Switching triode, analysis of accelerating capacitance!
1. Due to the charge storage effect, there is a capacitor between the transistors BE, and Rb constitutes an RC circuit, and the time constant greatly affects the conduction and cut-off speed of the transistor (ie, the switching speed).
2. Accelerating capacitance.
(1) When the control pulse is low, when the circuit reaches steady state, the transistor is turned off and the voltage across the capacitor is zero.
(2) When the high level of the control pulse comes, since the capacitor voltage cannot be abruptly changed, the capacitor needs to remain at zero. Thus, the base voltage of the transistor is abruptly changed to a high level, so that the transistor is turned on quickly; the capacitor is charged to the pulse level voltage. Entering the steady state, the capacitor voltage is the pulse level voltage.
(3) Thereafter, when the low level of the control pulse comes, since the capacitor voltage cannot be abruptly changed, the pulse level voltage needs to be maintained. Therefore, the base voltage jumps from zero (actually bebe voltage drop) to a negative pulse level. When the voltage is applied, the transistor quickly goes from saturation to the off state; after that, the capacitor discharges through R, and when the steady state is reached, the voltage at both ends is zero.
(4) Then, repeat the above process.
1. Typical Accelerating Capacitor Circuit T491D475M050AT Figure 2 shows the accelerating capacitor circuit in a pulse amplifier (an amplifier that amplifies the pulse signal).
The VT1 in the circuit is a triode, which is a pulse amplifying tube, Cl is connected in parallel with R1, and Cl is an accelerating capacitor. The role of Cl is to speed up the conversion speed of VT1 turn-on and turn-off, so it is called an accelerating capacitor.
2. Analysis of circuit working principle
The transistor VT1 in the circuit operates in the switching state (corresponding to a switch), and Ui is the input signal voltage applied to the base of the transistor VT1, which is a rectangular pulse signal. When U is high, transistor VT1 is saturated and turned on; when Ui is low, transistor VT1 is turned off.
The accelerating capacitor C1 and the triode VT1 input resistor Ri constitute an equivalent circuit as shown in FIG. 3(a).
(1) Accelerate the conduction process. When the input signal voltage Ui transitions from OV to high level, since the voltage across the capacitor C1 cannot be abruptly changed, the voltage applied to the base of VT1 is a apex pulse whose voltage amplitude is the largest, as shown in Fig. 3(b). This apex pulse is applied to the base of VT1, so that the VT1 base current rapidly increases from OA to large, so that VT1 quickly enters saturation from the off state, which accelerates the saturation conduction of VT1, ie shortens the VT1 saturation conduction time. (The time it takes for the triode to go from saturation to saturation).
(2) Maintain the conduction process. After t0, the charging of Cl is quickly ended. At this time, the voltage of the input signal voltage Ui applied to the base of VT1 is relatively small, maintaining the saturation conduction state of VT1.
(3) Accelerated cut-off process; when the input signal voltage U suddenly jumps from high level to OV, as shown in Figure 3(b), the polarity of the voltage originally charged on Cl is left and right. The voltage applied to the base of VT1 is a negative apex pulse. Since the voltage applied to the base of VT1 is negative, the process of extracting charge from the base region by VT1 is accelerated, and VT1 is switched from saturation to off state at a faster speed, that is, the time for VT1 to turn off is shortened.
Due to the access capacitor C1, VT1 enters the saturation state at a faster speed, and also enters the off state at a faster speed. It can be seen that the capacitance Cl has the function of accelerating the VT1 operation, so Cl is called an acceleration capacitor.
Such an accelerating capacitor circuit mainly appears in an electronic switching circuit (a circuit using a triode as a switch) or a pulse amplifier circuit, and an audio amplifier does not use such a circuit.
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