020 1-watt BTL audio amplifier

Source: Philips Semiconductors 5V

Preliminary Specification
The TDA8581(T) from Philips C3 C4 VP
UP
Semiconductors is a 1-watt 100n 220µ
VOLUME
Bridge Tied Load (BTL) audio CONTROL
25V

power amplifier capable of deliv- UP/DOWN VP

ering 1 watt output power into an DOWN
1 6
8-Ω load at THD (total harmonic
C1 VP
distortion) of 10% and using a 5- IN
4 VOLUME
V power supply. The schematic 330n CONTROL 8
OUT+

shown here combines the func- VP

tional diagram of the TDA8551

with its typical application cir- TDA 20k

cuit. The gain of the amplifier 8551 LS1

can be set by the digital volume

15k

5k
control input. At the highest vol-
SVR
ume setting, the gain is 20 dB. 3 R
8Ω
Using the MODE pin the device

15k
C2
can be switched to one of three R

modes: standby (MODE level 100µ

between Vp and Vp–0.5 V), STANDBY/
5
muted (MODE level between 5V MUTE/ OUT–
OPERATING/
1 V and Vp–1.4 V) or normal
(MODE level less than 0.5 V). STAND-BY 2 7
The TDA8551 is protected by an MODE GND
internal thermal shutdown pro- MUTE
tection mechanism.
OPERATING
The total voltage loss for both
MOS transistors in the comple-
mentary output stage is less than 984092 - 11
1 V. Using a 5-V supply and an
8-Ω loudspeaker, an output
power of 1 watt can be deliv- to 40 dB (low volume), so the
ered. gain of the total amplifier is then
The volume control has an –20 dB. Some positive pulses
attenuation range of between have to be applied to the
0 dB and 80 dB in 64 steps set UP/DOWN pin to achieve lis-
by the 3-state level at the tening volume. The graph shows
UP/DOWN pin: floating: vol- the THD as a function of output
ume remains unchanged; nega- power. The maximum quiescent
tive pulses: decrease volume; current consumption of the
positive pulses: increase volume amplifier is specified at 10 mA,
Each pulse at he Up/DOWN pin to which should be added the
causes a change in gain of 80/64 current resulting from the output
= 1.25 dB (typical value). offset voltage divided by the
When the supply voltage is first load impedance.
connected, the attenuator is set (984092-1, Gb)
021

simple electrification unit

From an idea by P. Lay quency) is determined by the tangular signal into narrow The output of IC1b is linked
The circuit is intended for car- time constant of network R1-C3 pulses. Differentiating network directly to the gate of thyristor
rying out harmless experiments in the feedback loop of op amp R2-C4, in conjunction with the THR1, so that this device is trig-
with high-voltage pulses and IC1a: with values as specified, it switching threshold of the gered by the pulses.
functions in a similar way as an is about 0.5 Hz. Schmitt trigger inputs of IC1b, The requisite high voltage is
electrified fence generator. The The stage following the determines the pulse period, generated with the aid of a small
p.r.f. (pulse repetition fre- op amp, IC1b, converts the rec- which here is about 1.5 ms. mains transformer, whose sec-

68 Elektor Electronics 12/98

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ondary winding is here used as 12V R3

TR1 reverse-biased, so that the cur-
the primary. This winding, in 12k rent loop to the transformer is
conjunction with C2, forms a broken, whereupon the capacitor
14
resonant circuit. IC1
is charged to 12 V again via R3.
Capacitor C3 is charged to 7
9V At the next pulse from IC1b,
1VA5
the supply voltage (12 V) via R2 this process repeats itself.
R3.When a pulse output by IC1b C2 Since the transformer after

22k
triggers the thyristor, the capac- 22µ
each discharge of the capacitor
itor is discharged via the sec- R1 25V at its primary induces not only a
ondary winding. The energy 10M primary, but also a secondary
THR1 D1
stored in the capacitor is, how- A
voltage, each triggering of the
IC1a IC1b
ever, not lost, but is stored in the C1 1 C4 5 thyristor causes two closely
3 4 G
magnetic field produced by the 2 & 6 & K 1N4148 spaced voltage pulses of oppo-
100µ BRX46
transformer when current flows 25V
100n site polarity. These induced volt-
through it. 10 IC1c 11 IC1d ages at the secondary, that is,
When the capacitor is dis- C3
& & the 230 V, winding, of the trans-
charged, the current ceases, 470n former are, owing to the higher
whereupon the magnetic field 8 9 12 13 turns ratio, much higher than
collapses. This induces a those at the primary side and
IC1 = 4093 984099 - 11
counter e.m.f. in the transformer may reach several hundred
winding which opposes the volt- volts. However, since the energy
age earlier applied to the trans- stored in capacitor C2 is rela-
former. This means that the tively small (the current drain is
direction of the current remains the transformer has returned the ing of the transformer. As soon as only about 2 mA), the output
the same. However, capacitor C2 stored energy to the capacitor, the capacitor begins to be dis- voltage cannot harm man or ani-
is now charged in the opposite the direction of the current charged, there is no current mal. It is sufficient, however, to
sense, so that the potential reverses, and the negatively through the thyristor, which cause a clearly discernible mus-
across it is negative. charged capacitor is discharged therefore switches off. When C2 cle convulsion.
When the magnetic field of via D1 and the secondary wind- is discharged further, diode D1 is [984099]
022

balanced amplifier for

photo-diode
Design: H. Bonekamp
R1
A photo-diode is a p-n diode 1M
whose reverse current depends R4
on the amount of light falling on 6 100k
its junction. The reverse current 7
R3
IC1b 100k
is greatly dependent on the tem- 5
13
perature since heat can liberate D1
14
more covalent bonds. As light IC1d
12
10
can also do this, the diode can R5
8
be housed in a transparent case. BPW34 IC1c 100k
9 R6
When a photo-diode is
82k

located at some distance from R2

the associated electronic cir- 1M
15V
cuits, noise may be picked up in P1
C1 C2 47k
the connecting cable, even when
this is screened. Such noise can, 10µ 100n
11 25V
fortunately, be suppressed eas-
IC1 2
ily, provided it is common mode,
4 1
that is, when the diode is not C3 C4
3
IC1a
connected to earth (‘floats’).
10µ 100n
A differential amplifier 25V IC1 = TL084
15V
enables a feedback signal to be 984096 - 11

amplified, but does not respond

to common-mode signals. In the
diagram, the differential ampli- The output voltage, Uo, of Uo=(R1+R2) · ID, nullified with P1 so as to achieve
fier consists of two op amps, the differential amplifier is optimum CMR. A Common
IC1b and IC1c, which convert where ID is the diode current. Mode Rejection Ratio, CMRR, of
the diode current into a voltage. Uo=(Uin1–Uin2) · R4/R3. The Common Mode Rejec- >60 dB is obtained when the
The current-to-voltage conver- tion, CMR, depends on the specified op amps are linked to
sion depends on R1 and R2, so When R3=R5=R4=R6+P1, equality of the resistors as stipu- the photo-diode by a twisted pair.
that gain setting in amplifier the amplification is unity. In that lated earlier. Their tolerances, The circuit draws a current
IC1d is not necessary. case, and those of R1 and R2, can be of about 10 mA. [984096]

Elektor Electronics 12/98 69