Consider the basic amplifier circuit. To form a transistor amplifier only is is necessary to connect an external load and signal source along with proper biasing.

**Midband analysis of BJT Single
Stage Amplifiers:**

Consider
the basic amplifier circuit. To form a transistor amplifier only is is
necessary to connect an external load and signal source along with proper
biasing.

We can
replace the transistor circuit as shown in the following figure.

Let us
analyze the hybrid model to find current gain, input resistance, voltage gain
and output resistance.

**Current gain (A _{i}):**

It is
defined as the ratio of output to input current. It is given by,

Here I_{L}
and I_{2} are equal in magnitude but opposite in sign. I_{L} =
-I_{2}

From
above circuit

**Current gain (A****IS)****: **

It is given by,

From above figure, using current divider rule,

**Input Impedance (Z _{i}):**

R_{i}
is the input resistance looking into the amplifier input terminals ( 1, 1’). It
is given by,

From the
input circuit,

Substituting
V_{2} = -I_{2}R_{L} = A_{i} I_{1} R_{L}
in the above equation,

Dividing
numerator and denominator by R_{L} we get,

From this
equation, note that the input impedance is a function offload impedance.

**Voltage gain (A _{v}):**

It is the
ratio of output voltage to input voltage. It is given by,

By
substituting V_{2} = -I_{2}R_{L} = A_{i} I_{1}
R_{L}

**Voltage gain (A _{vs}):**

It is
voltage gain including the source. It is given by,

From
above figure, applying potential divider rule, then we get,

Substituting
the value of V_{1}/V_{s} in the equation of

**Output Admittance (Y _{o}):**

It is the
ratio of output current to output voltage. It is given by,

Dividing
above equation by V_{2}, We get,

From
transistor amplifier in h-parameter model circuit, with V_{s} = 0,

R_{s}I_{1}
+ h_{i}I_{1} +h_{r}V_{2} = 0

(R_{s} + h_{i})
I _{1} = -h_{r} V_{2}

Substituting
the value of I_{1}/V_{2} from above equation in the equation of
Y_{o}. We obtain,

From this
equation, note that the output admittance is a function of source resistance.

**Power gain (Ap):**

It is the
ratio of average power delivered to the load to the input power. Output power
is given as,

Since the
input power is P_{1} = V_{1}I_{1}

The
operating power gain A_{p} of the transistor is given as,

**Relation between A _{vs} and A_{IS}**

From
equation,

Taking
ratio of above two equations we get,

**Method for analysis of a transistor circuit:**

The
analysis of transistor circuits for small signal behaviour can be made by
following simple guidelines. These guidelines are,

1. Draw the
actual circuit diagram

2. Replace
coupling capacitors and emitter bypass capacitor by short circuit

3. Replace
D.C. source by a short circuit

4. Mark the
points B, E, C on the circuit diagram and locate these points as the start of
the equivalent circuit

5. Replace
the transistor by its h-parameter model

**Problem 1:**

For the
common base circuit shown in figure, transistor parameters are h_{ib} =
22Ω,

h_{fb}
= -0.98, h_{ob} = 0.49µA/V, h_{rb} = 2.9*10 ^{-4}.
Calculate the values of input resistance, output resistance, current gain and
voltage gain for the given circuit.

**Solution:**

Change
the given figure into h-parameter equivalent model.

R_{o}'
= R_{o} || R_{L}’ = 1.21M || 5.45K = 5.425KΩ

**Problem 2: **

Consider
a single stage CE amplifier with Rs = 1KΩ, RL = 1.2KΩ. Calculate A_{i},
R_{i}, A_{v}, A_{is},
power gain and R_{o} if hie = 1.1k, h_{re} = 2.5*10^{-4},
h_{fe} = 50 and h_{oe} = 25µA/V.

**Solution:**

**Problem 3: **

Consider
a single stage CE amplifier with R_{s} = 1k, R_{1} = 50k, R_{2}
= 2k, R_{c} = 2k, R_{L} =
2K, h_{ie} = 1.1k, h_{oe} = 25µA/V, h_{fe} = 50
and hre = 2.5*10^{-4} as shown in the figure.

Find A_{i},
R_{i}, A_{v}, A_{i}, A_{vs} and R_{o}.

**Solution:
**

Since h_{oe}
R_{L}' = 25*10-6*(2K || 2K) = 0.25, which is less than 0.1, so use
approximate analysis.

Consider
the simplified hybrid model for the given circuit.

**Comparison of Transistor Configurations: **

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Electronic Circuits : BJT Amplifiers : Midband analysis of BJT Single Stage Amplifiers |

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