Datasheet

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Checking for Oscillations and Instability
10M100k 500M1M 100M10k1k10010
f − Frequency − Hz
Normalized Output Response − dB
V
CC
= ±15 V
R
L
= 100 Ω
Gain = 1 V/V
V
I
= 200 mV
−1
−3
−5
−7
−2
−4
−6
1
2
0
3
R
F
= 1 kΩ
R
F
= 620 Ω
R
F
= 820 Ω
10M100k 500M1M 100M10k1k10010
f − Frequency − Hz
Normalized Output Response − dB
V
CC
= ±15 V
R
L
= 100 Ω
Gain = 2 V/V
V
I
= 200 mV
−1
−3
−5
−2
−4
−6
1
2
0
3
R
F
= 430 Ω
R
F
= 1 kΩ
R
F
= 620 Ω
TPA6120A2
SLOS431 – MARCH 2004
APPLICATION INFORMATION (continued)
Special note regarding mono operation:
• If both amplifiers are powered on, but only one channel is to be used, the unused amplifier MUST have a
feedback resistor from the output to the negative input. Additionally, the positive input should be grounded as
close to the pin as possible. Terminate the output as close to the output pin as possible with a 25-Ω load to
ground.
• These measures should be followed to prevent the unused amplifier from oscillating. If it oscillates, and the
power pins of both amplifiers are tied together, the performance of the amplifier could be seriously degraded.
Checking the stability of the amplifier setup is recommended. High frequency oscillations in the megahertz region
can cause undesirable effects in the audio band.
Sometimes, the oscillations can be quite clear. An unexpectedly large draw from the power supply may be an
indication of oscillations. These oscillations can be seen with an oscilloscope. However, if the oscillations are not
obvious, or there is a chance that the system is stable but close to the edge, placing a scope probe with 10 pF of
capacitance can make the oscillations worse, or actually cause them to start.
A network analyzer can be used to determine the inherent stability of a system. An output vs frequency curve
generated by a network analyzer can be a good indicator of stability. At high frequencies, the curve shows
whether a system is oscillating, close to oscillation, or stable. Looking at Figure 27 through Figure 32 , several
different phenomena occur. In one scenario, the system is stable because the high frequency rolloff is smooth
and has no peaking. Increasing R
F
decreases the frequency at which this rolloff occurs (see the Resistor Values
section). Another scenario shows some peaking at high frequency. If the peaking is 2 dB, the amplifier is stable
as there is still 45 degrees of phase margin. As the peaking increases, the phase margin shrinks, the amplifier
and the system, move closer to instability. The same system that has a 2-dB peak has an increased peak when
a capacitor is added to the output. This indicates the system is either on the verge of oscillation or is oscillating,
and corrective action is required.
Figure 27. Normalized Output Response vs Frequency Figure 28. Normalized Output Response vs Frequency
15