The Figure shows the fixed bias circuit. It is the simplest d.c. bias configuration.

**Fixed Bias (Base Resistor Bias)**

The
Figure shows the fixed bias circuit. It is the simplest d.c. bias
configuration. For the d.c. analysis we can replace capacitor with an open
circuit because the reactance of a capacitor for d.c. is

**In the base circuit**,

Apply
KVL, we get

V_{CC}
= I_{B}R_{B} + V_{BE}

Therefore,

I_{B}
= (V_{CC} - V_{BE})/R_{B}

For a
given transistor, V_{BE} does not vary significantly during use. As V_{CC}
is of fixed value, on selection of R_{B}, the base current I_{B}
is fixed. Therefore this type is called *fixed
bias* type of circuit.

**In the Collector circuit**

Apply
KVL, we get

V_{CC}
= I_{C}R_{C} + V_{CE}

Therefore,

V_{CE}
= V_{CC} - I_{C}R_{C}

The
common-emitter current gain of a transistor is an important parameter in
circuit design, and is specified on the data sheet for a particular transistor.
It is denoted as β.

I_{C}
= βI_{B}

In this
circuit V_{E} =0

Stability
factor S for Fixed bias circuit

**Stability factor S**

**Merits:**

·
It is simple to shift the operating point anywhere
in the active region by merely changing the base resistor (R_{B}).

·
A very small number of components are required.

**Demerits:**

·
The collector current does not remain constant with
variation in temperature or power supply voltage. Therefore the operating point
is unstable.

·
Changes in V_{be} will change I_{B}
and thus cause R_{E} to change. This in turn will alter the gain of the
stage.

·
When the transistor is replaced with another one,
considerable change in the value of β can be expected. Due to this change the
operating point will shift.

·
For small-signal transistors (e.g., not power
transistors) with relatively high values of β (i.e., between 100 and 200), this
configuration will be prone to thermal runaway. In particular, the stability
factor, which is a measure of the change in collector current with changes in
reverse saturation current, is approximately β+1. To ensure absolute stability
of the amplifier, a stability factor of less than 25 is preferred, and so
small-signal transistors have large stability factors.

**Usage:**

Due to
the above inherent drawbacks, fixed bias is rarely used in linear circuits
(i.e., those circuits which use the transistor as a current source). Instead,
it is often used in circuits where transistor is used as a switch. However, one
application of fixed bias is to achieve crude automatic gain control in the
transistor by feeding the base resistor from a DC signal derived from the AC
output of a later stage.

**Problems**

1. Design
the fixed bias circuit from the load line given in the figure.

2. For
the circuit shown in figure. Calculate I_{B},I_{C},V_{CE},V_{B},V_{C}
and V_{BC}. Assume V_{BE}= 0.7V and β=50.

3. Design a fixed biased circuit using a
silicon transistor having β value of 100. Vcc is 10 V and dc bias conditions
are to be V_{CE} = 5 V and I_{C} = 5 mA,

**Solution**

Applying
KVL to collector circuit,

Applying
KVL to base circuit,

4. Calculate
the operating point (Q-point)

Base
biased CE connection

I_{C}
= β_{dc} * I_{B} = 100 * 29µA = 2.9 mA

V_{CE}
= V_{CC} - (I_{C} * R_{C}) = 15V - (2.9 mA * 3KΩ) =
6.3V

By
plotting I_{C} (2.9 mA) and V_{CE} (6.3V), we get the operation
point ----> Q-point (quiescent point).

Collector
curve with load line and Q – point

5. Draw
the load line and Q-point.

base
biased CE connection, β=50

__Solution:__

I_{C}
= I_{B} * β = 2.15 mA

V_{CE}
= V_{CC} - (R_{C} * I_{C})= 5.7V

` V_{CE
(cut)} = V_{CC} = 3.0V

Study Material, Lecturing Notes, Assignment, Reference, Wiki description explanation, brief detail

Electronic Circuits : Biasing of Discrete BJT and MOSFET : Fixed Bias (Base Resistor Bias) |

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