Power amplifier electronic circuits-tube audio rf amplifiers

Electronic Amplifier Circuits-Power amplifier, audio amplifier designs and circuits

March 20th, 2008


Cass D Amplifier.

The class D amplifier or switching amplifier uses the switching mode of operation of transistors to control the out power rather than using the linear operation of transistors (class AB).The cause of the high power efficiency is that the switching element (transistor or MOSFET) is never operated in the active region.They are operated in the switching mode only (either ON or OFF).So the out put has also two states OFF or ON.The result is greater power efficiency and low weight.

The amplifier uses pulse width modulation or sigma delta modulation for controlling the output power.The input signal is converted int o train of puses whose pulse width is proportional to the frequency and voltage of input signal.A power transistor switches the output to drive the speaker.In addition to the
switching transistors filter circuits have to be also included to filter off unwanted frequencies and harmonics.

The PWM frequency is generated by comparing the input signal with a high frequency comparator.The out put of the comparator will be a sequence of pulses whose pulse width is proportional to the frequency and voltage of the input signal.The PWM output of the comparator drives a switching controller made up of transistors or MOSFETS which generates a high power copy of the PWM signal.The output of the switching controller is filtered using a low pass filter and fed to the speaker.Some Class D designs incorporate a negative feed back too to reduce oscillations and improve overall gain.


  1. Low weight because heat sinks are smaller due to high power efficiency.Less power is wasted as heat.
  2. Overall power efficiency is very high (90%).


  1. Complicated circuitry
  2. Complex design.

Block Diagram of Class D Amplifier.


Authors view.

Class D Amplifiers stand above the conventional Transistor , MOSFET or even Valve amplifiers only on the parameters like power efficiency and low weight. I feel there is no advantage for Class D Amplifiers over conventional ones on parameters like fidelity, linearity,THD etc.Therefore Class D Amplifiers till now cannot cause much threat to our conventional transistor or MOSFET amplifiers.

March 20th, 2008

68 W amplifier circuit using LM3886

About the Circuit.

Wanna have some real punch? Then this is the ideal IC power amplifier  for you  .This amplifier circuit is based on National Semiconductor’s  LM 3886.

The LM3886 is a high-fidelity audio power amplifier IC  able of delivering a blasting 68W of continuous power on 4 Ohm load.  The performance of the LM3886, using its Self Peak Instantaneous Temperature  Protection Circuitry (SPiKE) makes it in a class above discrete and hybrid amplifiers. . SPiKe Protection refers to that the amplifier is  almost safe against problems like over voltage, under voltage, overloads,  shorts to the supplies,
thermal runaway, and  temperature peaks.The LM3886 maintains a good S/N Ratio of
above  92 dB . It showes extremely low Total Harmonic Distortion over the audio spectrum, and provides excellent linear behavior.

Circuit Diagram & Parts List.


Notes .

  • The circuit needs a well regulated and filtered 28V dual power supply.
  • If you need some more punch you can extend the supply voltage up to 35V.
  • Switch S1 is used for mute function. It can be opened to mute the amplifier.

LM 3886 Pin out.


LM 3886 Quick reference data 


March 19th, 2008

12 Watt Transistor Amplifier Circuit

 About the circuit.

This is a low cost amplifier circuit using 4 transistors and one op amp. The op amp uA 741 produces the required gain and the transistors wired in complementary Darlington configuration produces the drive for the speaker.

The input for the Darlington pairs are derived from the power supply as a voltage drop across R2 and R3.The voltage drop across these resistors will be proportional to the input signal because the supply current to the op amp varies according to the input signal .The negative feedback from the junction of Q4 and Q2 stabilizes the amplifier. This circuit will produce a decent 12W out put power on a 4 Ohm speaker .

Circuit diagram and parts list.



  1. Use a  +12 , -12 , 1A  dual power supply for powering the circuit.The power supply must be well regulated and filtered to reduce the noise.
  2.  Wire the board on a good quality PCB or common board.
  3. Do not give more than +15 , -15   volts to the circuit.