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.
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.
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.
Let me start with some definitions....
Here is what I mean by Vcc.
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.
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.
