36
PIN CONFIGURATION
BLOCK DIAGRAM
Q501 : LC72130M
35
7. SODA (Self-Oscillating class D
Amplifier)
7.1 Explanation of class D
7.1.1 Principle
Shown below is the basic schematic of a straight-through
standard class D amplifier:
A triangle wave oscillator feeds a computer which acts as a
slicer for the audio signal, taking the triangle wave as a
reference. The result is a square wave voltage with the same
frequency as the oscillator’s but with a duty cycle varying
proportionally with the audio input voltage. A power buffer
consisting of 2 switches reproduces the same signal with a
larger amplitude (i.e. switching between the power supply
rails). A LP filter passes only the average of the output wave,
which is an amplified version of the input signal.
The gain may be expressed as G = . It is clear that any
power supply variations will cause equal variations in gain,
which is intolerable.
This may be countered by scaling the amplitude of the triangle
wave exactly with the power supply voltage, thus keeping the
above ratio constant.
An amplifier as shown here will have some 1% distortion. In
order to keep this distortion low, negative feedback is usually
applied. Unlike with standard linear amplifiers this is quite a
difficult thing to do.
When the output signal is fed back to the comparator the
switching frequency should not be present.
If it is, it will cause the comparator to switch slightly too early
or too late (depending on the phase shift imparted by the
feedback loop), thus causing more distortion than it cures. A
low-pass filter is absolutely necessary but this will decrease
the effectiveness of the feedback loop.
The necessary trade-off can be determined using a simple
rule of thumb: the switching ripple from the output must be
attenuated such to a level of n dB below the audio input signal,
where n is the distortion figure envisaged.
One workaround is to build the feedback loop such that the
ripple fed back to the input somewhat resembles a triangle
wave, so that the timing error varies linearly with the audio
signal and thus cause only a minor gain error, not distortion.
This is what makes most class D amplifiers work satisfactorily,
although mostly their designers are quite unaware of it.
+V
-V
7.1.2 Concept
“Everything should be as simple as possible, but no simpler.”
Sir Isaac Newton.
With regard to cost, how simple can one make a class D
amplifier without sacrificing performance?
Power stage: the cheapest thinkable output stage is a pair of
complementary MOSFETs, AC coupled to the driver stage
with DC restoration. This eliminates the need for dead time
circuitry and level shifters. The driver stage is a complementary
pair of emitter followers, in turn driven directly by a low cost
comparator.
PWM generation: given that feedback will be necessary (in
order to have PSRR and useful distortion figures) and that
feedback is an unusually complex thing to do, a new oscillator/
PWM topology is in order. The solution was found to be building
a “free running multivibrator” around the comoutput
stage (the latter two being topologically equivalent to a
comparator simply) and injecting the audio signal into the
integrating feedback node. This effectively results in a Self-
Oscillating class D Amplifier (dubbed SODA) which behaves
like an inverting amplifier with feedback around it.
R1 and R2 provide positive feedback to create a hysteresis.
This should be set to about 100mVpp. Rint and Cint form an
integrating negative feedback loop. The circuit will start
oscillating as the comparator changes state each time the
voltage across Cint exceeds the hysteresis loop.
The graph below shows the operation of the circuit, with a
negative voltage on Vin.
Vint
Vf
Va
Vb
t1
t0
Vee
Vcc
Vin
Power Stage
Low-pass filter
Output
GND
Comparator
Rint
R1
Cint
R2
Rs
Vf
Vint
Vsw
Vout
V
su p
V
tr,p p
R2
R
2
+
R
2
V
f
iS V
SW
divided down by R1 and R2, i.e.
V
f
= V
SW
x =k x V
SW
