3.6M north10.pdf - Dean-O's Toy Box

Cathode Pulsers: Hard-Tube Modulators (10) 193
capacitance. The fall time, however, cannot be reduced by anything that can be
done to or with the switch tube. When shorter fall time is required, a “tail-biter”
or “active pull-up” stage is usually employed. (This featuw was illustrated in
Fig. 2-1, the transmitter block diagram.) This device, usually another switch
tube, is connected between the diode-gun cathode and ground and is pulsed into
conduction only during the desired fall time of the pulse. Its current adds to the
ever-diminishing current of the diode load to facilitate discharging of the stray
capacitance. A constant-current tail-biter of current I will produce a fall time, At,
that is no longer than AV x C~/1,where AV is the pulse voltage amplitude, even if
the diode-load current is negligibly small.
In the case of grid-driven tiiode switch tubes, which include the modulatinganode
type of tube as well, there is another aspect to rise-and-fall time: the effect
of “Miller” capacitance, which is related to the capacitance between anode and
control grid. Miller capacitance and the capacitance between grid and cathode
make up the static input capacitance of a triode. With the anode comected to a
fixed voltage source, the rate at which the voltage between grid and cathode can
be varied depends upon the driver stage’s source resistance, or current-delivery
capabili~, and the sum of the two input capacitance components. The anode,
however, is usually not connected to a fixed voltage. It is connected to the load,
and the load-voltage excursion is the same as the anode-voltage excursion. The
change in charge, or AQ, experienced by the anode-grid capacitance is not the
product of the grid-voltage change times anode-grid capacitance but is the anode-voltage
change times the anode-grid capacitance. In effect, the anode-grid
capacitance component of total triode input capacitance is multiplied by the
voltage gain of the triode. This fact must be considered in the design of a grid
driver. Moreover, the internal impedance of the driver stage in both directions
must be considered. This is because all amplifiers that have a single active device
have source impedances that are different, depending on whether the device is a
source or a sink for output current. For this reason, active pull-up/active pulldown
pairs connected in a “totem-pole” configuration are the preferred grid
driver stages in critical rise-and-fall-time applications. This is because they can
be designed to deliver equal output currents in both directions.
With high-voltage and high-current transistors now availabl~specially the
metal-oxide semiconductor field-effect transistor (MOSFET)-triodes do not have
to be grid-driven. They can be cathode driven by what is often called the
grounded-grid, or grid-separation, connection. The control grid can be connected
to a fixed source of positive grid bias, if necessary. A typical cathode-drive
connection is shown in Fig. 10-33. During the switch tube’s interpulse (or off)
state, the MOSFET is in the off state. To reduce cathode current to the same value
as the MOSFET’S drain-leakage current, which is negligibly small, the drain of
the MOSFET will automatically assume whatever positive voltage is required,
including an amount equal to whatever positive voltage is applied to the grid.
No negative grid bias supply is needed at all. To cause output current and load
voltage, the cathode MOSFET is turned on, pulling the switch-tube cathode to
within a few volts of ground (or deck) potential. The effective grid-cathode
voltage at this time is whatever positive voltage source is connected between
grid and ground. The “Miller” charge and discharge currents flow either into the