How to design discrete circuits for transistor beta changes?

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Discussion Overview

The discussion revolves around the challenges of designing discrete circuits that utilize transistors, particularly focusing on how to manage variations in the beta parameter of transistors such as BJTs, JFETs, and MOSFETs. Participants explore methods to compensate for these fluctuations in circuit design, including the use of feedback and specific resistor configurations.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant inquires about how to compensate for variations in the beta parameter when designing circuits with discrete transistors, questioning whether each transistor requires unique resistor and capacitor values.
  • Another participant suggests that negative feedback can reduce sensitivity to changes in beta, improving overall circuit performance.
  • A different contribution mentions using an emitter resistor to control gain in common emitter stages, indicating that the gain is approximately determined by the ratio of collector resistor to emitter resistor.
  • One participant notes that minimizing the effect of changing beta is a fundamental topic in BJT amplifier design, implying that strategies exist to address this issue.
  • Another participant recommends "The Art of Electronics" as a resource that covers practical approaches to transistor biasing and sensitivity reduction, highlighting its accessibility and real-world examples.

Areas of Agreement / Disagreement

Participants express various strategies for managing beta variations, but there is no consensus on a single best approach. Multiple competing views and techniques are presented without resolution.

Contextual Notes

The discussion does not resolve the specific methods for compensating beta variations, and assumptions about circuit design requirements and transistor characteristics remain unaddressed.

Who May Find This Useful

Individuals interested in circuit design, particularly those working with discrete transistors in amplifiers, may find the insights and resources shared in this discussion beneficial.

dpsciarrino
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In my circuits/active devices course we had to make amplifier designs with various type of transistors (BJT, JFET, MOSFET). In each design the transistor beta parameter played an important role in deciding which resistors to use in the final amplifier circuit. This beta parameter changes widely from one transistor to another...

My question is, if I wanted to manufacture a circuit that has discrete transistor components, how are these fluctuations in the beta parameter of all these transistors compensated?

For example: If two 2N3904s are used and their betas were, say, 100 and 130, would the resistors and caps in the design have to cater to each individual transistor, therefore CHANGING the design values of the circuit for each individual product? Or are there some useful circuits that get around this pitfall? Or are there other techniques that I don't see?
 
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Pretty much the first topic covered in BJT amplifiers in school was designing in such a way as to minimize the effect of changing beta.
 
Check out "The Art of Electronics" by Horowitz and Hill. They've just issued a new edition. It's full of helpful and practical approaches to problems like this. It starts right at the beginner level but works right through things like transistor & FET biasing and reducing beta and gm sensitivity. They cover a multitude of topics and have all kinds of real-world examples, part numbers, and practical tips. The tone is informal and entertaining too.
 
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