Biasing is used in transition amplifiers to

1. Stabilize the operating point against temperature variations.

2. Place the operating point in the linear region of the characteristics.

3. Make α, β and I_{CO} of the transistor independent of temperature variations.

4. Reduce distortion and increase dynamic range.

Option 2 : 1, 2 and 4 only

The purpose of Biasing in BJT amplifier:

i) To Stabilize Q-point in its linear region of operation (Q-point is an operating point).

[Statement 2 correct]

ii) To stabilize Q-point in against variation of temperature because

I_{c} = βI_{B} + (1 + β) I_{co}

I_{co} → leakage current.

Where I_{co}, β_{1,} and I_{B} are temperature dependent.

i.e. Temp ↑ → I_{co} ↑ → β ↑ → α ↑

\(\left(\because {\beta = \frac{\alpha }{{1 - \alpha }}} \right)\)

**[Statement 1 correct], [Statement 3 wrong]**

iii) To reduce distortion and increase dynamic range.

Since biasing fixed the Q-point so that there is no variation in the output so distortion is reduced hence dynamic range i.e. operating range of the amplifier is improved.

**[Statement 4 correct]**

So, statement 1, 2, 4 are correct.

iv) Biasing must provide the operating point independent of β of the transistor. So that variation in β due to temperature change does not affect the Q-point of an amplifier.

__More information:__

Types of biasing used in amplifier:

**1) Fixed bias (Base bias)**

Stability factor (S) = (β + 1)

Ideally, the stability factor should be zero.

S_{idereal} = 0 (∵ s→ large so I_{c} will be unstable).

**2) Collector to base bias:**

\(S = \frac{{\beta + 1}}{{1 + \beta \left( {\frac{{{R_C}}}{{{R_C} + {R_B}}}} \right)}}\)

→ It is more stable but

S depend on R_{C}, So,

If R_{C} ↓ → I_{C} ↑ → S↑ ⇒ So I_{c} does not remain constant.

**3) Self Bias (voltage divider circuit)**

→ The least stability factor and most widely used

s = 1 Least so most stable form.

Biasing is used in transition amplifiers to

1. Stabilize the operating point against temperature variations.

2. Place the operating point in the linear region of the characteristics.

3. Make α, β and I_{CO} of the transistor independent of temperature variations.

4. Reduce distortion and increase dynamic range.

Option 2 : 1, 2 and 4 only

The purpose of Biasing in BJT amplifier:

i) To Stabilize Q-point in its linear region of operation (Q-point is an operating point).

[Statement 2 correct]

ii) To stabilize Q-point in against variation of temperature because

I_{c} = βI_{B} + (1 + β) I_{co}

I_{co} → leakage current.

Where I_{co}, β_{1,} and I_{B} are temperature dependent.

i.e. Temp ↑ → I_{co} ↑ → β ↑ → α ↑

\(\left(\because {\beta = \frac{\alpha }{{1 - \alpha }}} \right)\)

**[Statement 1 correct], [Statement 3 wrong]**

iii) To reduce distortion and increase dynamic range.

Since biasing fixed the Q-point so that there is no variation in the output so distortion is reduced hence dynamic range i.e. operating range of the amplifier is improved.

**[Statement 4 correct]**

So, statement 1, 2, 4 are correct.

iv) Biasing must provide the operating point independent of β of the transistor. So that variation in β due to temperature change does not affect the Q-point of an amplifier.

__More information:__

Types of biasing used in amplifier:

**1) Fixed bias (Base bias)**

Stability factor (S) = (β + 1)

Ideally, the stability factor should be zero.

S_{idereal} = 0 (∵ s→ large so I_{c} will be unstable).

**2) Collector to base bias:**

\(S = \frac{{\beta + 1}}{{1 + \beta \left( {\frac{{{R_C}}}{{{R_C} + {R_B}}}} \right)}}\)

→ It is more stable but

S depend on R_{C}, So,

If R_{C} ↓ → I_{C} ↑ → S↑ ⇒ So I_{c} does not remain constant.

**3) Self Bias (voltage divider circuit)**

→ The least stability factor and most widely used

s = 1 Least so most stable form.

For the fixed bias circuit shown, assuming V_{BE} = 0.7 V and β = 60, the quiescent values of base current is 42.27 μA. The Quiescent values of base current and collector current respectively

when β = 110 are :

Option 1 : 42.27 μA, 4.65 mA

I_{BQ} is not affected by the change in β

I_{BQ} (old) = 42.27 μA

I_{CQ} (old) = 60 × 42.27 = 2.5 mA

∴ I_{BQ} (new) = 42.27 μA