AUDIO POWER AMPLIFIER Prof. Yosef PINHASI
Transcript of AUDIO POWER AMPLIFIER Prof. Yosef PINHASI
AUDIO POWER AMPLIFIER
Prof. Yosef PINHASI
In Figure 1, schematics of an audio amplifier aimed at delivering
50Watts of RMS power into 8Ω speakers, when a unipolar voltage of
65Volts is supplied to the circuit.
+V
+.6
V/2
V/2+.6
V/2+1.2
MJ15024
2N5416
MJ15024
2N3439
4.7u
1N4007
1N4007
V/2-1.2
0.5
5W
5W
220
+.6
2N3439
200 220
0.5
47u
1K10K
50p
2x 1,000u
1N4007
82
6.8K
22K
200u
10K
BC149
470K
2.2K
15K
470K
1u3.3K
220p
100K
VOLUME
Figure 1: Electronic scheme of the power amplifier (one channel).
The power amplifier is based on three main stages. The input stage
is a voltage amplifier, consisting of the transistor BC149. The input
amplifier is followed by a transconductance biasing stage, with current
flow of approximately:
mAV
V
IBE
CC
78.92.3
6.022
65
2.21
22
0 =⋅−
=+
⋅−=
The 200Ω resistor trimmer in the bias circuit should be adjusted to obtain
a quiescent current of 20-30mA from the power supply, without an input
signal.
The output section is in a quasi-complementary configuration. The
graphs in Figure 2 describe output power:
L
peak
outR
VP
2
2
=
The power consumption:
peak
L
CC
CC VR
VP ⋅=
π1
and the dissipated power:
L
peak
peakCCDISSR
VVVP ⋅
−=2
11
π
as a function of the voltage amplitude at the output when VCC=65V and
RL=8Ω.
Figure 2: Power vs. output amplitude for VCC=65V.
The maximum amplitude feasible on the load is Vpeak=VCC/2. The
maximum output power Pout that can be obtained for different supply
voltages is given in Table 1, including peak, DC and RMS currents.
Table 1: Maximum power and current for RL=8Ω load.
VCC
L
CC
outR
VP
8
2
=
L
CC
peakR
V
I 2= peakDC II
π2
= peakRMS II2
1=
L
CC
DISSR
VP
MAX
2
2
1
2
1
π=
30V 14W 1.88A 1.19A 1.33A 5.70W
40V 25W 2.5A 1.59A 1.77A 10.1W
50V 39.1W 3.13A 1.99A 2.21A 15.8W
70V 76.6W 4.38A 2.79A 3.09A 31.0W
Note that the dissipated power is for both power transistors and its
maximum is obtained when the amplitude is CCCCpeak VVV ⋅≅= 318.01
π (see
Fig. 2). The power and peak current of the driver transistors are
calculated in Table 2 assuming current gain of β=15 for the power output
transistors.
Table 2: Maximum current and power dissipation for driver transistors.
VCC
L
CC
peakR
VI
β1
= L
CC
DISSR
VP
MAX
2
2
1
2
11
πβ=
30V 0.125A 0.15W
40V 0.17A 0.67W
50V 0.21A 1.05W
70V 0.29A 2.07W
The transistors are all high voltage, bipolar (BJT) ones, with
maximum ratings summarized in Table 3.
Table 3: Maximum ratings of the transistors.
TRANSISTOR TYPE VCEO IC PDISS β fT
BC149 NPN 20V 625mW 240-900 300MHz
2N3439 NPN 350V 1A 10W 30-160 15MHz
2N5416 PNP -300V -1A 10W 30-120 15MHz
MJ15024 NPN 250V 16A 250W 15-50 4MHz
The schematics of the tone control and microphone pre-amplifier are
shown in Figure 3 and Figure 4 respectively.
10K
10K 10K
22K
22K
27n
27n
270p
270p
27n 2.7n
2.7n
1.5K
BASS MID TREBLE
47K
47K
100K
-
+
100u
3.3K
1u
25u
47K
47K
47K
TL071
1u
Figure 3: Electronic scheme of tone control.
+
180
10K
1.5M 150p
10K
.1u
.056u
BC149
Figure 4: Electronic scheme of microphone pre-amplifier.
The layout of the components on a single channel board is shown
in a picture Fig. 5, including the power supply.
Figure 5: Pictures of the amplifier board and power supply.