# Overall Voltage Gain 3 Stage Class B/AB Driver

#### NSCC_EETN

Joined Oct 6, 2010
17
Having an issue with a small homework assignment. The question asks what the overall voltage gain is for the three stage amplifier in the attached picture below.

We were told to assume Hfe = 200

Here's the calculations and formulas I have thus far:

Zin Base (1) = Rswamp(R4) x Hfe = 1k ohms x 200 = 200k ohms
Zin Base (2) = Rswamp(R8) x Hfe = 100 ohms x 200 = 20k ohms
Zin Base (3) = Rload x Hfe = 100 ohms x 200 = 20k ohms

Av (1) = R1//R2//Zin Base (1) = 10k//5.6k//200k = 3.53k ohms
Av (2) = R5//R6//Zin Base (2) = 12k//1k//20k = 882 ohms
Av (3) = Have not figured that out, kind of assuming R7//R8//Zin Base (3)

Now the back of the book gives me a gain of 1679, but various other calculations I've messed around with do not give me that answer.

Can anyone help clear this up for me?

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#### Jony130

Joined Feb 17, 2009
5,292
Voltage gain of a CE amplifier with CE capacitor is equal

AV =( RC||RL ) / re = 40 * Ic * Rc||RL

RL = R5||R6|| (β+1 * R8) ≈ 880Ω

Ic = 10V/1K = 10mA

Av1 ≈ 40*10mA * 1K||880 = 40mA * 468 = 187 V/V

The voltage gain of a Q2 stage is equal:

Av2 = (Rc||RL) / (Re + re) ≈ Rc/Re = R7/ R8 = 10 V/V (9.35V)

And the gain of last stage is equal:

Av3 = RL/( re + RL ) = R9/ ( re + R9 )

re = 26mV/Ic - but we don't know Ic of a last stage, so we can assume Av3 = 0.9V

The overall voltage gain is equal:

AV = 187 * 10 * 0.9 = 1683 V/V

#### NSCC_EETN

Joined Oct 6, 2010
17
Voltage gain of a CE amplifier with CE capacitor is equal

AV =( RC||RL ) / re = 40 * Ic * Rc||RL

RL = R5||R6|| (β+1 * R8) ≈ 880Ω

Ic = 10V/1K = 10mA

Av1 ≈ 40*10mA * 1K||880 = 40mA * 468 = 187 V/V

The voltage gain of a Q2 stage is equal:

Av2 = (Rc||RL) / (Re + re) ≈ Rc/Re = R7/ R8 = 10 V/V (9.35V)

And the gain of last stage is equal:

Av3 = RL/( re + RL ) = R9/ ( re + R9 )

re = 26mV/Ic - but we don't know Ic of a last stage, so we can assume Av3 = 0.9V

The overall voltage gain is equal:

AV = 187 * 10 * 0.9 = 1683 V/V
Nice, that makes a bit more sense. The only thing I am curious about (and I do understand there is no Ic of the last stage) is why automatically assume 0.9V for Av3? What's the reason for it?

#### Jony130

Joined Feb 17, 2009
5,292
And of course there is a current flow trough Q3, Q4 but we don't have enough information from your diagram to determine the magnitude of a Ic current.