Design a class AB audio amplifier

Question

I am required to design a class AB audio amplifier to deliver 10W to an 8Ω speaker as shown, but I don't know where to start. The bandwidth is to cover the range from 20Hz to 18kHz. Input voltage is 500mV rms.

_{Image source: Electronics Tutorials - Class AB Amplifier Driver Stage}

This is what I have done so far

CALCULATIONS.

• R_L = 8 Ω

• P_AVG = 10W

• Bandwidth from 20 Hz to 18 kHz

• V_(0 peak) = √(2×10×8) = 12.65v

• V_CC = V_(0 peak)×2+2 = 27.3V ≅ 28V

• I_(0 peak) = V_(0 peak)/R_L = 12.65/8 = 1.58A

Each transistor current is given by:

• I_(0 peak)/2 = 1.58/2 = 0.79A

Assuming β = 113

• I_(B Q1) = I_(B Q2) = I_E/(β+1) = 0.79/114 = 6.93mA

Assuming I_D = 1A, then:

• I_bias = 1mA+1mA+6.93mA+6.93mA = 15.86mA

Setting V_C = 14V, then:

• R4 = (28V-14v-0.7v-0.7V)/15.86mA = 794.5Ω

Setting V_E = 1V

• R_3+R_3^II = 1V/15.86mA = 63.05Ω

Input current is assumed to be 500mA Q_3 gain is given by:

• V_(O peak rms)/V_(in rms) = √(8×10)/500mA = 17.88Ω

Setting Q_3 gain to 20:

• R_3^II = ((28V-14V)/15.86mA)/20=44.14Ω
• R_3=63.05-44.14 = 18.92Ω
• I_B = I_C/β = 15.86mA/100 = 0.1586mA

Setting I_1 = 20 I_B:

• I_B = 20×0.1586mA = 3.172mA
• R_2 = 1.7V/3.172mA = 535Ω
• R_1 = (28V-1.7V)/3.172mA = 8291Ω

Designing for C_1 and C_3:

• r_e = 26mV/15.86mA = 1.64Ω
• R_1 ||R_2 = 502.57Ω
• R_3^II = 44.14Ω
• R_th = 41.96Ω
• C1 = C3= 1/(2×π×f_L×R_th) = 1/(2×π×20×44.14) = 189.65μF

Designing for C_2:

• R_th = 69.6759Ω
• C_2 = 10/(2×π×f_L×R_th) = 10/(2×π×20×69.6759) = 1142μF

When I simulate the circuit, the output voltage is quite small and the output power is much less than 10W.

What could be the issue?

The final circuit I came up with is shown.

Answer 1

Your output transistors are small signal transistors, not power transistors.

The base currents of your output transistors are loading the common emitter amplifier's output down and reducing the gain.

I have added a power output stage and the required extra two diodes for biasing. Also increased the power supply voltage a little to prevent clipping.

Answer 2

For 10 watts into 8 ohm speaker, you can do this system design

• P = Vrms * Vrms / Rload

• 10 watts = V * V / 8

• 10 * 8 = V * V thus Vrms_out ~~ 9 volts RMS, or 2.828 * 9 peakPead

You will need headroom. So have at least 40 volts VDD.

Now for the gain:

The topology of common_emitter amplifier, into a resistive load, has maximum voltage gain of VDD / 0.026; with 40 volts / 0.026 (or 40 * 39) , the maximum gain is nearly 1,600. You only need 9_rms/0.5_rms or 18X.

Now for the load on the collector. That is heavy. Many milliAmps, because of 1 amp+ Iout, and moderate beta of the two output transistors. One amp / Beta = 100 ==> Ibase of 0.010 amp = 10mA.

What to do? The base currents will tend to cancel, so assume 10mA (net) into the two output transistors. And assume another 10mA to provide pullup to the +40 volts. So plan to operate that first transistor at 0.020 amps.

WIth 20mA Icollector and 40 volts, the maximum power dissipation of your first transistor (that common emitter) will be 20mA/2 * 40 volts/2 (for passive loads), or 10mA * 20Volt = 0.2 watts.

So you need a transistor in the voltage amplifier (gain of 9/0.5 = 18x) able to dissipate 0.2 watts. Probabaly don't worry about that yet.

How to design that first (voltage gain) stage?

Answer 3

but I don't know where to start

Every design process starts with listing the requirements. It appears that you've listed those, so that's a start.

A Class-AB audio amplifier involves three stages:

• Input stage
• Voltage Amplification Stage
• Output stage.

And the design of each stage requires:

• Basic circuit theory,
• Transistor-based amplifier design,
• Filter design,
• And even a very basic understanding of feedback theory.

So, as you can see, a Class AB audio amplifier is not a "how can I design one?" thing. And it's almost impossible to learn how to design one from a simple answer posted here, believe me. We don't know your level of each topic above. If you had shown what you have tried so far then it would give a clue about your level and I'm sure that the experts here would show you where/what you did wrong and how to fix. For example,

I got Vo peak as √(10 * 8 * 2) which gives 12.65V. I then set Vcc=Vo peak*2 +1 which gives 27V.

You can't just put that +1V headroom randomly.

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Note to the mods: I know that this is not an answer. But it'd be too long to put in the comments. So please don't hit me!

Answer 4

Overview block diagram of single supply class AB amplifier. This is representative of a 4W into 8R amplifier which I built and so the shown 24V supply would be too low for your requirements.