How to analyze the function of triode in circuit — SUNMATE

How to analyze the function of triode in circuit

Triode, fully known as semiconductor triode, also known as bipolar transistor and crystal triode, is a semiconductor device for current control. Its function is to amplify the weak signal into an electrical signal with large amplitude value, and it is also used as a contactless switch. Crystal triode is one of the basic components of semiconductor. It has the function of current amplification and is the core component of electronic circuit.

Triode is to make two PN junctions close to each other on a semiconductor substrate. The two PN junctions divide the whole semiconductor into three parts. The middle part is the base area, and the two sides are the emission area and collector area. The arrangement modes are PNP and NPN.

Basic principle of triode:

For NPN: when the potential at point B is a few tenths of a volt higher than the potential at point E, the emitter junction is in the positive bias state. When the potential at point C is a few volts higher than the potential at point B, the collector junction is in the reverse bias state, and the collector power supply EC is higher than the base power supply EB.

According to the principle of current continuity:

Ie=Ib+Ic

That is, if a small IB is added to the base, a large IC can be obtained on the collector, which is the so-called current amplification.

IC and IB maintain a certain proportional relationship, that is:

β 1 = IC / IB, where: β 1 -- called DC magnification,

β= △ IC / △ IB, where: β-- It is called AC current magnification

[note]:

Due to low frequency β 1 and β There is little difference between the two values, so there is no strict difference between them.

Working state of triode:

Cut off state: when the voltage applied to the emitter junction of the triode is less than the on voltage of the PN junction, the base current is zero, and the collector current and emitter current are zero. At this time, the triode loses the current amplification effect. The connection between the collector and the emitter is equivalent to the off state of the switch. We call the triode in the cut-off state.

Amplification state: when the voltage applied to the emitter junction of the triode is greater than the on voltage of the PN junction and is at a certain appropriate value, the emitter junction of the triode is forward biased and the collector junction is reverse biased. At this time, the base current controls the collector current, so that the triode has the function of current amplification and its current amplification factor β=Δ Ic/ Δ IB, at this time, the triode is in the amplified state.

Saturated conduction state: when the voltage applied to the emitter junction of the triode is greater than the conduction voltage of the PN junction, and when the base current increases to a certain extent, the collector current no longer increases with the increase of the base current, but does not change much near a certain value. At this time, the triode loses the current amplification effect, and the voltage between the collector and the emitter is very small, The conduction state between the collector and the emitter is equivalent to that of the switch. This state of triode is called saturation conduction state.

Amplification of triode in circuit:

1) The triode works in the amplification state, that is, the emitter junction is positively biased and the collector junction is inversely biased.

2) The setting of the input circuit should make the input signal coupled to the input electrode of the triode to form a changing base current, so as to produce the current control relationship of the triode and become the change of collector current.

3) The setting of the output circuit shall ensure that the current signal amplified by the triode is transformed into the power form required by the load (output voltage or output current).

Switching control function of triode in circuit:

1) The base must be connected with resistance in series to protect the base.

2) The base adds pull-down resistance or pull-down resistance according to PNP or NPN tube.

3) The collector resistance value is adjusted according to the actual situation of the driving current.

The base and collector need series resistance, which is used to reliably cut off the transistor when the input is in a high resistance state. The heavier the load, the greater the allowable resistance, usually in the order of 10K.

Prevent the triode from misoperation due to the influence of noise signal, and make the transistor cut-off more reliable! The base of the triode cannot be suspended. When the input signal is uncertain (such as when the input signal is in high resistance state), add the pull-down resistance to make it effectively grounded.