GATE Electrical Engineering - Complete Master Library

 

Analog Electronics – Problems Page 29

h-Parameter Analysis & Gain Calculations

This section focuses on transistor amplifier analysis using hybrid h-parameters such as hie, hfe, hre and hoe. These parameters are widely used in small-signal analysis of BJT amplifiers.

                                       

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Basic h-Parameter Equations

V1 = h11 I1 + h12 V2

I2 = h21 I1 + h22 V2

For common emitter configuration:

h11 = hie (input resistance)

h21 = hfe (current gain)

h12 = hre (reverse voltage gain)

h22 = hoe (output admittance)

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Voltage Gain Formula

The approximate voltage gain of a CE amplifier using h-parameters is:

Av ≈ − (hfe RL) / hie

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Problem 1

A transistor amplifier has hfe = 100, hie = 1 kΩ and load resistance RL = 4 kΩ. Calculate voltage gain.

Solution

Av = − (hfe × RL) / hie

Av = − (100 × 4000) / 1000

Av = −400

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Problem 2

If hie = 2 kΩ and input voltage Vin = 10 mV, find base current.

Solution

Ib = Vin / hie

Ib = 10mV / 2kΩ

Ib = 5 μA

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Problem 3

If hfe = 120 and base current Ib = 20 μA, find collector current.

Solution

Ic = hfe × Ib

Ic = 120 × 20 μA

Ic = 2.4 mA

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Problem 4

A CE amplifier has hie = 1.5 kΩ, hfe = 80 and RL = 3 kΩ. Calculate voltage gain.

Solution

Av = − (hfe × RL) / hie

Av = − (80 × 3000) / 1500

Av = −160

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Problem 5

If reverse voltage gain hre is very small, what is the effect?

Solution

• Reverse feedback becomes negligible
• Amplifier analysis becomes simpler
• Voltage gain mainly depends on hfe and hie