Tips and advice

Transistor (bipolar and field effect)

Transistors are among the basic electronic components in hi-fi and audio technology. Basically, you can think of a transistor quite simply as an electrically controllable switch. However, the operating principle of a transistor is not based on a mechanical switching function, as known from a light switch or relay, but on semiconductor technology. In addition – and this is a particularly important point – transistors are not only suitable for switching currents , but also for amplification .

Bipolar Transistor:

bipolar transistor photo
Bipolar Power Transistor:

bipolar power transistor
The points just mentioned are explained in more detail below:

A semiconductor is a material that, under certain conditions, can function like an electrical conductor but also like a non-conductor. If these two states can be controlled from the outside, then in principle you have the above-mentioned switch with which you can switch electricity “on” and “off”. The controlled activation works through external manipulation of the semiconductor at the atomic level (doping), in the case of the transistor this is implemented by applying a control voltage or the flow of a control current.

The situation can be explained clearly using the circuit symbol of a bipolar npn transistor. Its connections are called collector (C), base (B) and emitter (E):

transistor npn circuit diagram
If there is a voltage between C and E – analogous to a switch – the area between C and E becomes conductive by deliberately driving the base with a sufficiently large control current: Current flows between C and E, which is the “switch”. closed so to speak. If the current flow at B is interrupted, the switch blocks again. As mentioned, this all works without any mechanics.

In addition to the states just described of complete conduction (in technical jargon: saturation range) and non-conduction (blocking range), a transistor also has a third, very important range, which also completely distinguishes it from a trivial switch:

Namely the amplification range, which is to a certain extent between the saturation and stopband range. Although the current flow at B is greater than zero in this area, it is smaller than in the saturation state.

In the amplification area, the distance between C and E is already conductive, but not yet complete. Small changes in the control current have a direct effect on the conductivity of the section between C and E. In other words: By means of a small control current (IB) one is able to control a (theoretically arbitrarily large) power current (IC on the path CE) accordingly: The base signal (IB) is thus amplified analogously. This is where the gain factor B comes into play:

IC= B x IB

In addition to the bipolar npn transistor just described, there are a large number of other types of transistors which, in principle, have the same functional pattern in their application. For example, there is the bipolar pnp transistor, which only works with reversed polarities compared to the npn transistor. Above all, the group of field effect transistors (FET) deserves special mention:


mosfet transistor photo
A FET is characterized by the following connections:

Source S, Drain D, Gate G

mosfet circuit diagram
What is special about FETs is that they only work with a control voltage or an electric field – an actual control current does not flow . The control of the power current between D and S – and thus the process of amplification – takes place without power (on the relationship between power, current, voltage: basic electrotechnical terms ).

In addition, unlike bipolar transistors, FETs can also be used for AC amplification . However, they are also more susceptible to interference with regard to electrostatic charges and their switching speeds are often somewhat slower.