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Because Beta (current gain) in a transistor is inversely proportional to frequency.
Hence it increases as frequency decreases.
Also the ac voltage gain is directly proportional to Beta.
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How do you calculate the current gain of a transistor?
Bipolar transistor current gain is also called "Beta," or the h-parameter "hfe." beta = current_out / current_in The beta of a BJT is mostly determined by the thickness of the Base region, and by the excess doping in the Emitter relative to the Base. A thin Base and a heavily-doped Emitter leads to a high value for current gain. In a BJT, beta = Ic / Ib In a FET, beta is usually taken as infinity, since no current flows in or out of the gate. Beta is an impirical number. It means nothing unless the Ic is known or the load. It can have a beta from 1000 to 10 it all depends on the load.
How do you design a bjt amplifier with gain 10?
To design a bjt amplifier with gain 10, consider that gain in a class A common emitter configuration is collector resistance divided by emitter resistance. So, pick the two resistors so that their ratio is ten. Bias the base so that the collector is at the center of the operating voltage range, along with the transistor being at the center of its linear region. Of course, you need to consider impedance and operating current, so you need to choose carefully. Also, the beta (hFe) of the transistor must be substantially more than ten for this to work reliably. Often, this voltage gain stage is followed by a class C common collector emitter follower to give you current gain as well.
What is btj transistors?
They are NPN and PNP. BJT mean bipolar junction transistors. there are two P-N junctions in BJT transisters.
Why FET produces less noise than BJT?
because the fet is made out of carbon metal oxide and the bjt is made out of silicon or germanium
How do you use BJT as a switch?
sit on dick
What are the steps involved in finding the signal gain for a common-emitter BJT amplifier?
The signal gain of a CE BJT amplifier is hFe or collector resistance divided by emitter resistance, whichever is less.
What is thermal runaway in BJT?
Thermal runaway is where the biasing and operating point is such that the temperature causes the gain to increase, which causes the temperature to increase, which causes the gain to increase, in a vicious circle, leading to destruction of the BJT. Proper biasing and gain management can prevent this from occurring.
What is the effect of bypass capacitor on frequency response?
Gain in a CE configuration of a BJT is collector resistance divided by emitter resistance, subject to the limit of hFe. The emitter bypass capacitor will have lower impedance at high frequency, so the gain will be higher at higher frequency, making this a high-pass amplifier.
Can you replace mosfet with an bjt transistor?
A MOSFET (metal oxide semiconductor field effect transistor) can be used INSTEAD of a BJT (bypolar junction transistor, so transistor is redundant in your question), if the circuit in question is modified to allow it and the MOSFET is chosen appropriately. BJT's will usually have a higher intrinsic gain, but have lower input resistance. Also a BJT in general will work better at higher frequencies than a MOSFET (unless you choose a high frequency MOSFET) due to the capacitive nature of MOSFETs.
Why electrolytic capacitors are used for frequency response of BJT as amplifier?
It can take a lot of capacitance to present a low impedance to a low frequency. Electrolytics offer lots of capacitance for a low price.
What are the frequency limitations for transistor FET's and BJT's?
As with any solid state device the ratings vary greatly depending on the manufacture. For exact statistic consult the component manufacturer. For FET: A gate length of 1µm limits the upper frequency to about 5 GHz, 0.2µm to about 30 GHz. For BJT: ??? could not find a general limit
Frequency response of bjt amplifier?
All amplifier typically exhibit a band-pass frequency response. The cut off frequency in the low end is usually determined by the coupling band bypass capacitor .and the high frequency limit is typically determined by internal capacitances in the transistor itself.
Why bjt is bipolar?
Bjt is bipolar because in bjt the conduction of current is due to the electrons as well as holes
How do you calculate the current gain of a transistor?
Bipolar transistor current gain is also called "Beta," or the h-parameter "hfe." beta = current_out / current_in The beta of a BJT is mostly determined by the thickness of the Base region, and by the excess doping in the Emitter relative to the Base. A thin Base and a heavily-doped Emitter leads to a high value for current gain. In a BJT, beta = Ic / Ib In a FET, beta is usually taken as infinity, since no current flows in or out of the gate. Beta is an impirical number. It means nothing unless the Ic is known or the load. It can have a beta from 1000 to 10 it all depends on the load.
How do you design a bjt amplifier with gain 10?
To design a bjt amplifier with gain 10, consider that gain in a class A common emitter configuration is collector resistance divided by emitter resistance. So, pick the two resistors so that their ratio is ten. Bias the base so that the collector is at the center of the operating voltage range, along with the transistor being at the center of its linear region. Of course, you need to consider impedance and operating current, so you need to choose carefully. Also, the beta (hFe) of the transistor must be substantially more than ten for this to work reliably. Often, this voltage gain stage is followed by a class C common collector emitter follower to give you current gain as well.
What are the uses of BJT?
Bipolar junction transistors has two junctions base emitter junction, base collector junction. Accordingly there are four different regions of operation in which either of the two junctions are forward biased reverse biased or both. But the BJT can be effectively operated in there different modes according to the external bias voltage applied at each junction. i.e. Transistor in active region, saturation and cutoff. The other region of operation of BJT is called as inverse active region.
What is thermal runaway in a BJT?
Thermal runaway is where the biasing and operating point is such that the temperature causes the gain to increase, which causes the temperature to increase, which causes the gain to increase, in a vicious circle, leading to destruction of the BJT. Proper biasing and gain management can prevent this from occurring.
