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