Basic Operation Of Horizontal Drive - Toshiba CN27E90 Technical Training Manual

3. BASIC OPERATION OF HORIZONTAL
DRIVE
A sufficient current must flow into base of the horizontal
output transistor to rapidly make it into a saturated (ON)
condition or a cut off (OFF) condition. For this purpose, a
drive amplifier is provided between the oscillator circuit and
the output circuit to amplify and to waveshape the pulse
voltage.
3-1. Theory of Operation
(1) The horizontal drive circuit works as a so called switching
circuit which applies a pulse voltage to the output
transistor base and makes the transistor on when the
voltage swings in forward direction and off in reverse
direction.
(2) To turn on the output transistor completely and to make
the internal impedance low, a sufficiently high, forward
drive voltage must be applied to the base and heavy base
current ib must be flown. On the contrary, to completely
ib
V
(a)
turn off the transistor, a sufficiently high, reverse voltage
must be applied to the base.
(3) When the transistor is on (collector current is maximum)
condition with the sufficiently high forward voltage
applied to the base, the transistor can not be turned off
immediately, if a reverse base bias is applied to the base
because minority carriers storaged in the base can not
be reduced to zero instantly. That is, a reverse current
flows through an external circuit and gradually reduces
to zero. The time lag required for the base current to
disappear is called a storage time and falling time.
(4) To shorten the storage time and the falling time, a
sufficiently high reverse bias voltage must be applied to
allow a heavy reverse current to flow. This operation
also stabilizes operation of the horizontal output
transistor.
On period
+
0
-
+
Forward
current
0
Reverse
current
-
Storage
time
Fig. 11-2
71
OFF period
t Input waveform (b)
t Base current (c)
Falling
time