Accelerating capacitor action? Circuit operation principle analysis
Due to the charge storage effect, a capacitor exists between the base and emitter of the transistor, along with a resistor Rb, forming an RC circuit. The time constant of this RC circuit significantly influences the switching speed—both the turn-on and turn-off times—of the transistor.
2. Accelerating Capacitor
(1) When the control pulse is at a low level, and the circuit reaches a steady state, the transistor is in the off state, and the voltage across the capacitor is zero.
(2) As the control pulse transitions to a high level, the capacitor voltage cannot change abruptly. Therefore, it remains at zero momentarily, causing the base voltage of the transistor to jump to a high level almost instantly. This rapid rise in base voltage turns the transistor on quickly. At the same time, the capacitor begins to charge up to the level of the control pulse voltage. Once the circuit reaches steady state, the capacitor voltage stabilizes at the pulse level.
(3) When the control pulse returns to a low level, the capacitor voltage again cannot change abruptly, so it maintains its previous value. This causes the base voltage to drop suddenly from zero (or slightly above due to the BE junction voltage) to a negative voltage. This negative pulse helps the transistor transition rapidly from saturation to the off state. After that, the capacitor discharges through the resistor R, and eventually, the voltage across the capacitor returns to zero as the circuit reaches steady state again.
(4) The process then repeats continuously.
[Image: http://i.bosscdn.com/blog/o4/YB/AF/q4VoOAEKWcAACX3uOAnOg404.png]
1. Typical Accelerating Capacitor Circuit – T491D475M050AT
Figure 2 shows an accelerating capacitor circuit used in a pulse amplifier, which is a type of amplifier designed to amplify pulse signals. In this circuit, VT1 is a triode, also known as a pulse amplifying tube. C1 is connected in parallel with R1, and it serves as the accelerating capacitor. Its main function is to increase the speed at which the transistor switches between on and off states, hence the name "accelerating capacitor."
[Image: http://i.bosscdn.com/blog/o4/YB/AF/q4VoOACanqAAAwyZ5asxI073.png]
2. Circuit Working Principle Analysis
The transistor VT1 operates in a switching mode, similar to a switch. The input signal voltage Ui is applied to the base of VT1 and is a rectangular pulse. When Ui is high, the transistor saturates and turns on; when it is low, the transistor turns off.
The accelerating capacitor C1 and the input resistor Ri of the triode VT1 form an equivalent circuit, as shown in Figure 3(a).
(1) Accelerating the Turn-On Process: When the input signal voltage Ui transitions from 0V to a high level, the voltage across the capacitor C1 cannot change instantaneously. As a result, the base of VT1 receives a peak pulse with the maximum amplitude, as shown in Figure 3(b). This peak pulse increases the base current of VT1 rapidly, allowing it to enter saturation quickly from the off state, thus speeding up the turn-on process and reducing the time it takes for the transistor to reach saturation.
(2) Maintaining the Turn-On State: After a short time t0, the capacitor C1 is fully charged. At this point, the base voltage of VT1 is lower but still sufficient to maintain the transistor in the saturated on state.
(3) Accelerating the Turn-Off Process: When the input signal voltage Ui suddenly drops from a high level to 0V, the polarity of the voltage stored on C1 remains unchanged. This results in a negative peak pulse being applied to the base of VT1. This negative voltage accelerates the removal of charge from the base region, allowing the transistor to turn off more quickly. Thus, the turn-off time is significantly reduced.
[Image: http://i.bosscdn.com/blog/o4/YB/AF/q4VoOADYJeAAAiHjfc9hM317.png]
Because of the presence of the accelerating capacitor C1, VT1 can both turn on and turn off more rapidly. It is clear that the capacitor plays a key role in speeding up the operation of the transistor, which is why it is referred to as an accelerating capacitor.
This type of accelerating capacitor circuit is commonly found in electronic switching circuits or pulse amplifiers where transistors are used as switches. However, such circuits are not typically used in audio amplifiers.
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