Switch driver circuits

Revised January 24, 2007

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Here we will compare driver circuits for PIN diodes with those needed for switch FETs. The driver circuit that is needed for PIN diodes almost precludes the use of this technology on volume-limited applications such as T/R modules , where element spacing is less than the width of available diode driver circuits. Because of the complexity of designing a high-speed current driver circuit, the FET switch typically has an advantage in switching speed as well.

PIN diode switch driver examples

PIN diode driver circuits are a pain to design and build. There are some great vendors out there that can supply you with many types of PIN switch drivers (commercial, military), but they will remain nameless until they pay for an ad on this page! (Contact us if you really want to know...)

National Semiconductor offers a monolithic solution for driving PIN diodes, DH0035. We have no experience with it.

FET switch driver examples

Remember that the FETs used in switches are depletion-mode devices. Typical switch FETs need zero volts to turn on, and -5 volts to turn off (to be pinched off). You can use certain types of silicon logic to drive switch FETs as described below, but you will have to play one of two games to get the negative voltage to the switch FET. You can operate the logic using a negative supply, by grounding the V+ pin and connecting the normally grounded pin(s) to a negative voltage. You will eventually have to deal with the problem somewhere a logical interface, or you will have to operate all of your system logic using negative voltage. The second method is to "float" the switch by biasing the sources to a positive voltage. A negative voltage across VGS will result when the control voltage is brought low. More on floating a switch later...

CMOS-type gates are often used to drive FET switches. The HCT logic family makes a good choice, such as the 74HCT04 hex inverter, available at DigiKey. This device switches in about 10 nanoseconds, and the gates can be daisy-chained to provide the complimentary signal voltages that many FET switches require. The feature that makes the HCT series a good bet for switching FETs is that the outputs operate almost to the voltage rails, which will give you the best switching performance. Note that HCT only operates reliably at 5 volts, so don't use this stuff if your switch needs seven volts to switch. If you need higher voltage levels, try the CD4041 quad complimentary buffer. The CD family can operate up to 15 volts. However, switching time is on the order of 100 nanoseconds.

Positive voltage biasing for FET switches

Ordinarily a FET switch will need control voltages of 0 and -5 volts. "Floating" a FET switch to change to positive logic can be done in two ways. Perhaps the more elegant solution is to bypass the FETs onboard the MMIC chip, so that the FETs are RF grounded but a positive DC voltage is injected as shown below. The second method is to float the entire switch. For a surface mount switch, this means that instead of grounding the device through via holes you need to RF ground it through appropriate capacitors.

Either method of floating a switch adds an additional terminal to a switch, and a high-pass response results from the DC blocking capacitors that are required.

Need some figures here!