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Why to short circuit capacitor in small signal analysis of BJT?
Wiki User
∙ 12y ago
if the input signal passes through the biasing resistors, the biasing conditions get altered . To prevent this, the input signal should be directly sent to the amplifier (BJT) .Since a capacitor acts as a short circuit for ac signals,capacitors are placed both in the input side and the output side.
Wiki User
∙ 12y ago
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What is the purpose of a blocking capacitor?
At high frequency, capacitor can be considered as 1. Short Circuit in AC analysis. 2. Open Circuit in DC analysis. {because Xc= 1/(2*f*pi) where f= supply frequency,pi=3.14} As at high frequencies, in DC analysis, capacitor will be open circuited & can block the DC signal while AC signal is allowed to pass through.. Hence, this capacitor will act as a blocking capacitor for DC supply.
How are the capacitors treated in the dc equivalent circuit of an amplifier?
In dc equivalent circuit of an amplifier all capacitors are replaced by open circuit because capacitor block dc. As , Xc=1/2πfC We know that that frequency of dc is zero so Xc will infinite so we replace all capacitors with open circuit.
What is a reverberating circuit?
a closed circuit, in which the signal continually travels around a circular path over and over until one of the components (usually neurons) stops functioning and the signal is not transmitted to the next component; proposed by Hebb as the mechanism for short-term memory
Which is faster to discharge or to charge a capacitor?
It depends on the amount of current available to charge or discharge. On the surface, I would say that you can discharge a capacitor faster than you can charge it, because the charge rate is limited by the current available in the power supply, while the discharge current could be quite high, because you could just apply a short circuit conductor around the capacitor. Of course, this could damage the capacitor. In truth, the equation of a capacitor is...dv/dt = i/C, which means that, given the same charge or discharge current, the rate of change of voltage would be the same.AnswerThe time taken to fully charge, or to fully discharge, a capacitor is given by the equation: time = 5 CR, where C represents its capacitance, in farads, and R represents the resistance of the circuit supplying the capacitor, in ohms. By 'fully charge', we mean bring the potential-difference across the capacitor's plates to the same value as the applied potential difference.If the external voltage source is replaced with a short circuit then, providing the resistance of the circuit hasn't changed, the discharge time will be exactly the same as the charging time. If the resistance is changed, then the same equation applies, but you need to insert the new value of resistance.
What is the difference between a standard capacitor and a coupling capacitor?
Coupling capacitors are used to couple different stages so as to prevent DC from the o/p of one stage to go into the i/p of the next stage. For instance in coupling two BJT (bipolar junction transistors) it is required to use coupling capacitor to allow only ac signal from the o/p of fisrt stage to go to i/p of next BJT as incoming dc can distrub the biasing of the other BJT. Bypass capacitors are used to bypass the ac signal to ground. A capacitor is connected b/w the gnd and the wire. For ac signal capacitor will behave as short and will bypass it. However dc will not be bypassed as capacitor will behave as open for DC.
How capacitor behave in alternating current?
Capacitors have an equivalent reactance of 1/jwC (ohms) where w is the angular frequency of the AC signal and C is the capacitance. As the frequency of the signal across the capacitor increases, the capacitor reactance approaches 0 (capacitor acts like a short circuit). As the frequency of the signal across the capacitor decreases, the capacitor reactance approaches infinity (capacitor acts like an open circuit). So, if you have a high frequency signal (like a step input) the capacitor will momentarily act like a short.
What is the purpose of a blocking capacitor?
At high frequency, capacitor can be considered as 1. Short Circuit in AC analysis. 2. Open Circuit in DC analysis. {because Xc= 1/(2*f*pi) where f= supply frequency,pi=3.14} As at high frequencies, in DC analysis, capacitor will be open circuited & can block the DC signal while AC signal is allowed to pass through.. Hence, this capacitor will act as a blocking capacitor for DC supply.
Effect of shorting out the capacitor in a clamping circuit?
Any circuit using a capacitor will not work if the cap is short-circuited.
Why would the gain go up in a circuit board when you put an emitter capacitor in the circuit?
The emitter resistor is there to provide DC bias to the base. If it is not bypassed, then the AC output signal is also dropped across this resistor, effectively lowering the output swing. When a bypass capacitor is added, the DC bias still flows through the resistor, but the capacitor acts as a short circuit for the AC signal, so that the AC signal is not reduced. The capacitor selected must be large enough so it appears as a very low resistance at the lowest frequency the amplifier will pass.
Why capacitor behave as open circuit against alternating current?
Capacitors store electrical charge. Imagine we have a capacitor. At time 0 seconds we connect a DC voltage across the capacitor - immediately as the voltage is connected the capacitor is at 0 volts and the maximum current (relative to the circuit resistance) flows. At this extreme the capacitor can be treated as a short circuit, so for high frequency AC volts we should treat a capacitor as being a short circuit. As time passes the current in the circuit will go down and the voltage of the capacitor will go up - this is because as the capacitor gains more charge it gains more voltage, lowering the voltage across any resistance in the circuit consequently lowering the current in the circuit. When the capacitor is virtually full no current will flow at all and the voltage across the capacitor will equal the DC source voltage. At this extreme the capacitor can be treated as an open circuit, so for low frequency AC (allowing the capacitor to fill up before the current alternates) we can treat the capacitor as being an open circuit. Technically, it is not an open/closed circuit when it comes to AC because the capacitance will results in a signal lag or lead. However, if the frequency is low/high enough the lag/lead is often negligable.
When DC power is first applied to an uncharged capacitor it appears as a?
When DC power is first applied to an uncharged capacitor it appears as a short circuit.
1998 cavalier and why is it blowing fuses when we use it signal lights?
There is a short in the signal light circuit.
What is the meaning of a capacitor been shorten?
A shorted capacitor is one where the gap between the plates is damaged, and the plates are touching each other, creating a short circuit.
Can a 440 volt capacitor be used for a 230 volt application?
No. The capacitor will short out and possibly explode. On the other hand, the 440 Volt Cap can be used in a 220 Volt circuit.
What does cap do in a circuit?
Cap is short for capacitor, so what it does really depends on the specific circuit it is being used in. It could be for power supply smoothing, noise reduction, part of a timing circuit to mention just a couple. It depends what other components are being used with the cap(capacitor) and how they are connected to each other.
An uncharged capacitor acts like a short circuit when voltage is first applied to it?
Yes it does. Capacitors and capacitor banks have a high inrush current when first energized. As an example this is the reason that VFD's place a resistor in series with the capacitors for a short period of time when they are first energized to prevent damage to the rectification componente. They call it a precharge circuit. Current leads voltage by 90 deg in a purely capacitive circuit.
How are the capacitors treated in the dc equivalent circuit of an amplifier?
In dc equivalent circuit of an amplifier all capacitors are replaced by open circuit because capacitor block dc. As , Xc=1/2πfC We know that that frequency of dc is zero so Xc will infinite so we replace all capacitors with open circuit.
