GATE Electrical Engineering - Complete Master Library

 

Common Emitter Amplifier Gain using h-Parameters

The h-parameter model is widely used to analyze transistor amplifiers. For a Common Emitter (CE) amplifier, four parameters are used:

                                        

• hie → input resistance
• hre → reverse voltage gain
• hfe → forward current gain
• hoe → output admittance

---

Step 1 : Input Voltage Equation

From the h-parameter model:

V1 = hie I1 + hre V2

---

Step 2 : Output Current Equation

Output current is given by:

I2 = hfe I1 + hoe V2

---

Step 3 : Output Voltage

The output voltage across load resistor RL:

V2 = − I2 RL

---

Step 4 : Substitute I2

Substituting the current equation:

V2 = − RL ( hfe I1 + hoe V2 )

---

Step 5 : Solve for Voltage Gain

After simplification:

Av = V2 / V1

For practical CE amplifier where hre and hoe are very small:

Av ≈ − hfe RL / hie

---

Important Result

Voltage Gain (CE amplifier)

Av ≈ − (hfe × RL) / hie

---

Important GATE Points

• CE amplifier gain is negative (phase inversion).
• Higher hfe increases gain.
• Larger load resistance increases gain.
• Small hre and hoe usually neglected.