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E E 339 - CN17-BJTs - Transistor Transistor introduction...

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1 Transistor introduction and Bipolar Junction Transistors (BJTs) 1 Brief introduction to the transistors (in general) Bipolar Junction transistors (BJTs) Qualitative analysis of current amplification in BJTs Outline Figures of merit Quantitative analysis (Small signal) ac frequency limitations (Large signal) cutoff saturation and switching Non-ideal behaviors Heterojunction BJTs Summary 2
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2 “Transistors ” are three-terminal devices (in contrast to two- terminal diodes) … with current flow between two of the terminals controlled by application of a voltage field-effect transistors (FET) or Transistor introduction (see sections 6.1 and 6.2 of text) field effect transistors (FET) current bipolar junction transistors (BJT) signal to the third terminal . E.g., a (n-channel, short-channel MOS) FET (TBD) V control (“gate” voltage) I terminal V control = 4V terminal current depends on control gate not terminal V terminal (“drain” voltage) (“source”) transistor V terminal I terminal V control = 2V V control = 3V transistor characteristics voltage 3 This control by the third terminal can provide amplification of the analog control/input signals switching between “off” (e.g., 0) and “on”/saturation (e.g., 1) levels of the current through, or voltage drop across, the transistor (digital) Note that even in digital circuits, amplification or “gain” relative to some reference is a critical feature of transistors: It allows, e.g., a perhaps an nominally 1.5 Volt (0.0 Volt) signal representing a “one” (“zero) that has been degraded down to a perhaps 1.0 Volt (up to perhaps 0.5 V) signal at some point in the circuit for whatever reason, to still switch the output of the next transistor-based logic gate between 0 and 1.5 Volts, e.g., between “zero” and “one,” which is signal restoration. It allows the output of one transistor to drive and quickly switch the input of many others fan out . 4
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