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Collector current and emitter current are related by Ic = α Ie with α ≈ 1, so increase in emitter current with temperature is opposed, and operating point is kept stable.
Similarly, if the transistor is replaced by another, there may be a change in IC (corresponding to change in β-value, for example). By similar process as above, the change is negated and operating point kept stable
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How do NPN bipolar junction transistors turn on and off?
In order to bias a bipolar junction transistor on, you need to forward bias the base-emitter junction at the same time you forward bias the collector-emitter junction, and the ratio of collector current over base current must be somewhat less than hFe, the transistor's gain. This is known as saturated, or non-linear mode, operation. In practice, we drive the base much harder than the calculated required current, so as to minimize dependency on varying hFe's for various transistors.Turning the transistor off is a simple matter of eliminating the base current.In the case of the NPN transistor, the base and collector would need to be more positive than the emitter. In the case of the PNP, they would need to be more negative.
How do you design active single transistor low pass filter?
Using an appropriate transistor, set up the base bias in the normal fashion, same for the emitter resistor.You then need RC feedback emitter to base. This will take the form of a t filter: base input>c1>r1(to emitter)>c2(to base)C2 = 2×C1R1=R2×R3/(R1+R2)Fo=1.414/(4π×R1×C2)See external link for more.
Explain the operation npn transistor when used as amplifier.. and explain the basic operation of NPN when used as switch?
The NPN transistor when used as an amplifier is operating in linear mode, and, when operating as a switch, in saturated mode.In the following discussion, base currrent means base-emitter current, while the base is more positive than the emitter, and collector current means collector-emitter current, while the collector is more positive than the emitter. There is base-collector current, but we are going to ignore it for now - besides, we are going to discuss class A, common emitter, configuration.The PNP transistor is very similar. Everything is backwards, including Vcc, which is now -Vcc, or appropriate reconfiguration. The rules are the same - just backward.In switched or saturated mode, the ratio of base to collector current is far greater than beta-dc, or hFe, so the transistor is operating way out of its linear mode. We call that saturated mode, and the transistor is essentially either fully on or fully off, and therefore operating as an on-off switch.The rest of this discussion will focus on linear or amplilfier mode.If the ratio of base to collector current is less than beta-dc, or hFe and, if both base and collector voltage are greater than cutoff voltage, then the transistor is operating in linear mode. Well, sort of, for best linear mode, we look at the data sheet, or make empirical observations, and we pick the base and collector currents that are centered between the base knee and the collector knee, i.e. "in the middle of" the linear region.In this mode, a very small base current can control a much larger collector current, and, most importantly, a very small change in base current can create a much larger change in collector current.In the theoretical case, for example, where the emitter is grounded and where hFe is 100, then 1 mA of base current translates to 100 mA of collector current, and 2 mA of base current translates to 200 mA of collector current. Problem is, that hFe varies amongst even so called identical transistors, and hFe varies as a function of temperature as well.So, in the practical case, an emitter resistor is added to stabilize the transistor and place limits on the need for hFe of a particular value. Done properly, this will yield predictable gain for various transistors and for various temperatures.Now, lets look at how gain works in the practical sense. The base voltage is also a known delta above emitter voltage. Yes, temperature will affect this, but proper design can make this a negligable factor. The emitter current times the emitter voltage results in a known voltage. By Norton's current law, the base current and the collector current add up to be the emitter current, but by hFe, the base current is very much smaller than collector current, meaning that the really important part is that collector and emitter current are the same for all practical purposes.So, now add a collector resistor. Ignoring base current, the collector/emitter circuit is a series circuit, and Norton's current law, reinterpreted for series circuits, says the two resistors have the same current. Think about what that means; if the current in both resistors is the same, then the ratio of the voltage across the two resistors is proportional to their value. The gain of the amplifier is collector resistor divided by emitter resistor. That is critical knowledge. Again, base current enters into the equation but, if hFe is high enough, it does not matter.All that is left, then, is to bias the base. You want to pick a base voltage (current) that places the collector current in the center (or in an appropriate point) of the linear region. Choose a nominal hFe, divide by collector current, and you get an approximation of what base current bias should be. Choose a resistor divider to match, keeping in mind that the two resistors (base to Vcc and base to Gnd) in parallel will reflect your effective input impedance.Review everything, particularly your power levels. To calculate the power through the collector/emitter junction subtract collector resistor voltage from emitter resistor voltage from Vcc, and you get collector/emitter voltage. Multiply that by collector current, and you get power dissipated by the transistor in nomial bias condition.Play with the values until you have what you want. You could even set this up in a spreadsheet.Last, but not least, there is a base bias voltage. If you are going to amplify something, you need to maintain that nominal bias voltage. Connect a series capacitor between the base and the input point and you will be able to operate from an AC signal that is zero referenced. Just pick the RC time constant appropropriate for your application.Similarly, there is a collector bias, so, if you want an AC output zero referenced, use a series capacitor also in between the collector and the ouput.This is an AC coupled, inverting amplifier. There are DC coupled non-inverting versions, but they are more complicated, requiring more than one transistor, and this answer does not address them. Good luck!
How transistor produce 180 degree phase shift?
In the common emitter configuration, a class A amplifier, an increase in base voltage (the input) leads to an increase in base-emitter current which leads to a proportionately larger increase in base collector current. That pulls the collector towards the emitter, which decreases the collector voltage. Since the collector is the output, this configuration is an inverting amplifier.
Working of npn transistor?
N-p-n transistor is made by sandwiching thin layer of p-type semiconductor between two layers of n-type semiconductor. It has three terminals, Emitter, Base and collector. The npn transistor has two supplies, one is connected through the emitter base and one through the collector base. The supply is connected such that emitter-base are forward biased and collector base are reverse biased. It means , Base has to be more positive than the emitter and in turn, the collector must be more positive than the base. The current flow in this type of transistor is carried through movement of electrons. Emitter emits electrons which are pulled my the base as it is more positive. these end up in the collector as it is yet more positive. In this way, current flows in the transistor. Transistor can be used as an amplifier, a switch etc.
Why emitter bias is more stable than fixed bias?
negative feedback
Why emitter follower is stable?
The emitter follower, or Class C amplifier, is stable because offset does not depend too much on temperature, nor is there a bias circuit that affects gain. The emitter voltage is about 0.7V less (NPN) or more (PNP) than the base. So long as the ratio of collector-emitter current over base-emitter current stays in the linear region and is less than the transistor's gain (hFe) the emitter will "follow" the base with the junction offset. There will be some drift with temperature, but the circuit will remain stable so long as temperature stays within the transistor's specifications. If you need a circuit that is even more stable, you drive the base with an op-amp that compensates for the error between emitter and the reference input. Many linear power supplies are built this way. This technique nulls out the small changes to offset caused by temperature.
How do NPN bipolar junction transistors turn on and off?
In order to bias a bipolar junction transistor on, you need to forward bias the base-emitter junction at the same time you forward bias the collector-emitter junction, and the ratio of collector current over base current must be somewhat less than hFe, the transistor's gain. This is known as saturated, or non-linear mode, operation. In practice, we drive the base much harder than the calculated required current, so as to minimize dependency on varying hFe's for various transistors.Turning the transistor off is a simple matter of eliminating the base current.In the case of the NPN transistor, the base and collector would need to be more positive than the emitter. In the case of the PNP, they would need to be more negative.
How do you design active single transistor low pass filter?
Using an appropriate transistor, set up the base bias in the normal fashion, same for the emitter resistor.You then need RC feedback emitter to base. This will take the form of a t filter: base input>c1>r1(to emitter)>c2(to base)C2 = 2×C1R1=R2×R3/(R1+R2)Fo=1.414/(4π×R1×C2)See external link for more.
What is the effect of emitter resistance in common emitter amplifier?
The gain of a common-emitter amplifier is collector resistor divided by emitter resistor, or hFe, whichever is less. Since hFe depends on temperature, designing the amplifier to be dependent on resistance ratio makes it more stable. As such, the emitter resistance serves to stabilize the amplifier.
How do you identify a pnp or npn transistor?
If you know the base of the transistor, and you have an ohmmeter that puts out more than about 0.7 volts, you can check base to emitter or base to collector as if it were a diode, and it will conduct when the more positive lead of the ohmmeter is connected to the P junction. That will tell you if the transistor is NPN or PNP. If you don't know the base, you can check all six directions. Only two should conduct, the two that are forward biased towards the base.
What is the working of a transistor?
For proper working of a transistor,the voltage at the base region must be more positive than that of the emitter region.The voltage at the collector region, in turn, must be more positive than that of the base region.when voltage is applied to transistor, the emitter supplies electron,which is pulled by the base from the emitter as it is more positive than the emitter.This movement of electrons from emitter to collector creates as flow of electricity through the transistor.The current passes from the emitter to the collector through the base.Thus, adjustment of voltage in the base region modifies the flow of the current in the transistor by changing the number of electron in the base region. In this way, small changes in the base voltage can cause large changes in the current flowing out of the collector. We have three transistor element, a.)Emitter b.)Base c.)Collector
What is reason of invertong output of common emitter amplifier?
The output of the common emitter amplifier is inverted because increasing the base-emitter current causes a proportional increase in collector-emitter current. That increase in collector-emitter current pulls the collector towards the emitter, so the voltage on the collector will go down when the biased base voltage goes up, and vice versa. This is the characteristic of the Class A Common Emitter amplifier. Responding to a request for more details... This is for the NPN transistor. It applies to the PNP transistor as well, but directionality of voltage and current increases is reversed in that case. Start with the base-emitter circuit. You have some kind of bias network holding the base at a certain voltage. That voltage represents a certain current, which goes through the base-emitter junction and emitter resistor, if there is one. Typically, you consider that the emitter voltage is less than the base voltage by the amount of one diode junction, or about 0.7 volts. If you were to increase the input voltage, you would cause a corresponding increase in base-emitter current. The transistor has gain, beta-dc or hFe, which is basically the ratio of collector-emitter current over base-emitter current, so the base-emitter current is controlling a larger collector-emitter current. Now, focus on the collector-emitter circuit. You have some kind of resistor in the collector, and you might have some kind of resistor in the emitter. (More on the emitter resistor later.) Think of this circuit as three resistors in series, the collector resistor, the equivalent resistance of the collector-emitter junctions, and the emitter resistor. This also represents a current, one that is being controlled by the base-emitter resistance. Note that the base-emitter current is being added to the collector-emitter current, so the emitter current, by Kirchoff's current law, is the sum of the base and collector currents. Since the gain is relatively high, however, the contribution from the base is generally negligible. (In high power transistor amplifiers, gain is usually low, so base current is not negligible, so we do take it into account.) The crucial factor here is that the collector current is proportional to the base current, in the ratio of beta-dc, or hFe. If, for instance, base current were increased by 1 ma, with an hFe of 200, then the collector current would increase by 200 ma. Well, sort of.... You have to consider the transistor's limits, and you have to consider whether or not you are opereating in linear mode. Limits are easy, just check the specs. Lets look at linear mode... If you attempt to pull more collector current than the collector-emitter circuit would allow, i.e. to make the equivalant collector-emitter resistance go to zero, then the transistor starts operating in saturated mode. In saturated mode, the transistor is acting as a switch, and it is distinctly non-linear. Even if not saturated, the transistor can be poorly linear when operating at the ends of the linear range. This is why any good design includes consideration of linear mode range. You want to operate in the center of the linear range, which simply means that we bias the base to cause the collector to be in the middle of its optimal range, giving maximum linearity. Summarizing so far, we have a transistor that is multiplying its base current by some factor, beta-dc or hFe, causing a proportional collector current. With this viewpoint, the amplifier is non-inverting because increasing base current causes collector current to increase. We call the circuit inverting, however, because we want to think of the collector voltage rather than the collector current. Remember that the transistor has an equivalent resistance. In particular, the collector-emitter resistance changes in response to stimuli on the base. In order for the collector current to increase, the equivalent resistance must decrease. Looking at the collector-emitter circuit, you have a voltage divider, collector resistance at the top, and the sum of equivalent collector-emitter resistance and the emitter resistance at the bottom. It is easy to see that, if the collector current increases, the collector voltage must decrease. That is why we call this an inverting amplifer. Back to the emitter resistor... When we say "common emitter", we mean that the emitter is common and we analyze everything else. You can design and operate this amplifer with no emitter resistor, and that would be a true common (or grounded) emitter configuration. Problem is the circuit will not be stable... First, gain varies amongst transistors, even amongst transistors of identical design. It is common to state that hFe ranges from 80 to 400, as an example. The circuit design must consider this variability. If you want predictable and stable gain, you must compensate for gain variation. You design the circuit for minimum hFe, but you look at what happens with maximum hFe. To make matters worse, the junction voltage at any particular current varies with temperature, sometimes substantially. This means that your beautifully designed circuit is unpredictable when it gets warm, and all circuits that manipulate power, even small amounts of power, get warm. Its all a matter of degree. There are many ways to compensate for gain variations. One of them is to use an emitter resistor. This effectively places a limit on gain by moving the primary factor for gain from the transistor to the circuit. The gain of a common emitter amplifier is hFe. When there is an emitter resistor, however, the gain is collector resistance divided by emitter resistance. If that ratio is less than hFe, then hFe variability will not affect gain.
Would a PNP transistor be conducting in this situation 0 volts at collector 1 volt at base and 2 volts at the emitter?
So for a PNP transistor to conduct the Emitter is always more positive with respect to both the Base and the Collector.
For normal operation of an npn transistor the base must be ground true or false?
False. For normal operation, an NPN transistor will have the base be more positive than the emitter and less positive than the collector, with the collector more positive than the emitter. Whether the base is grounded or not depends on the chosen design configuration of the circuit.
Why is collector wider than emitter and base?
actually in the case of transistors there are two concepts that are often misleaded those are 1. order of doping 2. order of the size of various regions order of doping emitter>collector>base order of size of various regions collector>emitter>base now the reason for this as CB junction is reverse biased more heat is dissipated at this junction so if the collector junction has large area the heat can be dissipated easily there by the transistor is saved from the burning of CB junction
What is ''base of support?
Base of support is the area between your feet. The wider and more solid your base of support is the more stable you will be. Base of support is crucial when you what to stay stable and perform well.
