Switch
driver circuits
Revised January
24, 2007
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here to go to our main page on microwave switches
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!
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