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3 Watt PWM stereo amplifier

The most remarkable feature of this small stereo amplifier is the use of a stepper motor bridge driver IC as a stereo power output stage. The circuit diagram shows a three-stage pulse-width modulation (PWM) converter/amplifier for each channel. The left (L) and right (R) audio input signals are first converted into triangular waveforms by opamps IC1 and IC2, which are wired as integrators, with feedback from the power output stage via resistors R7 and R11. Next, the triangular waveforms are converted into rectangular signals with a variable pulse width by opamps IC3 and 1C4, which drive the digital inputs of the bridge power amplifier, IC5.

The L6203 from SGS-Thomson is actually a full bridge driver for motor control applications. Its DMOS output transistors have an RDs(on) resistance of only 0.3 0, which results in low dissipation and, consequently, high efficiency. Each channel (half bridge) of the device is controlled by a separate logic input, while a common ‘enable’ (pin 11) is available to switch both channels on and off. In the present application, the channels are enabled by a +5.1 V level derived from the ic’s voltage reference output. Although the L6203 con-tains a bridge circuit, the loudspeakers are connected single-ended for a stereo outputarrangement.

The test results obtained with a prototype ofigi the amplifier are not spectacular but non. the less worth mentioning.

3 Watt PWM stereo amplifier Schematic diagram

With a supply voltage of 12 V and an input frequency of 1 kHz, a maximum output power of 2 W was supplied into a load of 4 O. The nominal input level required for this output power was 2 V. A damping factor of 20 was measured, while the distortion was about 1.5%.

These figures change slightly when the supply voltage is increased to 14.4 V. Maximum output power, for instance, increases to about 2.8 W with a distortion of about 1.5%. The input drive level required for this output power is 2.25 Vrms• With full drive, the amplifier achieves an efficiency of about 73%, with PWM clock intermodulation products down to -40 dB.

At lower input signal levels, the distortion of the amplifier remains around 0.3% and the noise level at about -80dB.

3 Watt PWM stereo amplifier Schematic diagram

Parts list

R1 , R2 = 47 kΩ.

R3, R4 = 33kΩ.

R5, R6 = 2.2 kΩ.

R7, R11 = 18 kΩ.

R8,R12 = 22 kΩ.

R9, R13 = 100 kΩ.

R10,R14 = 470kΩ.
R15,R16 = 10kΩ.
R17, R18 = 1 kΩ.


C1 = 10 uF, 25 V

C2 = 10 uF, 25 V radial

C3 = 100 uF, 25 V radial

C4, C5 = 560 pF, polystyrene

C6 = 220 uF, 25 V radial

C7, C8 = 22 nF

C9 = 220 nF

C10, C11 = 470 nF

C12, C13 = 2200 uF, 25 V, radial


L1,L2 = 30 uH/3 A toroid choke

D1, D2 = 1N4148

Integrated circuits:

IC1, IC2 = CA3100*

IC3, IC4 = LM311N

IC5 = L6203**
* Harris Semiconductor                                                        **SGS Thomson Microelectronics

3 Watt PWM stereo amplifier Schematic diagram

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