Toshiba TLP411E, Technical Training Manual

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4-3
2-2. Gamm
a (
g g
g g
g
) C
ircuit
The circuit consists of a current source consisting of R983,
R984, Q954, full feedback amplifier Q955 – Q960, the gain
variation circuit Q961 – Q963 and R991. (The circuit in-
cluding Q955 – Q963 is called the gamma circuit.)
A current of (12.7V – 0.7V)/(1.3 k W + 15 W ) = 9.15 mA is
flown into the current source for Q954 and the same cur-
rent is also flown into Q956 (Q947). At the same time, a
current of 9.15 mA
x
(15/20) = 6.9 mA is flown into Q960.
The followings are described referring to Fig. 4-3.
The signal from the level shifter is supplied to the base of
Q955. If the device elements and currents of Q955 and Q957
are exactly the same, the base state of Q955 is same as that
of Q957. However, Q955 and Q957 are not paired in their
characteristics, the actual base state will be different. The
base state of Q957 is described by referring to the triangu-
lar waveform of 4.06V – 5.36V.
When the base of Q963 develops 3.55 V
DC
, the emitter of
Q961 develops 4.25 V
DC
. In the signal area where the base
of Q963 is higher than 4.25V, the current of 7.7 mA is flown
into R988 because of Q961 turned off, and the emitter volt-
age of Q956 increases by amount of 1.7V, 7.7 mA
x
220
W
= 1.7V, from the base of Q957. As the base voltage of Q957
is close to 4.25V, Q961 turns on and the current is flown
into the collector of Q960 through R991.
When the base voltage of Q957 develops 4.06V, the cur-
rent flowing into R991 is 2.8 mA, (4.25V – 4.06V)/68 =
2.8 mA. The current flowing into R998 decreases by that
amount and the voltage shifting amount also decreases by
the same amount, 2.8 mA x 220
W
= 0.62V. The operation
is shown in Fig. 4-4.
Fig. 4-4 Gamma circuit operation
Q955
Q956 Q954
9.15 mA 7.7 mA 0.5 mA
5.36V
4.06V
Q956
Q961
Q962
Q963
4.25V
R988
220
R991
68
Q957
Q960
7.06V
5.14V
5.36V
4.06V
3.55 V
DC
R1004
Q968
1.3k
15
Fig. 4-3 Gamma circuit
7.06V
5.76V
5.14V
0.62V
(5.94V)
5.36V
(4.24V)
4.06V
1.7V
Q957
Base voltage
Q956
Emitter voltage