FET and Bipolar Transistors

The transistor has been around since the nineteen forties. In fact, the FET (Field Effect Transistor) was actually developed before the more common bipolar type. Bipolar transistors use semiconductor material such as germanium. A process known as doping adds impurities to the germanium so it can have an abundance of free electrons or lack of free electrons. Materials with an abundance of free electrons is known as N-type material while materials with a lack of free electrons is known as P-type.

The purpose of a transistor is to control the flow of current from one point to another. To accomplish this, we must construct a two-junction device known as a bipolar transistor. Both P (positive) and N (negative) types of material are used to make a sandwich, either PNP or NPN. In the diagram, we see 2 thick layers separated by a thin layer of the opposite material.

bipolar transistor

The thin layer acts as an analog control. In the case of the NPN unit, the more positive biasing current to the thin layer, the more current can pass through the transistor. Negative biasing produces the same effect in the PNP unit. Biasing current is not passed along, it just controls the flow between the thick layers.

Transistor biasing

Bipolar transistors do not respond until a threshold voltage (approximately .7 volts), is supplied. This is done using a biasing resistor as shown in the illustration. More

biasing transistor

accurate biasing techniques for specific circuit requirements use two resistors as a voltage divider.

transistor construction


Although FETs are similar to bipolar transistors, there are some important differences. In the schematic illustration, the leads are labeled Drain, Source, and Gate. The collector and emitter leads are differentiated by an arrow point, indicating the direction of current flow.

bipolar fet

There is no graphic differentiation between drain and source leads of an FET because the leads are interchangeable. Current flow through an FET can be in both directions, making it function like a current controlled resistor.

jfet construction

Another important difference is, FET transistors respond exponentially. This feature makes them ideally suited for music since musical envelopes as well as hearing characteristics are for the most part exponential. The FET is also a low noise device because of its greater resistance.

transistor response

This unique response is very valuable to the expansion of electronic design capabilities. Non-linear response is a necessary element to the development of important new devices for future electronic devices. Combined with linear devices, little is left to be desired, when designing trajectory modifications are combined with linear processing techniques.

Biasing for FET transistors is quite different than for bipolar transistors. FETs do not have a threshold requirement and since they can respond to the electrical signal in the air, it is necessary to desensitize the FET with a reverse biasing resistor.

biasing fet

As we see in the J-FET transistor construction diagram, the to N sections are not totally separated. There is a narrow channel of N type material connecting the two main N sections.

Bipolar amplification

A bipolar amplification circuit is essentially transparent to the input. This means that exponential signal input will be an exponential signal out. The same applies to a linear signal. This is valuable when all you require is an amplified version of the input signal. The Bipolar transistor makes no wave changes in the input.

bipolar amplification

Field effect amplification

The Field Effect Transistor is an exponential device, so its output is exponential regardless of the input.

fet amplification

Whenever an exponential signal is amplified by an exponential amplifier the curve is changed to a deeper slope. the can be an advantage when processing digital signals. Each consecutive stage will be a more sharply angled wave.

exponential amplification

Some FET output amplifiers use corrective feedback circuits to make sure that a linear output is assured.

Case types

There are many case types for the wide variety of transistors and some of them are shown in the illustration below.

case types

What ever your needs, whether you need a linear or exponential device, a bipolar or FET device should solve your circuit problems. Just keep in mind that different manufacturers may have different pin arrangements.