The characteristic frequency of triode (hereinafter referred to as hfe) should be selected according to the working frequency of the circuit. When the voltage Vce (hereinafter referred to as hfe) between the collector C (hereinafter referred to as hfe) and the emitter E (hereinafter referred to as hfe) of the transistor is constant and the temperature is constant, the frequency characteristics of HFE are different when Ic changes, and when Ic approaches the maximum value, the frequency characteristics of HFE drop sharply.
Hfe can refer to the data sheet in the transistor manual, and choose the appropriate lamp according to the requirements of the circuit, usually leaving a margin (peak margin of AC voltage and current), and choose the lamp with smooth hfe curve under the condition that Vce and Ic are unchanged.
It is meaningless to look at hfe alone. For example, the withstand voltage of socket tube 100V, current 1A and power of 0.5W Even if hfe reaches 100 MHz, it cannot be used in the circuit of 200V, 1w and 50MHz. In other words, it can be used in the amplifier circuit.
There are many forms of transistor circuits, such as amplification, shaping, filtering and extrapolation.
High frequency electric field is one of the important parameters of AC and DC circuits, and it is also one of the conditions for AC amplifier to obtain low distortion in the whole frequency range. For DC circuits, transistors are usually used as switching tubes, and its hfe is an important parameter to determine whether this switch can pass the normal pulse duty cycle and time.
The following examples illustrate the application of transistors in AC, DC and hybrid circuits respectively: (Take NPN tube as an example)
For example, when the traveling wave tube of TV is working in the on-off state, besides the requirements of voltage, power and current, the appropriate hfe tube should be selected according to the line frequency. The DC path of the row tube is that the main power supply is applied to the C pole, the B pole is driven by the row pulse, and the amplified signal is generated at the C pole to control the pulse applied by the main power supply to the row transformer, and the AC path is that the amplified row pulse is applied to the deflection coil.
Another example is the power tube of switching power supply on the computer. The pmw wave (DC signal) from the control circuit is applied to the base of the transistor. There is about 300 volts DC on the C pole of the triode. The pmw pulse is used to control the turn-on or turn-off of the lamp tube, and a pulse voltage with the same frequency as the pmw wave is generated at the C pole and sent to the switching transformer. Then, ...
To sum up, if the hfe is lower than the operating frequency of the circuit or the hfe is not within the optimal curve range, the circuit will not work normally, resulting in distortion, loss of duty ratio, increased loss and overheating. And even cause damage to pipes and even circuits.
Therefore, the hfe of triode has an influence on both AC signal and DC signal (DC pulse signal), which is self-evident for AC circuit ... For DC amplifier, hfe determines whether the high-level DC signal can be changed by the pulse level added by the control electrode.
Because I can't map the explanation, it's difficult to express it. You can refer to the triode manual and refer to my answer. I hope it will help you.