An audio power amplifier (or power amp) is an electronic device that amplifies or strengthens low-level electronic audio signals, such as the signal from radio receiver, pre-amplifier, and other audio sources. Signal bandwidth is between 20 - 20,000 Hz, the human range of hearing. The goal of an amplifier is to amplify input level to a level that is strong enough for driving (or powering) loudspeakers. Power amplifiers make the signal--whether it is recorded music, a live speech, or an electric guitar--audible to listeners. It is the final electronic stage in a typical audio playback chain before the signal is sent to the loudspeakers.
Input into the amplifier is 0.1V AC sine-wave. The input signal is fed into C1 10uF DC blocking capacitor.
R4 and R5 add 6V DC shift to the input sine wave. U1A has a gain of 33 and feedback around the amplifier
is AC coupled. This allows to block 6V DC shift introduced by R4/R5 resistor network. U1B is a buffer stage
which can be converted into a second gain stage if needed. C5 is a DC blocking capacitor.
The output stage stabilizes the quiescent current.
U2A and U2B are voltage-followers through its corresponding power transistor,
holding the differential voltage across the two 1 ohm emitter resistors to a
precise value set by the selected 220 ohm across the inputs.
With a 12 volt supply, it is about 13 mV drop
across the 220 ohm resistor.
Iquiescent = 0.013 / 2 = 6.5 mA (Current through 1 Ohm resistors)
Iquiescent can be changed by changing the 220 resistor. The 1N4148 diodes limit negative output swing when the related transistor turns off.
U1A and U1B are the Noninverting Op Amp. The noninverting op amp has the input signal connected to its noninverting input, thus its input source sees an infinite impedance. There is no input offset voltage because VOS = VERROR = 0, hence the negative input must be at the same voltage as the positive input. The op amp output drives current into RF until the negative input is at the voltage, VIN. This action causes VIN to appear across RG.
In our case it is going to be: 33k/1k + 1 = 34
U1B an be changed to a gain stage as follows:
For the above configuration the total gain is 11 x 11 = 121