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Do you know your transistors from your FET's and MOSFET's?

You can amplify an audio signal with a bipolar junction transistor, FET or MOSFET. Which should you choose, and why?


If you're not using vacuum tubes to amplify and process your electronic signals, then you are using transistors.

The original transistor, developed way back in the 1940s, is more formally known as the 'bipolar junction transistor'. Poles are like the poles of a battery - positive and negative. Nothing complicated there.

The way a bipolar junction transistor works is quite easy to understand. A potentially large current flowing between two of the terminals - the collector and the emitter - is regulated by a much smaller current flowing into the third terminal, which is the base.

So a small current can control a larger current. Hey - we have amplification!

But for some applications, even that small current is too large. The capsule of a capacitor (condenser) microphone for example can only deliver a tiny current - far too small to get a transistor interested.

So from the bipolar junction transistor was developed the 'Field Effect Transistor' or FET.

An FET works by electrically 'pinching' the channel through which the current flows. The terminals here have different names - the current to be regulated flows between the source and the drain terminals. A control voltage is applied to the gate.

Now here, hardly any current at all flows into the gate. The voltage however causes an electric field in the channel between the source and the drain. The stronger this field, the less current can get through.

So an FET only requires a minutely tiny current to work.

Going one step further is the MOSFET - 'Metal Oxide Semiconductor Field Effect transistor'. The MOSFET is like an FET, but now there is a further layer of insulation on the gate, so the current flowing into the gate is reduced to near zero.

The advantage of the FET and MOSFET over the bipolar junction transistor is that they are both highly linear. This means if you put a voltage in, you can get a bigger voltage out, and always in proportion to the input.

The reaction of a bipolar transistor to voltage is interesting...

If you put a small voltage in, nothing happens. If you put a slightly larger voltage in, nothing happens. But when you reach a certain point, suddenly the transistor will 'open up' and a massive current will flow between the collector and the emitter. It is a highly non-linear device, hence transistor circuits require additional complexity to overcome this feature.

FET and MOSFET circuits are simpler in comparison. The FET has found its principal role as a high impedance input stage, where little current is required of the supplying device. The FET can also be used as an electronically controlled variable resistor.

The MOSFET has taken off as an output device in power amplifiers. Here the MOSFET has a significant advantage...

A bipolar junction transistor has the irritating property of conducting electricity more easily when it gets hotter. And more electricity makes it get hotter still. It is difficult to keep this in check and a transistor power amplifier requires careful control over temperature.

MOSFET's on the other hand become less conductive as they get hotter. Therefore there is no 'thermal runaway'.

Having said all of this, the bipolar junction transistor is damned versatile and is manufactured in an incredible variety of specifications. So most electronic circuits do depend on transistors (even integrated circuits, where the transistors are encapsulated within the chip).

FET's are used where high input impedance (low current requirement) is a priority, and they also make a good electronic switch due to their linearity.

MOSFET are often used in power amplifiers for their thermal stability.

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By David Mellor Friday May 13, 2005
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