Advantages that a bipolar junction transistor has over a MOSFET?
- The collector current follows an exponential function of the base-emitter voltage very precisely and over a wide range (can be over ten orders of magnitude), especially if the current is not too high.
- This function can be used for logarithmic amplifiers and bandgap references
- Hence the transconductivity is simply Ic / Ut, where Ut = kB T / e is the thermal voltage
- This is the highest possible transconductivity of all devices, which makes them suitable for broadband amplifiers
- As a further consequence, the current ratio of a differential pair is given very precisely by exp ((V1-V2) / Ut), where V1 and V2 are the two base voltages. This applies to a wide frequency range.
- This function can be used as a precisely adjustable current divider.
- Or in Gilbert cell multipliers.
- They often have lower capacitances for a given nominal current, which is useful with broadband amplifiers
- Their voltage and current noise is easily predictable given the emitter current and beta. The noise figure is lower than that of FETs for low source impedance (but much higher for high impedance).
- They turn on at lower VBE than FETs (although very low threshold FETs are now available). This function is used in the popular Joule thief circuit
How is the concept of saturation in MOSFET different from BJT?
(1) At BJT, the base area is flooded with carriers so that it becomes saturated. However, there is no region of saturation in the mosfet.
(2) There is an ohmic region in Mosfets. If a higher gate voltage is applied, the MOSFET switches on in the ohmic range. However, this region does not exist in the BJT.
(3) BJT is activated in the saturation region and MOSFET is activated in the ohmic region. The ohmic area induces a channel in the area of the MOSFET body. The current then flows from the drain to the source.
(4) In BJT, saturation means that the base is completely flooded with beams. Therefore, current flows from the collector to the emitter.