Discrete Semiconductor Devices and Circuits

Class A BJT Amplifiers

62 questions By Tony R. Kuphaldt

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Question 25 of 62

Identify the type of transistor amplifier configuration in these schematic diagrams as either common-emitter, common-collector, or common-base.

Reveal answer

Notes:
I have seen more than one method for determining the “common-ness” of an amplifier configuration, and not all are satisfying. Rather than telling your students how to distinguish one amplifier type from the others, let them examine the different configurations in the answer and figure out their own method(s)!
Question 26 of 62

Common-emitter, common-collector, and common-base amplifier circuits are sometimes referred to as grounded-emitter, grounded-collector, and grounded-base, respectively, because these configurations may actually be built with those respective terminals connected straight to ground.

Although this may not be very practical for ease of biasing, it can be done. Draw the rest of the circuit necessary to provide class-A operation for each of these (partial) transistor circuits. Be sure to show where the DC power source, signal input, and signal output connect:

Reveal answer

Notes:
Although it is more common in modern times to refer to the three BJT amplifier configurations as common-(e, c, b) rather than as grounded-(e, c, b), it may help some students grasp why the word “common” came to be used. “Grounded” makes more literal sense, and seeing these three circuit configurations with directly grounded terminals may serve as a starting point for identifying configurations where the “common” terminals are not directly grounded.
Question 27 of 62

Define what a common-collector transistor amplifier circuit is. What distinguishes this amplifier configuration from the other single-BJT amplifier configurations, namely common-emitter and common-base?

Also, describe the typical gains (voltage and current) of this amplifier configuration, and whether it is inverting or noninverting.

Reveal answer

The common-collector amplifier configuration is defined by having the input and output signals referenced to the base and emitter terminals (respectively), with the collector 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-collector amplifiers are characterized by high current gains, voltage gains of 1 or (slightly) less, 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.”