Question
Download Solution PDFThe collector to base bias circuit in BJT has better stability than fixed bias circuit as __________.
Answer (Detailed Solution Below)
Detailed Solution
Download Solution PDFThe collector to bias circuit in BJT has better stability than a fixed bias circuit, as its stability factor is lower than that of fixed bias by a factor of (β + RB / RE).
For fixed bias: \(S(V_{BE}) = \frac{\partial I_C}{\partial V_{BE}} = -\frac{β}{R_E}\)
For the collector to base bias: \(S(V_{BE}) = \frac{\partial I_C}{\partial V_{BE}} = -\frac{β}{R_B+β R_E}= -\frac{β/R_E}{β + R_B/R_E}\)
Stability factor for different configurations
| Biasing Configuration | Biasing Circuit | Stability Factor w.r.t VBE |
| Fixed Bias |
\(I_C = β I_B = β \left(\frac{V_{CC}-V_{BE}}{R_B}\right)\) \(S(V_{BE}) = \frac{\partial I_C}{\partial V_{BE}} = -\frac{β}{R_E}\) |
|
| Collector to base Bias |
Assuming IE = β IB,
\(V_{CC} - I_BR_B-V_{BE} - β I_BR_E=0\) \(I_C = β I_B = β\left(\frac{V_{CC}-V_{BE}}{R_B+β R_E}\right) \) \(S(V_{BE}) = \frac{\partial I_C}{\partial V_{BE}} = -\frac{β}{R_B+β R_E}= -\frac{β/R_E}{β + R_B/R_E}\) |
|
| Voltage Divider |
Assuming IE = β IB, \(V_{TH}-I_BR_{TH}-V_{BE}-β I_BR_E=0\) \(I_C = β I_B = β\left(\frac{V_{TH}-V_{BE}}{R_{TH}+β R_E}\right)\) \(S(V_{BE}) = \frac{\partial I_C}{\partial V_{BE}} = -\frac{β}{R_{TH}+β R_E}= -\frac{β/R_E}{β + R_{TH}/R_E}\) |
|
| Feedback Bias |
\(V_{CC}-β I_BR_C-I_BR_B-V_{BE}=0\) \(I_C = β I_B = β\left(\frac{V_{TH}-V_{BE}}{R_{B}+β R_C}\right)\) \(S(V_{BE}) = \frac{\partial I_C}{\partial V_{BE}} = -\frac{β}{R_B+β R_C}= -\frac{β/R_C}{β + R_B/R_C}\)
|
Last updated on Jun 7, 2025
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