It has three RC networks that affect its gain as the frequency is reduces below midrange.

**Low frequency analysis of BJT:**

From
above figure, it has three RC networks that affect its gain as the frequency is
reduces below midrange. These are,

·
RC network formed by the input coupling capacitor C_{1}
and input impedance of the amplifier.

·
RC network formed by the output coupling capacitor
C_{2}, resistance looking in at the collector and load resistance.

·
RC network formed by the emitter bypass capacitor C_{E}
and resistance looking in at the emitter.

**Input RC network**:

The
following figure shows the input RC network formed by C_{1} and the
input impedance of the amplifier.

The
resistance value is R_{in} = R_{1} || R_{2} || R_{in}(base)

A
critical point in the amplifier response is generally accepted to occur when
the output voltage is 70.7 % of the input. At critical point,

At this
condition, R_{in} = X_{c1}

Overall
gain is reduced due to attenuation provided by the input RC network. The
reduction in overall gain is given by,

The
frequency f_{c} at this condition is called lower critical frequency
and it is given by,

If the
resistance of input source is taken into account the above equation becomes,

The phase
angle in an input RC circuit is expressed as

**Output RC network:**

The above
figure shows the output RC network formed by C_{2}, resistance looking
in at the collector and load resistance.

The
critical frequency for this RC network is given by,

The phase
angle in output RC network is given as,

**Bypass network:**

From
above figure,

is the
resistance looking in at the emitter. It is derived as follows, R= (V_{b}
/ βI_{b}) + h_{ie} / β

Where R_{TH}
= R_{1} || R_{2} || R_{s}. It is the thevenin’s
equivalent resistance looking from the base of the transistor towards the
input.

The
critical frequency for the bypass network is

**Problem:**

Determine
the low frequency response of the amplifier circuit shown in the figure.

**Solution:**

It is
necessary to analyze each network to determine the critical frequency of the
amplifier.

The above
analysis shows that the input network produces the dominant lower critical frequency.
Then the low frequency response of the given amplifier is shown in the
following figure.

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

Electronic Circuits : Frequency Analysis of BJT and MOSFET Amplifiers : Low frequency analysis of BJT |

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