For the BJT to be biased in its linear or active operating region : (a) the base-emitter Junction must be forward biased
(b) the base-collector Junction must be reverse biased
(c) the base-emitter Junction must be reverse biased
(d) the base-collector Junction must be forward biased
(a) and (b) are correct
A BJT has two junctions, so four bias combinations exist, and each names one region of operation.
| Emitter junction | Collector junction | Region | Use |
|---|---|---|---|
| Reverse | Reverse | Cut-off | Switch OFF |
| Forward | Reverse | Active | Amplification |
| Forward | Forward | Saturation | Switch ON |
| Reverse | Forward | Reverse active | Rarely used, very low gain |
The active region is therefore (a) and (b) — option 3.
Why each condition is needed, physically. Forward-biasing the emitter junction lowers its barrier so the heavily doped emitter can inject carriers into the base — without this there is no current to control. Reverse-biasing the collector junction creates a strong field that sweeps those carriers out of the base and across to the collector as soon as they arrive, and it keeps the collector from injecting anything back. The base is deliberately made thin and lightly doped so that almost all the injected carriers survive the crossing rather than recombining, which is what makes
\(\alpha=\dfrac{I_{C}}{I_{E}}\approx0.98\ \text{to}\ 0.995\qquad \beta=\dfrac{\alpha}{1-\alpha}\)
so large.
What goes wrong in the other regions. In saturation the collector junction is also forward biased, so the collector begins injecting carriers back into the base; the collector current stops obeying \(I_{C}=\beta I_{B}\) and is set by the external circuit instead, with VCE collapsing to about 0.2 V. In cut-off both junctions are reverse biased and only leakage flows. Both are perfectly useful as the two states of a switch, but neither is linear.
The practical consequence for a designer is that the quiescent point must sit comfortably between the two, typically with VCE near half the supply, so that the signal can swing in both directions without clipping against saturation at one end or cut-off at the other.
Hence, the correct conditions are (a) and (b).
Similar Questions
Match the following :
List – I (Biasing of BJT) List – II (Functions) a. E-B junction forward bias and C-B junction reverse bias i. Very low gain amplifier b. Both E-B and C-B junctions forward bias ii. Saturation condition c. E-B junction reverse bias and C-B junction forward bias iii. High gain amplifier d. Both E-B and C-B junctions reverse bias iv. Cut-off condition Codes :
The doping concentration of n-p-n transistor are
i. 5 × 1018 / cm3
ii. 1017 / cm3
iii. 2 × 107 / cm3Identify the regions in the above order i, ii and iii.
Assertion (A) : Two P-N diodes connected back to back cannot be used as a transistor.
Reason (R) : Fabrication of a transistor requires controlled doping of Emitter, base and collector regions.
Select your answer using the codes given below :
Assertion (A) : Considering two p-n-p and n-p-n transistors of identical construction as far as shape, size and doping are concerned, the n-p-n transistor will have a better frequency response.
Reason (R) : The electron mobility is higher than that of the hole mobility.
Select your answer using the codes given below :
Important Questions from Bipolar Junction Transistor
Photo transistor is used for:
What happens if a voltage of about 0.7 V is applied across the base and emitter of the NPN transistor?
In a junction transistor, recombination of electrons and holes occurs in
A bipolar junction common emitter transistor is operating in saturation mode, identify the correct statement.
In BJT when both the junctions are forward biased, then its operating mode is called:
