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Learn about the purpose and design of bias circuits for BJT transistors, including the drawbacks of bad bias circuits and the advantages of better bias circuit designs. Explore classic 4-resistor bias circuits and diode bias circuits, and understand the importance of negative feedback in creating stable and reliable transistor biasing.
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ECE 255:BJT Bias Circuits(Sedra and Smith, 7th Ed., Sec. 6.3)Mark LundstromSchool of ECEPurdue UniversityWest Lafayette, IN USA ECE 255: Fall 2019 Purdue University Lundstrom: Fall 2019
DC bias circuits Purpose of bias circuits Bad bias circuit Better bias circuit Classic 4-resistor bias circuit Lundstrom: Fall 2019
Diode bias circuits Definition: Bias: A DC voltage applied to a device (such as a transistor control electrode) to establish a reference level for operation.
Diode bias circuits Q Lundstrom: Fall 2019
Transistor bias circuits Q Lundstrom: Fall 2019
DC bias circuits Purpose of bias circuits Bad bias circuit Better bias circuit Bias circuit design Classic 4-resistor bias circuit Lundstrom: Fall 2019
Transistor bias circuit design Design for Lundstrom: Fall 2019
Bias circuit design Lundstrom: Fall 2019
Bias circuit design large! Lundstrom: Fall 2019
741 Op amp David Fullagar IEEE Spectrum, “Chip Hall of Fame: Fairchild Semiconductor μA741 Op-Amp,” June 30, 2017
Why is this a bad bias circuit? Lundstrom: Fall 2019
Why is this a bad bias circuit? Lundstrom: Fall 2019
Why is this a bad bias circuit? What if beta doubles? Saturation! Lundstrom: Fall 2019
DC bias circuits Purpose of bias circuits Bad bias circuit Better bias circuit Bias circuit design Classic 4-resistor bias circuit Lundstrom: Fall 2019
A better bias circuit Apply KVL and find an expression for IC. Lundstrom: Fall 2019
A better bias circuit Independent of beta! Lundstrom: Fall 2019
DC bias circuits Purpose of bias circuits Bad bias circuit Better bias circuit Bias circuit design Classic 4-resistor bias circuit Lundstrom: Fall 2019
Design Choose RB and RC to get the specified IC and VCE. Lundstrom: Fall 2019
Design (Results) Lundstrom: Fall 2019
Analysis for beta = 200 (close)
Analysis for beta = 200 Lundstrom: Fall 2019
Why does it work? Negative feedback Lundstrom: Fall 2019
DC bias circuits Purpose of bias circuits Bad bias circuit Better bias circuit Bias circuit design Classic 4-resistor bias circuit Lundstrom: Fall 2019
Classic 4-resistor Bias circuit design Design for Lundstrom: Fall 2019
Classic Bias circuit design Start at the output: Have 7 V to split between 2 resistors. Let:
Classic Bias circuit design Now move to the input:
Classic Bias circuit design Choose:
Classic bias circuit analysis Now that everything is specified, we can analyze the circuit.
Classic Bias circuit analysis Lundstrom: Fall 2019
Classicbias circuit design Lundstrom: Fall 2019
Classic Bias circuit design What if beta doubles? Why is the circuit so good?
From the equations Lundstrom: Fall 2019
Summary Good transistor bias circuits make use of negative feedback to deal with variations in beta. For IC design, we don’t use the 4-resistor bias circuit because high value resistors are “expensive”. Instead, we make use of transistors, because they are cheap. Lundstrom: Fall 2019
BJT bias circuits Purpose of bias circuits Bad bias circuit Better bias circuit Bias circuit design Classic 4-resistor bias circuit Lundstrom: Fall 2019
