The Transistor Amplifier

Posted on Feb 5, 2014

Time would be much better spent on explaining transistor and MOSFET behaviour in a simpler way and getting on with digital circuitry and microcontroller projects. You simply cannot put a transistor into a circuit and expect it to produce the calculated results. The gain of a transistor can be from 100 to 200 in a batch and this changes the outcome by 50%!

The Transistor Amplifier
Click here to download the full size of the above Circuit.

There are thousands of transistors and hundreds of different makes, styles and sizes of this amazing device. But there are only two different types. NPN and PNP. The most common is NPN and we will cover it first. There are many different styles but we will use the smallest and cheapest. It is called a GENERAL PURPOSE TRANSISTOR. The type-numbers on the transistor will change according to the country where it was made or sold but the actual capabilities are the SAME. It is also referred to as a BJT (Bi-polar Junction Transistor) to identify it from all the other types of transistors (such Field Effect, Uni-junction, SCR, ) but we will just call it a TRANSISTOR. Most small transistors have a plastic case and the leads are in a single line. The side of the transistor has a "front" or "face" with markings such as transistor-type. Fig 10 has a capacitor on the input and output. This means the stage is separated from anything before it and anything after it as far as the DC voltages are concerned and the transistor will produce its own operating point via the base resistor and LOAD resistor. We have already explained that the value of the two resistors should be chosen so the voltage on the collector should be half-rail voltage and this is called the "idle" or "standing" or "quiescent" conditions. When the voltage on the collector is mid-rail, the transistor can be turned off a small amount and turned on a small amount and the...

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