DN3

Design Note 2
Issue 2 June 1995
Sonar Transducer Driver using TO92 Style Bipolar
Transistors
Design Note 3
Issue 2 June 1995
High Current High Voltage
Analog Switches
+ 100V
R7
C4
Input
ZVN4306
ZVN4306
Output
R6
ZTX
855
C2
R8
VR1
ZTX
108
Output
R9
R1
R4
Control
R5
ZTX
955
C1
C3
ZTX
504
Input
R2
Commercial analog switch ICs are in the
main designed for low power signal
applications, and often cannot be used
due to:
ZTX
455
R3
0V
Depth/Echo sounders require high
power linear amplifiers for driving the
transducer. Since these amplifiers
operate in pulsed mode, the average
power dissipation experienced by the
output transistors is low, but the value of
the pulsed current can be very high. This
would normally dictate that high power,
large package power transistors would
be required, which tends to increase the
overall circuit size.
packaged in the TO92 style E-Line
package, and permit an output power of
100W to be obtained from this design.
Larger and more costly TO220
transistors have been eliminated. The
devices possess an FT figure of over
90MHz, enabling a high bandwidth and
slew rate specification.
BVCBO BVCEO
ZTX855
With a peak current capability of 10A, the
ZTX855/ZTX955 transistors are
IC(M)
IC
(DC)
Package
250V
150V
10A
4A
TO92
ZTX955 180V
140V
10A
3A
TO92
DN2-1
a) not possessing a wide enough swing
in signal voltage;
b) size problems;
c) current limitations due to their
on-state resistance.
Single MOSFET devices cannot be used
as alternatives because their body diode
(shown dotted in the diagram) will
conduct part of any AC signal even when
the device is turned off. However two
low resistance ZVN4306A TO92 style
MOSFETs in the simple arrangement
shown here, will give a compact high
performance 0.45Ω, 1A analog switch
without this restriction. With the control
input driven 2V more positive than the
peak of the analog signal to turn the
switch on, and as negative to turn the
s w i tc h o ff , th e c i r c ui t a b o v e w i l l
accommodate signals up to 18V peak to
peak. Substituting Zetex 2N7000
MOSFETs in the circuit allows signals up
to 38V ptp and still handles currents up
to 200mA. Higher voltage signals may be
switched using isolated drive circuits.
For applications where size is critical,
high performance Zetex SOTFETs can be
used. The SOT23 ZVN4106F will produce
an analog switch possessing an
on-resistance of only 4Ω.
DN3-1
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