A3 Amplifiers with MOSFET

Topic

This MOSFET—already described in the context of advanced semiconductor device modeling—exhibits virtually infinite input resistance at its gate terminal (drawing no DC control current), making it particularly attractive for most modern analog applications. In its common-source configuration, it inverts and amplifies the input signal, much like the common-emitter configuration found in BJT amplifier models.

In contrast, in the source-follower configuration, the voltage gain is approximately unity—meaning it does not amplify voltage—yet the circuit provides high current capability and very low output resistance; it thus serves as a buffer to couple a weak signal source to a load requiring higher current. The differential amplifier—consisting of two matched transistors that amplify the difference between two input signals while rejecting their common components, such as noise picked up equally by both lines—serves as the input stage for virtually all operational amplifiers; the amplifier's ability to reject the common-mode signal relative to the differential signal is quantified by a single figure: the common-mode rejection ratio. By combining a differential pair with an active load stage, enormous voltage gains can be achieved in a single stage.

Output stages built using complementary MOS transistor pairs—an N-type and a P-type working in tandem—achieve high energy efficiency without the crossover distortion issues that plague the simpler Class B designs described earlier.

Thus, this module—which begins by introducing a gate that draws virtually no current—concludes by demonstrating that this input characteristic is no mere detail; rather, it is the decisive factor driving modern analog design to favor the MOSFET over the BJT in most cases, even though the BJT remains more linear in certain respects.

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