Stereo Amplifier Essentials
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How the Amplifier Works
At the centre of the circuit is an audio amplifier Integrated Circuit or IC. Inside the IC are lots of transistors, which are
connected together to allow the small input signal to be amplified into a more powerful output that can drive a
speaker.
All amplifiers need to use feedback to ensure that the amount of gain stays the same. This allows the output to be an
exact copy of the input, just bigger. The gain is the number of times bigger the output is compared to the input. So if
an amplifier has a gain of 10 and there is 1 volt on the input, there will be 10 volts on the output. An operational
amplifier has two inputs, these are called the inverting (-) and non-inverting (+) inputs. The output of the operational
amplifier is the voltage on the non-inverting input less the voltage on the inverting input, multiplied by the
amplifier’s gain. In theory, an operational amplifier has unlimited gain so if the non-inverting input is a fraction
higher than the inverting input (there is more + than -), the output will go up to the supply voltage. Change the
inputs around and the output will go to zero volts. In this format the operational amplifier is acting as a comparator,
it compares the two inputs and changes the output accordingly.
With an infinite gain the amplifier is no good to amplify audio, which is where the
feedback comes in. By making one of the inputs a percentage of the output the
gain can be fixed, which allows the output to be a copy of the input but bigger.
Now when the two inputs are compared and the output is adjusted, instead of it
going up or down until it reaches 0 volts or V+, it stops at the point when the two
inputs match and the output is at the required voltage.
Looking at the circuit diagram for the audio amplifier – it’s not obvious where the feedback is, this is because it is
inside the IC. The TDA2822M or NJM2073D chip has fixed the gain so the output is about 90 times bigger than the
input. To make the gain useful in our application, there is a potential divider on each channel that is fed into the IC
(R1+R5 and R2+R6). Each of these reduces the input signal to only 10% of the original signal. This, therefore, reduces
the overall gain of the amplifier to about 9 times the input to the circuit.
C3 is connected across the supply to make sure that it remains stable. The other capacitors have a filtering role,
either to cut out high frequency noise or get the best out of the speaker.
Battery
(15V max)
Speaker 1
R1
10K
C3
10 F
Input
C1
100 F
C2
100 F
C4
470F
C5
470 F
C6
100nF
C7
100nF
R2
10K
R5
100K
R6
100K
Speaker 2
R3
4R7
R4
4R7
TDA2822
Out 1
Out 2
In 1 +
In 1 -
In 2 +
In 2 -
90%
Input
10%
Output
Amplifier
X10 gain
