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What will happen to voltage divider circuit if emitter resistance is greater than collector resistance?
Wiki User
∙ 12y ago
The gain of a class A, common emitter BJT amplifier, a fairly standard configuration, is defined as collector resistance divided by emitter resistance, or as hFe, whichever is less. Assuming that we are operating in a linear mode, and hFe is not a limiting factor, then the emitter resistance being greater than the collector resistance simply means that the gain is less than one.
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∙ 12y ago
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The dc input resistance of MOSFET is?
it depends on the type of the circuit you are analyzing..it could be a voltage divider, emitter follower, be specific on what type of circuit and maybe i can help you aobut this question.
True or false a high resistance between the collector and emitter when a transistor is switched off?
yes , it has high resistance between collector and emitter on the off state.
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.
What is meant by biasing in a single stage ce amplifier?
Biasing in a single stage common-emitter amplifier means to place the base-emitter current at a point where the collector-emitter current is in the middle of the transistor's linear range.First, you pick the target range and output impedance of the amplifier, picking the collector and emitter resistors. The gain of the stage is collector resistance divided by emitter resistance, limited by available hFe. You want to try to pick a resistor pair that will place the collector voltage in the center of the desired range, while keeping the desired operating current where you want it.Then, you pick the base resistor divider pair such that the base voltage is the forward bias drop of the base-emitter above (NPN) or below (PNP) the emitter voltage. You find that emitter voltage by considering the collector voltage, along with the operating current and the collector and emitter resistors. (Its straightforward Ohm's law, considering that the collector-emitter forms the third resistor in the divider chain.) You have to consider hFe in this calculation, as well as realizing that the two base resistors will form the input impedance of the stage. (Well, actually, base-emitter current is included in the input impedance calculation, but that is usually a small contribution if the hFe is high enough.)Then you need to consider the power dissipation in the stage, and make sure that the transistor can handle that, and that hFe will not drift unacceptably under temperature. (Stable designs are such that the hFe is far greater than the ratio of collector resistance over emitter resistance, so that your limits are based on ratio, and not on hFe. Problematic designs are when the desired gain is greater than hFe, such as when the emitter resistance is zero - this makes gain equal to hFe, and introduces the possibility of thermal runaway.)
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.
Why a capacitor is used for amplification in common emitter circuit?
A: The ratio of emitter/collector resistance is the gain. by adding a capacitor on the emitter the AC parameters will shift as a function of frequency
What is the Effect of collector resistance in emitter follower circuit?
Colector resistance in an emitter follower circuit serves to place a limit on how much current can be supplied by the transistor. Often, the resistor is sized so that a short circuit in the load does not cause the transistor to fail.
Why would there be a gain when you add the emitter capacitor to a circuit board?
A capacitor has lower resistance (impedance) as frequency increases. Adding an emitter capacitor effectively lowers the emitter resistance as frequency increases. Since gain in a typical common emitter amplifier is collector resitance divided by emitter resistance, this decrease in emitter resistance will increase gain as frequency increases.
The dc input resistance of MOSFET is?
it depends on the type of the circuit you are analyzing..it could be a voltage divider, emitter follower, be specific on what type of circuit and maybe i can help you aobut this question.
Explain why the collector voltage is approximately zero when a transistor has a collector-emitter short?
The collector voltage is not necessarily approximately zero when a transistor has a collector-emitter short. It depends on whether or not there is an emitter resistor.A typical collector-emitter circuit has two resistors, one in the collector and one in the emitter. One or both of them might be zero, i.e. not present, depending on design requirements. The collector-emitter junction represents a third resistor, the value of which is dependent on base-emitter vs collector-emitter current ratios and hFe.If the collector-emitter junction is shorted, then this circuit degrades to a simple voltage divider, or single resistor, and the collector-emitter voltage differential will be approximately zero. Simply calculate the voltage based on the one or two resistances.Results could be different than calculated, if the resistors are small in camparision to the shorted impedance, and it could be different depending on the base to emitter or collector relationship in that fault state, though the latter case is usually negligible due to the relatively high resistances of the base bias circuit.
True or false a high resistance between the collector and emitter when a transistor is switched off?
yes , it has high resistance between collector and emitter on the off state.
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.
What is meant by biasing in a single stage ce amplifier?
Biasing in a single stage common-emitter amplifier means to place the base-emitter current at a point where the collector-emitter current is in the middle of the transistor's linear range.First, you pick the target range and output impedance of the amplifier, picking the collector and emitter resistors. The gain of the stage is collector resistance divided by emitter resistance, limited by available hFe. You want to try to pick a resistor pair that will place the collector voltage in the center of the desired range, while keeping the desired operating current where you want it.Then, you pick the base resistor divider pair such that the base voltage is the forward bias drop of the base-emitter above (NPN) or below (PNP) the emitter voltage. You find that emitter voltage by considering the collector voltage, along with the operating current and the collector and emitter resistors. (Its straightforward Ohm's law, considering that the collector-emitter forms the third resistor in the divider chain.) You have to consider hFe in this calculation, as well as realizing that the two base resistors will form the input impedance of the stage. (Well, actually, base-emitter current is included in the input impedance calculation, but that is usually a small contribution if the hFe is high enough.)Then you need to consider the power dissipation in the stage, and make sure that the transistor can handle that, and that hFe will not drift unacceptably under temperature. (Stable designs are such that the hFe is far greater than the ratio of collector resistance over emitter resistance, so that your limits are based on ratio, and not on hFe. Problematic designs are when the desired gain is greater than hFe, such as when the emitter resistance is zero - this makes gain equal to hFe, and introduces the possibility of thermal runaway.)
Why output of common emitter amplifier is inverted?
In a common emitter amplifier, the base-emitter current causes a corresponding collector-emitter current, in the ratio of hFe (beta gain) or collector resistance over emitter resistance, which ever is less. Since this ratio is usually greater than one, the differential collector current is greater than the differential base current. This results in amplification of the base signal. As you increase the base-emitter current, the collector-emitter current also increases. This results in the collector being pulled towards the emitter, with the result that the differential collector voltage decreases. This results in inversion of the base signal.
What is swamping resistance?
(Electronics) Resistor placed in the emitter lead of a transistor circuit to minimize the effects of temperature on the emitter-base junction resistance and its resistance is called swamping resistance.
What is diffrance resistance of emitter and base or base and collector?
Maybe you ought to not take emitter and base into consideration prior to concluding
How do you design a voltage divider bias circuit with certain specifications for Vcc Ic and beta-dc?
When you design a voltage divider bias circuit for a BJT amplifier, you must consider the base current, because that represents a resistance which is in parallel with the lower leg of the divider. To determine the base current, select the desired operating point, and calculate the emitter (collector) current. Divide that by beta-dc, and you have base current. Back calculate the effective base resistance, and build the divider accordingly. Note that in a silicon BJT, the base voltage is about 0.7 V higher (NPN) or lower (PNP) than the emitter. Note also that these calculations only work correctly when the BJT is in linear mode. Note also that beta-dc varies amongst BJT's, even though with identical designs, so your design must consider these variations - you can compensate with an emitter resistor, but variations still have an impact.
