Assertion (A) : A self-biased circuit has a better stability than a fixed bias circuit. Reason (R) : It provides negative feed back by the use of an additional resistor between the base and ground.
(A) is true, but (R) is false.
The assertion is true, but the reason misplaces the resistor that does the work — option 3.
The assertion is standard. Fixed bias sets the base current with a single resistor from the supply, so \(I_{B}\) is fixed and
\(I_{C}=\beta I_{B}\)
follows \(\beta\) directly. Since \(\beta\) varies by three to one between samples of the same part number and rises steeply with temperature, the operating point wanders badly. Its stability factor is \(S=1+\beta\) — as bad as it can be. Self-bias holds the operating point far more firmly.
Where the reason goes wrong. The negative feedback in a self-biased (voltage-divider) stage comes from the emitter resistor, not from a resistor between base and ground. The mechanism is a loop:
\(I_{C}\uparrow\ \Rightarrow\ V_{E}=I_{E}R_{E}\uparrow\ \Rightarrow\ V_{BE}=V_{B}-V_{E}\downarrow\ \Rightarrow\ I_{B}\downarrow\ \Rightarrow\ I_{C}\downarrow\)
The divider merely holds \(V_{B}\) steady; it is \(R_{E}\) that senses the collector current and feeds the correction back. Remove \(R_{E}\) and the divider alone stabilises nothing.
| Resistor | Role |
|---|---|
| R1, R2 divider | Fixes the base voltage — a reference, not feedback |
| RE (emitter to ground) | Provides the negative feedback |
The resulting stability factor shows how much is gained:
\(S=\dfrac{1+\beta}{1+\beta\dfrac{R_{E}}{R_{E}+R_{TH}}}\)
which for \(R_{TH}\ll\beta R_{E}\) approaches \(1+R_{TH}/R_{E}\) — a small number independent of \(\beta\), against \(1+\beta\) for fixed bias.
A charitable reading, and why it still fails. "Between base and ground" could be meant as R2 of the divider — but R2 provides no feedback whatever; it is part of a fixed reference. Alternatively the phrase might be a garbled reference to collector-to-base bias, where a resistor from collector to base does provide genuine feedback. Either way, as written R is false, and the answer is flagged only because of that ambiguity.
The cost of the emitter resistor is signal gain, since the same feedback that stabilises the DC point also reduces AC gain — which is why RE is usually bypassed by a capacitor, keeping the DC feedback and removing the AC feedback.
Hence, (A) is true, but (R) is false.
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