bias stability lab2a
Figure 1. Bias circuit schematics

1. Lab goals

  • 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 relatively not sensitive 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


Discussion of several biasing schemes and the equations resulting from circuit analysis.

3. Transistor parameters

Obtain 10 NPN transistors of the same model (2N3904). Use the LCR-T4 semiconductor + LCR tester to measure each transistor’s forward βF and the VBE at the measurement conditions. Use small pliers to handle the transistors to avoid heating them with your fingers in order to make the measurements more consistent. Remember that, for example, VBE changes by -2 mV / °C.

Use the tester’s short manual LCR-T4_semiconductor-tester_short.pdf on Google Drive to help compute the base current for each transistor under the measurement conditions. Note: the voltage between the + and - nodes is set to 5.0 V by a voltage regulator and the two low-value resistors are 680 Ω instead of the 700 Ω in Figure 1.1 (a) and (b).

Use 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).

[ ] Record the measurements from the transistor tester
  • Transistor ID number

  • Reported βF

  • Reported VBE

  • Calculated IB

4. Measurements