bias stability lab2a
Figure 1. Bias circuit schematics

1. Lab goals

Topics

  • Transistor (DC) bias conditions and circuit sensitivity to transistor parameter variations.

  • Rules-of-thumb for designing a bias circuit to be insensitive to transistor variations.

Your task is to study the effect that transistor parameters have obtaining a robust circuit design. A robust circuit design is insensitive to the specific value of transistor parameters, especially the ones which vary widely (like β, IS, and variations with temperature).

This study includes circuit analysis and examining the mathematical properties of the resulting equations, and building example circuits and measuring how the resulting bias conditions change when replacing the transistor.

2. References

https://www.allaboutcircuits.com/textbook/semiconductors/chpt-4/biasing-calculations/

Discussion of several biasing schemes and their circuit equations.

3. Transistor parameters

  • Obtain 10 NPN transistors of the same model (2N3904).

bjt beta test
Figure 2. BJT beta test jig

Use Figure 2 as a simple test jig for measuring transistor Beta. Select base and collector resistors so that VC is around 7 V with a collector current of 300 μA if you guess that the transistor’s Beta value is 150. Use VCC=10 V.

Construct this circuit and use it to infer the beta of your batch of transistors. this Google Sheet to record your data. Each group should use a different 100’s position digit for the transistor IDs (the N in the integer Nxx).

Ensure that each spreadsheet row is for measurements of the same physical transistor. → keep them in order!

4. Measurements

  • Build the circuit of Figure 1 (a), measure VB and VC, and compute IC for each of your 10 transistors.

  • Build the circuit of Figure 1 (b), measure VB and VE and VC, and compute IC for each of your 10 transistors.

References