Main parameters of transistor

Main parameters of transistor

1. DC parameters
(1) Collector-base reverse saturation current Icbo, when the emitter is open (Ie=0), the collector reverse current when a specified reverse voltage Vcb is applied between the base and the collector. It is only related to temperature and is a constant at a certain temperature, so it is called the collector-base reverse saturation current. For a good triode, Icbo is very small. The Icbo of a low-power germanium tube is about 1 to 10 microamperes, and the Icbo of a high-power germanium tube can reach several milliamperes, while the Icbo of a silicon tube is very small, at the nanoampere level.
(2) Collector-emitter reverse current Iceo (penetration current) When the base is open (Ib=0), the collector current when a specified reverse voltage Vce is applied between the collector and the emitter. Iceo is approximately β times of Icbo, that is, Iceo=(1+β)Icbo o Icbo and Iceo are greatly affected by temperature. They are important parameters for measuring the thermal stability of the tube. The smaller the value, the more stable the performance. The Iceo of a low-power germanium tube is larger than that of a silicon tube.
(3) Emitter-base reverse current Iebo When the collector is open, the emitter current when a specified reverse voltage is applied between the emitter and the base. It is actually the reverse saturation current of the emitter junction.
(4) DC current amplification factor β1 (or hEF) This refers to the ratio of the DC current output by the collector to the DC current input by the base when there is no AC signal input in the common emitter connection, that is:
β1=Ic/Ib
2. AC parameters
(1) AC current amplification factor β (or hfe) This refers to the ratio of the change in collector output current △Ic to the change in base input current △Ib in the common emitter connection, that is:
β= △Ic/△Ib
The β of a general transistor is approximately between 10-200. If β is too small, the current amplification effect is poor. If β is too large, the current amplification effect is large, but the performance is often unstable.
(2) Common base AC gain α (or hfb) This refers to the ratio of the change in collector output current △Ic to the change in emitter current △Ie when the common base connection is used, that is:
α=△Ic/△Ie.
Because △Ic＜△Ie, α＜1. When α of a high-frequency triode is greater than 0.90,
the relationship between α and β can be used :
α = β/(1+β)
β = α/(1-α)≈1/(1-α)
(3) Cut-off frequency fβ, fα When β drops to a low frequency, the frequency of 0.707 times is the cut-off frequency fβ of the common emitter; when α drops to a low frequency, the frequency of 0.707 times is the cut-off frequency fα of the common base. fβ and fα are important parameters that indicate the frequency characteristics of the tube. The relationship between them is:
fβ≈(1-α)fα
(4) Characteristic frequency fT Because β decreases when the frequency f increases, when β decreases to 1, the corresponding fT is an important parameter that fully reflects the high-frequency amplification performance of the transistor.
3. Limit parameters
(1) Maximum allowable collector current ICM When the collector current Ic increases to a certain value, causing the β value to drop to 2/3 or 1/2 of the rated value, the Ic value at this time is called ICM. Therefore, when Ic exceeds ICM, although the tube will not be damaged, the β value will drop significantly, affecting the amplification quality.
(2) Collector-base breakdown voltage BVCBO When the emitter is open, the reverse breakdown voltage of the collector junction is called BVEBO.
(3) Emitter-base reverse breakdown voltage BVEBO When the collector is open, the reverse breakdown voltage of the emitter junction is called BVEBO.
(4) Collector-emitter breakdown voltage BVCEO When the base is open, the maximum allowable voltage applied between the collector and the emitter. If Vce>BVceo during use, the tube will be broken down.
(5) Maximum allowable collector dissipation power PCM When the collector current exceeds Ic, the temperature will rise. The maximum collector dissipation power when the parameter change caused by the heat of the tube does not exceed the allowable value is called PCM. The actual dissipation power of the tube is the product of the collector DC voltage and current, that is, Pc=Uce×Ic. When in use, make sure that Pc PCM is related to heat dissipation conditions. Adding heat sinks can improve PCM.