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we generally used Mosfet where we need high input impedance..whereas BJT has low i/p imp than mosfet.MOsfet is voltage controlled device whereas BJT is current controlled device.MOSFEt is less noisy than BJT.
bjt's are more suited for high frequency and high gains. its also far more advanteageous than the mos in the implementation of differential amplifiers due to its faster switching between the 2 transistors and also due to bjt's lower input offset voltages.
however mosfets are preffered in cascodes due to its limitless output resistance increase by cascading one on top the other. but when a bjt is cascaded it's output resistance increases only upto (beta*r0).
and the input impedance of mos is infinite. so better matching between stages. there will be still more...
Most of today's ICs use MOS technology cause they're easier to implement (so they can be reduced to smaller sizes than BJTs) and consume less power (when used as switches). These advantages are very important when you have to design digital circuits, and since digital circuits have overtaken the market nowadays MOS technology is prefered over BJTs. Nontheless, there are a couple of aplications that use BJTs (since they are faster than MOSFETs), like RF circuits. BJTs, used as amplifiers, can generate much higher gain than MOSFETs, and that's why there are also technologies that use both MOSFETs and BJTs (BiCMOS) to take advantage of both kind of transistors.
It is true that MOSFET is easier to implement because it is voltage controlled, where as BJT is current controlled.
However, many analog IC still use BJT. This is espeacially in high power devices such as multi voltage regulator and audio and video equipments.
The is because as a whole, BJT can handle high voltage and current and have better THD.
CMOS are mainly for low power applications and digital circuits.
mosfet are slower than bjt's and tehy consumes less power and less aea on the silicon and eaier to fabricate.
so if u want to make a high frequency circuit use bjt otherwise mos is the best solution
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