.When pb1 is pressed transistor is fully saturated and near 1 amp flows on Ic ?


2.When pb4 is pressed transistor operates in active region and makes Ic = 500ma ?


3.When pb3 is pressed transistor operates in active region and makes Ic = 10ma ?


4.When pb2 is pressed transistor operates in active region and makes Ic = 0.1ua ?


The transistor is a Bc548A npn bjt the circuit is purely just to learn about a transistor active region and how to calculate required biasing current.


*Circuit diagram is attached*

 

[oddjob]

The circuit you have shown is very dependent on the current gain Hfe of the transistor; this is an unknown and variable value. To provide a better solution, negative feedback needs to be used by way of either an emitter resistor or a resistor between the collector and base.

 

[JCLrd]

Hi,

You are learning how to bias a transistor.

Here is 2 examples of how to practice biasing a transistor.

First into its saturation region.

Refer to the data sheet, given below.


DATA SHEET
--------------



Saturation design.
---------------


Now how to use the same value for RC, but put the transistor into its linear region.

Linear design
----------------


These are very basic ways of designing this.

There is a lot more detail involved but this should give you some practice, on biasing transistors.

Experiment with different values, to see how it works out.

 

[tron87]

Thanks JCLrd for taking time to explain !

Question though what does ID mean in your design examples ?

 

[ericgibbs]

Thanks JCLrd for taking time to explain !

Question though what does ID mean in your design examples ?

ID = Icurrent thru the Divider
The divider is the two resistors to the transistor base.
Usually you calculate ID to be 10 times Ib

 

[audioguru]

The BC547 transistor you selected has a max allowed collector current of only 100mA.
It is very difficult to see the numbers on your small and negative schematic but it looks like a 10V supply and a collector resistor of only 10 ohms.
Ohm's Law calculates the saturated collector current to be almost 1A which will blow up the transistor.

I made your awful schematic a normal positive image and removed the pastel colours.
I couldn't remove the chicken-pox dots all over it.