EC-6.6 The Bipolar Transistor: Operation and Models
You can explain how a small base current controls a much larger collector current, name the four regions of operation, and use the small-signal model an amplifier is designed from.
Before:EC-1. Circuit AnalysisEC-5. Physics and Materials for ElectronicsUnlocks:S4-1. Analog CircuitsS5-4. Linear and Digital Integrated CircuitsPE1-1. VLSI TechnologyPE1-2. Fiber Optic CommunicationPE2-1. CMOS Analog and Digital IC DesignEC-12. Electrical Energy, Machines and Power Electronics
This is the device the analog course has always started after. Two junctions back to back, with a base region deliberately made thin enough that most carriers injected into it survive the trip and arrive at the collector. Everything else — the current gain, the four operating regions, the small-signal model — follows from that one construction decision. The hybrid-pi model is included here rather than left to the analog course, because a model met before the device it models is a set of symbols.
Work through these
Two junctions, and the thin base that makes it a transistor
Back-to-back junctions on their own would do nothing useful; the device works because the middle region is thin compared with the diffusion length. Most carriers injected into it reach the far junction instead of recombining.
NPTEL: Introduction to Semiconductor Devices · CourseWhere the current gain actually comes from
The base current is only the small fraction of carriers that recombine on the way across, so collector current is many times larger. Gain is a consequence of the geometry rather than a property that was designed in directly.
The four regions, and which junction is biased which way
Forward-reverse is active and gives amplification, forward-forward is saturation and behaves as a closed switch, reverse-reverse is cut-off, and reverse-forward is the reverse-active region nothing uses deliberately. Naming the two biases identifies the region every time.
NPTEL: Solid State Devices · CourseReading the output characteristics, and what the slope means
Collector current is almost independent of collector voltage in the active region, which is what makes the device a current source. The slight upward slope is base-width modulation, and it sets the largest gain a single stage can reach.
The small-signal idea: a curve replaced by its tangent
For signals small enough, a nonlinear device behaves like the straight line touching its curve at the operating point. That single move is what lets a transistor circuit be analysed with the linear methods from circuit analysis.
MIT OpenCourseWare 6.012: Microelectronic Devices and Circuits · CourseTransconductance and the hybrid-pi model
Transconductance is how much the output current moves per volt of input, and it depends on the operating current rather than on the device. The hybrid-pi model is that number plus an input resistance and an output resistance, and it is what an amplifier is designed from.
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