Audio Amplifier has an Input signal of 10 mV (input signal of pre-amplifier) and output power is 50 watt (output power of power amplifier) then design the pre-amplifier and power amplifier stage of audio amplifier (calculate gain)?
Answers
Explanation:
The Voltage Amplification (Av) or Gain of a voltage amplifier is given by:
form-Vgain.jpg
With both voltages measured in the same way (i.e. both RMS, both Peak, or both Peak to Peak), Av is a ratio of how much bigger is the output than the input, and so has no units. It is a basic measure of the Gain or effectiveness of the amplifier.
Because the output of an amplifier varies at different signal frequencies, measurements of output power, or often voltage, which is easier to measure than power, are plotted against frequency on a graph (response curve) to show comparative output across the working frequency band of the amplifier.
Logarithmic Scales
Response curves normally use a logarithmic scale of frequency, plotted along the horizontal x-axis. This allows for a wider range of frequency to be accommodated than if a linear scale were used.
log-lin-scales.gif
Fig. 1.3.1 Logarithmic and Linear Scales Compared
The vertical y-axis is marked in linear divisions but using the logarithmic units of decibels allowing for a much greater range within the same distance. The logarithmic unit used is the decibel, which is one tenth of a Bel, a unit originally designed for measuring losses of telephone cables, but as the Bel is generally too large for most electronic uses, the decibel (dB) is the unit of choice. Apart from providing a more convenient scale the decibel has another advantage in displaying audio information, the human ear also responds to the loudness of sounds in a manner similar to a logarithmic scale, so using a decibel scale gives a more meaningful representation of audio levels.
power-response.jpg
Fig 1.3.2 Audio Power Response Curve
Power Gain in dBs
To describe a change in output power over the whole frequency range of the amplifier, a response curve, plotted in decibels is used to show variations in output. The powers at various frequencies throughout the range are compared to a particular reference frequency, (the mid band frequency). The difference in power at the mid band frequency and the power at any other frequency being measured, is given as so many decibels greater (+dB) or less (-dB) than the mid band frequency, which is given a value of 0dB. Notice that, on the logarithmic frequency scale in Fig 1.3.1 the middle of the 10Hz to 100kHz band is 1kHz and frequencies around this figure (where the output is usually at its maximum) are normally chosen as the reference frequency.
Converting a power gain ratio to dBs is calculated by multiplying the log of the ratio by 10:
Power-dB.jpg
Where P1 is the power at mid band and P2 is the power being measured.
Note:
When using this formula in a calculator the use of brackets is important, so that 10 x the log of (P1/P2) is used, rather than 10 x the log of P1, divided by P2.
e.g. if P1 = 6 and P2 =3
10 x log(6/3) =3dB (right answer), but 10 x log 6/3 = 2.6dB (wrong answer).
Voltage Gain in dBs
Although it is common to describe the voltage gain of an amplifier as so many decibels, this is not really an accurate use for the unit. It is OK to use decibels to compare the output of an amplifier at different frequencies, since all the measurements of output power or voltage are taken across the same impedance (the amplifier load), but when describing the voltage gain (between input and output) of an amplifier, the input and output voltages are being developed across quite different impedances. However it is quite widely accepted to also describe voltage gain in decibels.
voltage-dB-curve.gif
Fig 1.3.3 Audio Voltage Response Curve