Question

In Emitter current bias, if \[ R_1||R_2 \] is very much larger than \[ R_E, \] then the circuit performance becomes like

• A. Remains in the same bias circuit
• B. Fixed current bias circuit
• C. Collector to base bias circuit
• D. Voltage divider bias circuit

Hint:

For maximum bias stability: \[ R_1||R_2 \ll (\beta+1)R_E. \] If \(R_1||R_2\) becomes very large, the circuit gradually behaves like a fixed-bias transistor circuit.

Answer 1

The Correct Option is B

Concept: Emitter-bias circuits are often analyzed using the Thevenin equivalent of the voltage divider network formed by \(R_1\) and \(R_2\). The stability of the bias depends on the relative magnitudes of \[ R_B=(R_1||R_2) \] and \[ R_E. \]

Step 1: Recall the voltage-divider bias condition.
For good stabilization, \[ R_1||R_2 \ll (\beta+1)R_E. \] Under this condition the base voltage remains nearly constant and the circuit behaves as a voltage-divider bias circuit.

Step 2: Analyze the given condition.
The question states \[ R_1||R_2 \gg R_E. \] This means the divider network becomes very weak and provides negligible stabilization. The emitter resistor can no longer effectively control the operating point.

Step 3: Determine the equivalent behavior.
Under such circumstances the circuit loses the advantages of voltage-divider bias and behaves similarly to a fixed-bias arrangement. Hence the performance approaches that of a fixed current (fixed bias) circuit. \[ \boxed{\text{Fixed current bias circuit}} \]