Lets talk about common emitter BJTs

Thread Starter

Wendy

Joined Mar 24, 2008
21,938
The common transistor is a very undervalued component. It is cheap and easy to use and gives good results. Much of what I'm going to talk about is taught in tech school. If you are weak on Bipolar Junction Transistors (BJT) I highly encourage people to build what I show to apply their use and learn this interesting little component for fun.

Let me start with some definitions....
Here is what I mean by Vcc.
Vcc.png
Note: My choices arbitrarily are meant to give us a point of discussion. 9V batteries are cheap and easy and I always try to use a filter capacitor as they almost always increase the performance of the power source.

My favorite transistor is usually a PN2222. They are cheap and common NPN transistors with a typical β (gain) of 50. One of the disadvantages of BJTs is gain for a series is pretty loose. Transistors from the same bag can have a beta of 50 to 90. There are ways to control this but it's a start with quick and dirty approaches first.

Basic biasing means we want the collector voltage Vc at ½Vcc. Many DVMs have a beta measuring option which can help pin this number down for a specific component.
Q2.png
This diagram shows roughly how this was done. Note that the R2 value was arbitrary. However the DC values may interfere with the rest of the project. So we get rid of it with capacitor like this...

We now have a fully functional AC amplifier and gain is that of the transistor. The AC can be audio or RF. The maximum frequency is dependent on the transistor. The ½Vcc will help prevent clipping which can reduce the amps usefulness.

The price tag of the circuit is small and it is simple. The other problem you have to watch out for is thermal run-away. Most transistors treat heat like base current which will move Vc from its sweet spot of ½Vcc.Q0.png
 
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Bordodynov

Joined May 20, 2015
2,501
Resistor in the base you have too little value of resistance (for transistors with low gain). Because of this, the transistor will be in saturation mode.
Draft566.png
 

Thread Starter

Wendy

Joined Mar 24, 2008
21,938
Not if the beta is 50, what is your assumed beta?I will follow up w/ biasing schemes that tend to ignore beta variation. It is one of the stated problem w/ this approach Try the second picture down instead
 

OBW0549

Joined Mar 2, 2015
3,208
@Bordodynov is correct. If β = 50 and you want Q1's collector to sit at Vcc/2, the base bias resistor must be ≈100X the collector load resistor, not 50X.

EDIT: For example, if the base resistor is 100 kΩ the base current will be (9V - 0.65V)/100 kΩ = 83.5 μA. Ic = Ib * β = 83.5 μA * 50 = 4.175 mA. With Rc = 1 kΩ, Vc = 9V - Ic * Rc = 4.825 V, or approximately Vcc/2.
 
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