T E C H N I C A L I N F O R M A T I O N
Theoretical Efficiency Of A TA3020 Amplifier
The efficiency, η, of an amplifier is:
η = POUT/PIN
The power dissipation of a TA3020 amplifier is primarily determined by the on resistance, RON, of the
output transistors used, and the switching losses of these transistors, PSW. For a TA3020 amplifier,
PIN (per channel) is approximated by:
PIN = PDRIVER + PSW + POUT ((RS + RON + RCOIL + RL)/RL)2
where: PDRIVER = Power dissipated in the TA3020 = 1.6W/channel
P
SW = 2 x (0.01) x Qg (Qg is the gate charge of M, in nano-coulombs)
RCOIL = Resistance of the output filter inductor (typically around 50mΩ)
For a 125W RMS per channel, 8Ω load amplifier using STW34NB20 MOSFETs, and an RS of 50mΩ,
PIN = PDRIVER + PSW + POUT ((RS + RON + RCOIL + RL)/RL)2
= 1.6 + 2 x (0.01) x (95) + 125 x ((0.025 + 0.11 + 0.05 + 8)/8)2 = 1.6 + 1.9 + 130.8
= 134.3W
In the above calculation the RDS (ON) of 0.065Ω was multiplied by a factor of 1.7 to obtain RON in order
to account for some temperature rise of the MOSFETs. (RDS (ON) typically increases by a factor of 1.7
for a typical MOSFET as temperature increases from 25ºC to 170ºC.)
So,
η = POUT/PIN = 125/134.3 = 93%
Performance Measurements of a TA3020 Amplifier
Tripath amplifiers operate by modulating the input signal with a high-frequency switching pattern.
This signal is sent through a low-pass filter (external to the TA3020) that demodulates it to recover
an amplified version of the audio input. The frequency of the switching pattern is spread spectrum
and typically varies between 200kHz and 1.5MHz, which is well above the 20Hz – 22kHz audio
band. The pattern itself does not alter or distort the audio input signal but it does introduce some
inaudible noise components.
The measurements of certain performance parameters, particularly those that have anything to do
with noise, like THD+N, are significantly affected by the design of the low-pass filter used on the
output of the TA3020 and also the bandwidth setting of the measurement instrument used. Unless
the filter has a very sharp roll-off just past the audio band or the bandwidth of the measurement
instrument ends there, some of the inaudible noise components introduced by the Tripath amplifier
switching pattern will get integrated into the measurement, degrading it.
Tripath amplifiers do not require large multi-pole filters to achieve excellent performance in listening
tests, usually a more critical factor than performance measurements. Though using a multi-pole filter
may remove high-frequency noise and improve THD+N type measurements (when they are made
with wide-bandwidth measuring equipment), these same filters can increase distortion due to
inductor non-linearity. Multi-pole filters require relatively large inductors, and inductor non-linearity
increases with inductor value.
TA3020, Rev 2.1, 01.01
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TRIPATH [ TRIPATH TECHNOLOGY INC. ]
