AUDIO POWER AMPLIFIER Prof. Yosef PINHASI

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AUDIO POWER AMPLIFIER Prof. Yosef PINHASI

Transcript of AUDIO POWER AMPLIFIER Prof. Yosef PINHASI

Page 1: AUDIO POWER AMPLIFIER Prof. Yosef PINHASI

AUDIO POWER AMPLIFIER

Prof. Yosef PINHASI

Page 2: 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).

Page 3: AUDIO POWER AMPLIFIER Prof. Yosef PINHASI

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.

Page 4: AUDIO POWER AMPLIFIER Prof. Yosef PINHASI

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.

Page 5: AUDIO POWER AMPLIFIER Prof. Yosef PINHASI

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.

Page 6: AUDIO POWER AMPLIFIER Prof. Yosef PINHASI

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.

Page 7: AUDIO POWER AMPLIFIER Prof. Yosef PINHASI

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.