FM Radio Essentials
www.kitronik.co.uk/2135
Audio amplification
The audio amplification is performed by another Integrated Circuit or IC called a TBA820M. Inside the TBA820M 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. Before looking at
how the feedback works, we first need to understand how a standard amplifier works. 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 input 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 inside
the IC is a 6K resistor between the Output pin and the Gain setting input pin. The internal 6K resistor and the 100
resistor (R9) on the gain setting pin make up a potential divider that feeds back approximately a sixtieth of the
output. This fixes the gain so that the output is about 60 times bigger than the input.
The rest of the components are needed as follows:
C13 removes any DC offset from the audio signal from the radio IC.
R3 and R8 reduce the audio signal from the radio slightly so that when it is at full volume there is less chance of any
distortion on the sound from the speaker.
C18 & C22 are connected across the supply to make sure 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 voltage
Speaker 1
R3
100K
C22
470uF
Audio
input
C21
100uF
C14
470pF
C15
100nF
R8
10K
R10
1R
TBA820M
Output
BS
FC
Gain
C18
100nF
C13
220nF
R9
100R
C23
220uF
Input
+
-
90%
Input
10%
Output
Amplifier
X10 gain
