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ECE 340 Lecture 41 Other Modes of BJT Operation; Common-Emitter Gain

ECE 340 Lecture 41 Other Modes of BJT Operation; Common-Emitter Gain. So far we’ve studied BJT only in “ normal active ” mode (for PNP, V EB >0 forward biased, V CB < 0 reverse biased). Here, well-designed devices have high gain ( β = 50-300); most useful.

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ECE 340 Lecture 41 Other Modes of BJT Operation; Common-Emitter Gain

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  1. ECE 340 Lecture 41Other Modes of BJT Operation; Common-Emitter Gain • So far we’ve studied BJT only in “normal active” mode (for PNP, VEB >0 forward biased, VCB < 0 reverse biased). Here, well-designed devices have high gain (β = 50-300); most useful. • What about operation in other bias conditions? • “Inverted” mode: CB forward-biased, EB reverse-biased. • “Cut-off” mode: both junctions reverse-biased. Negligible currents (~1 pA), nearly an open circuit  useful.

  2. “Saturation” mode: Both EB and CB junctions are forward-biased. Base is flooded with minority carriers, i.e. _________ for a PNP. • No isolation, slow recovery, in practice  avoid. • But, interesting to study because it is a superposition of “forward” and “inverted” modes of operation • We can still write down the minority carrier concentration in the Base, at the E and C side: ΔpE = ΔpC = Where pn = __________.

  3. So I can write the hole diffusion current in the Base: • And the Base current due to recombination: • Key observation: minority carrier diffusion equation is linear, so all BJT operation modes can be written as a superposition of the “normal active” and “inverted” modes. (note that IC and IE can have either sign depending on voltages.)

  4. Ex: Common-Emitter configuration. Draw circuit: • Draw current-voltage diagram: • At low VEC both CB and EB diodes are _________________. Consequently the transistor is in ________________. • At higher VEC, CB diode goes into deeper and deeper reverse bias, and EB stays forward-biased with roughly constant VEB ~ _________.

  5. As the CB diode is pushed into deeper reverse bias, the CB depletion region _____________. • This effectively modulates (decreases) the ___________. • Which also modulates (increases) the _____________. Experimental Data

  6. Ideally, we do not want the gain to change due to Base width modulation. During design, we have three options: 1) Increase the Base width, WB 2) Increase the Base doping, NB 3) Decrease the Collector doping, NC • Which one is the most acceptable in practice? • What if we really push VEC until the transistor breaks? • Punch-through (WB 0) • Avalanche breakdown

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