Main parameters of transistor

The parameters of a transistor can be divided into DC parameters, AC parameters, limit parameters, and characteristic frequency. The parameters of a transistor are an important basis for the use and selection of a third stage transistor. Therefore, understanding the parameters of a transistor can avoid damage to the transistor caused by improper selection or use.

(1) DC parameters.

1) Collector base reverse current I (CBO). When the emitter is open and a specified reverse voltage is applied between the collector and base, the leakage current in the collector junction is called I (CBO). The smaller the value, the better the thermal stability of the transistor. Generally, low-power transistors are around 10 μ A, while silicon transistors are smaller.

2) Collector emitter reverse current I (CBO), also known as penetration current. It refers to the leakage current of the collector when a specified reverse voltage is applied between the collector and emitter when the base is open circuited. The smaller the value, the better. Silicon tubes are generally small, around 1 μ A or less. If this value is found to be large during testing, the tube will not be easy to use.

(2) Extreme parameters.

1) When the β value of the transistor drops to half of its maximum value, the maximum allowable collector current I (CM) of the transistor is called the maximum allowable collector current. When the collector current Ic of the transistor exceeds a certain value, it will cause changes in certain parameters of the transistor, the most obvious of which is a decrease in β value. Therefore, in practical applications, Ic should be less than I (CM °)

2) The maximum allowable dissipation power P (CM) of the collector electrode. When the transistor is working, the collector electrode needs to dissipate a certain amount of power, which causes the collector junction to heat up. When the temperature is too high, it can lead to changes in parameters and even burn out the transistor. The maximum dissipated power of the collector is defined as the power consumed when the collector temperature of a transistor rises to a level that does not burn out the collector. To increase the P (CM) value during use, heat sinks can be added to high-power tubes, and the larger the heat sink, the more the P (CM) value will increase.

3) The reverse breakdown voltage BU (CEO) of the collector emitter electrode is the maximum voltage allowed to be applied between the collector and emitter electrodes when the base is open. In practical applications, the voltage applied between the collector and emitter must be less than BU (CEO), otherwise it will damage the transistor.

(3) Current amplification factor.

1) DC amplification factor β "or expressed as h (FE). It refers to the ratio of the collector output DC I (B) to the base input DC in a common emitter circuit when there is no AC signal.

Namely, β=I (c)/I (B) β is an important parameter for measuring the current amplification capability of a transistor, but for the same transistor, there are different β values at different collector currents

2) The amplification factor β for communication can also be expressed as h (FE); express. This parameter refers to the ratio of the change in collector current △ AIc to the change in base current △ I (B) in a common emitter circuit when an AC signal is input.

Namely, β=△ Ic/△ I (B)

The above two parameters respectively indicate the amplification ability of the transistor for DC current and AC current. However, since these two parameter values are approximately equal, i.e. β ≈ β, they are generally not distinguished in practical use. Due to the production process, even pipes produced in the same batch have different beta values. For convenience and practicality, manufacturers sometimes mark the beta value on the transistor for users to choose from.

(4) The characteristic frequency product decreases with the increase of operating frequency, and the higher the frequency, the more severe the decrease in β. The characteristic frequency light of a transistor refers to the frequency value at which the body value drops to 1. That is to say, the transistor operating at this frequency has lost its amplification ability, and f (T) is the limit frequency of the transistor in use. Therefore, when selecting a transistor, the characteristic frequency of the transistor is generally at least three times higher than the operating frequency of the circuit. But it's not that the higher f (T), the better. If you choose too high, it will cause oscillation in the circuit.