Sansui AU-X1 Service Manual page 3

AU-X1
2. THE OPERATION & CHARACTERISTIC OF
HIGH SPEED POWER TRANSISTOR
In order to up-grade the so und quality, AU-Xl employs the cir-
cuitry of the push-pull drive DC comfiguration from MC head amp
to po'wer amp stage, which prov ide superior dynamic and open-loop
characteristics.
The improvement was made for parts such as volumes, wires, relays,
electrolytic capacitors and semi-conductors. In particular, newly
developed linear high speed device, LAPT (Linear Ampl ifier Power
Transistor) is adapted in power stage, and the combination use of
this high speed power transistor and diamond differential circuit
allows slew rate of 260 V
Ills
and wide range low distortion amplifi-
cation.
2-1. High Speed Power Transistor
The requirements for power trnasistor in order to elevate sound
quality are;
*
to have high cut'<lff frequency, fT (high speed device) .
*
to make voltage dependance of DC forward-current transfer
ratio , hFE be little and have excellent linearlity.
*
to have a large A.5.0. (Area of Safe Operation) and strong en-
durance against break-down .
The use of high speed power transisto r with requirements above
together with circuit improvement is effective for achievement of
wide range an d low distortion' amplification . Transistors to meet
with these requirements are both bi-polar and field effect transistor
(FET).
Note: Cut-off frequency, fT shows the frequency at which the
small signal current gain will be 1 (0 dB) in common emitter
circuit.
Comparison between bi-polar and field effect type as power
transistor.
Table 1; Comparison of bi-polar and field effect type as a power
device.
Field effect
Characteristics
Bi-polar device junc-
MOS FET
type tion type
FET SIT
Input impedance low
high high
Thermal coefficient of
positive negative negative
operating current
StOlied carrier effect
exist none none
(Carr ier strage effect)
Transfer character istic
exponential square square
Idring current as a
little large large
output device
Output characteristic
pentode triode pentode
State at no input bias off
on on
Current path (Channel) vertical vertical
vertical &
horizontal
Threshold voltage low
inter-
high
mediate
Cost
low
high high
The advantages of FET are high input impedance and no thermal
run-away because of negative thermal coefficient of operating cur·
rent however there still remains price problem on FET, in addition,
bi-p~lar
type' high speed power transistor are variously developed
and becoming common so that AU·X 1 employs bi-polar type power
trnasistor (LAPT) .
2-2. The Configuration of High Speed Transistor
I n order to increase the cut-off frequency, fT, it is necessary to
make transistor base as thin as possible, but it oppose to A.S.O.
(Area of Safe Operation) to enlarge. When making the transistor
base thill, the injected current density is increased so that junction is
destroied by second-breakdown. To solve above problem, the con-
struction of power transistor (LAPT) is parallel connection of
devided 35 of small signal transistors, in addition, the defusion
resistor is made at each emitter of devided transistor to equalize
the flowing current.
Fig. 1 Illustration of emitter ballast resistor made by defusion.
Base emitter
n"
Fig.2 Equivalent circuit of ballast resistor defused
to emitter of transistor.
"
r §"'1COllector
Base
ball ast resistor
,. . / (def f u sion resi s t o r )
. .. emitter
Fig. 3 Surface of LAPT before evaporation.
emitter contact
I
By connecting many small signal transistors with high fT in parallel ,
power transistor with high fT and large A.S.O . can be obtained.
If there is current-concentration to one of small signal transistors,
the voltage between emitter and base is decreased by the ballast
resistor (deffusion resistor) at emitter, which causes internal
negati~e
feedback and relieves the current-con~entration.
2-3. The Characteristic and Merit of use of High
Speed Power Transistor
• The relation between switching time, fT, and hFE of power
transistor.
Fig.4 Switching t ime
i n put ~
: : t
st~
-- d
ON t--
--1,1- I
t 1
~
I - t l 0
Qutputwavefo rm- ,
- ~-----:-:-
-
-T
lOYo
: : I
90%
Fig. 5 Influence of switching time against sine-wave (of half cycle)
~ ,_t s~g
.. ton ... ... .. turn-ontime l
tON I \
tt
I ..
" \ : ".. t f ·· .. · · ··fall tim e _ switch ing time
.J ..
~
..
tstg ··· ·· ·· ·storage tim e
The more the switching time is shortened (high speed) the more the
frequency range characteristic of power transistor is improved; this
relation is indicated in Fig. 4), 5). Fig . 6), 7),8) show the more the
fT is raised the more the switching speed is quickened.
2