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 voltag field-effect transistors (FET or Transistor introduction (see sections 6.1 and 6.2 of text) voltage field effect transistors (FET) or 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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3 Where the transistor circuit operates depends on the “ transistor characteristics and the external circuit : The “ transistor characteristics ” are the I-V relations for the transistor between two of the terminal as a function of control signal to the third. The “ load line ” is the I-V terminal relations allowed/imposed by the external circuit or “load.” The intersectionof the transistor characteristics controlled by the gate voltage here and of the load line is where the combined system/circuit will operate. 5 load line is V I D D K 1 V 15 transistor characteristics load V Example : Amplification … Figure 6—2, modified (a) biasing circuit; (b) I–V characteristic and load line. If v G = 0.5 V, the d-c values of I D and V D are as shown by the dashed lines.
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E E 339 - CN17-BJTs - Transistor Transistor introduction...

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