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Class A BJT Amplifiers
62 questions By Tony R. Kuphaldt
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Question 28 of 62
Define what a common-emitter transistor amplifier circuit is. What distinguishes this amplifier configuration from the other single-BJT amplifier configurations, namely common-collector and common-base?
Also, describe the typical gains (voltage and current) of this amplifier configuration, and whether it is inverting or noninverting.
Reveal answerThe common-emitter amplifier configuration is defined by having the input and output signals referenced to the base and collector terminals (respectively), with the emitter terminal of the transistor typically having a low AC impedance to ground and thus being “common” to one pole of both the input and output voltages.
Common-emitter amplifiers are characterized by moderate voltage and current gains, and an inverting phase relationship between input and output.
Notes:The answers to the question may be easily found in any fundamental electronics text, but it is important to ensure students know why these characteristics are such. I always like to tell my students, “Memory will fail you, so you need to build an understanding of why things are, not just what things are.”
One exercise you might have your students do is come up to the board in front of the room and draw an example of this circuit, then everyone may refer to the drawn image when discussing the circuit’s characteristics.
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Question 29 of 62
Define what a common-base transistor amplifier circuit is. What distinguishes this amplifier configuration from the other single-BJT amplifier configurations, namely common-collector and common-emitter?
Also, describe the typical gains (voltage and current) of this amplifier configuration, and whether it is inverting or noninverting.
Reveal answerThe common-base amplifier configuration is defined by having the input and output signals referenced to the emitter and collector terminals (respectively), with the base terminal of the transistor typically having a low AC impedance to ground and thus being “common” to one pole of both the input and output voltages.
Common-base amplifiers are characterized by high voltage gains, current gains less than unity, and a noninverting phase relationship between input and output.
Notes:The answers to the question may be easily found in any fundamental electronics text, but it is important to ensure students know why these characteristics are such. I always like to tell my students, “Memory will fail you, so you need to build an understanding of why things are, not just what things are.”
One exercise you might have your students do is come up to the board in front of the room and draw an example of this circuit, then everyone may refer to the drawn image when discussing the circuit’s characteristics.
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Question 30 of 62
Compared to common-collector and common-emitter amplifiers, common-base circuits have few practical applications. Explain why.
Reveal answerCommon-base amplifier circuits are typified by sub-unity current gains and very low input impedances.
Notes:Perhaps the most frequent application of the common-base amplifier topology is the so-called cascode amplifier circuit, where a common-emitter stage acts as a “front-end” buffer to the common-base stage to provide reasonable input impedance and current gain. The grounded-base configuration of the final output stage virtually eliminates the undesirable effects of Miller (collector-to-base) capacitance, resulting in an amplifier capable of high-frequency operation with little or no neutralization required.
