Field effect transistor. Oscilloscope applications - theoretical introduction
Main topics:
- the principle of operation of the oscilloscope,
- the oscilloscope as a characterograph,
- field effect transistor and his parameters.
The principle of operation of the oscilloscope
An oscilloscope is an electronic device used to study voltage waveforms. Thanks to its universal capabilities, it can also be used in the measurement of time, frequency, phase of electrical waveforms and other quantities.
The basic element of the oscilloscope is the oscilloscope tube (see figure below). The heated cathode emits electrons, which are then accelerated and focused into a beam by the anode system. A beam of electrons falls on a phosphor-coated screen, making a spot visible on the other side. By applying voltage to the horizontal and vertical deflection plates, the direction of the electron beam can be changed. In this way, the position of the spot on the screen changes and an image is formed.
The waveform under test is provided to the vertical deflection plates, suitably modified (usually amplified) by the vertical deflection path. The type of signal connected to the horizontal deflection plates depends on the operating mode. If the periodic waveform y=f(t) is studied, the time base is used. The time base is an electronic circuit that periodically generates a linearly increasing voltage over time, i.e. a sawtooth waveform. As the voltage builds up, the dot on the screen moves from left to right, and then returns to its starting point when the voltage decreases. In order for the image on the screen to remain still, it is necessary to synchronize the time base waveform with the input signal. Synchronization and triggering systems are used for this purpose. Another mode of operation is also possible, in which an external signal is connected to the horizontal deflection system. This makes it possible to record the tested waveform as a function of the voltage connected from the outside and to observe the functional relationship y=f(x).
Oscilloscopes differ in the number of channels, which determines the number of possible waveforms. Recording multiple waveforms is most often accomplished by using an electronic switch that connects the input of the vertical deflection path to the signal sockets of successive channels (this is called multiplexing, or division in the time domain). Two- or four-channel oscilloscopes are most commonly used.
Amplifiers and voltage dividers (attenuators) are placed between the deflection plates and the oscilloscope input to extend the range of recorded voltages while fully utilizing the screen. A simplified diagram of the oscilloscope is shown below:
