here to go to our main page on microwave switches
New for February
2011! We have more discussion of "floating FETs for positive
bias control", see below.
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.
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.
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.
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 (0 and +5 volts) can be done
in two ways. One solution is to bypass the individual FETs and add
DC blocking caps to the RF terminals. This can be done either on-board
the chip (the most elegant solution) or off-board. The result is
that the FETs are RF grounded but a positive DC voltage is injected.
This is illustrated in the figure below; we borrowed it from M/A-COMs
app note S2079, Drivers for GaAs FET Switches And Digital Attenuators.
There a funny mistake on this app note figure which we must point
out after all these years... The 5VDC connection to FETs Q2 and
Q3 is on the wrong side of capacitor C1. Doh!
A friend asked us a great question
from this schematic. If you actually hooked it up like this, would
it work? The concern is that when Q1 and Q3 are off, Q2 would have
no path to the bias voltage, and similarly, when Q2 and Q4 and off,
Q3 also has no reference. You know what? It will work anyway!
Here's what happens, Grasshopper:
when a FET is OFF, it is still slightly ON. Indeed, a 1 mm FET might
leak 1 mA when pinched off at -5V. When you observe the IV curves
you would not notice this unless you zoomed in. If it is leaking
1 mA, it behaves like a 5K resistor. This is more than enough conductivity
to bleed off any trapped charge on the "isolated" FETs
Q2 and Q3 when you switch back and forth, but if you were looking
for the fastest switching speed, you might want to put a voltage
on R1 but hook it to the correct side of C1.
Time for two more Microwaves101
Rule of Thumb!
You should plan on the off-state resistance
to somewhere between 5000 ohm-mm and 50,000 ohm-mm. In most designs
you can just ignore it, but in the example we just cited, it's important.
While were on the subject...
The minimum size for a gate choke resistor is on the order of 500
ohms. Many designers use thousands of ohms, this merely slows down
the switch. The gate is already (at least partially) decoupled from
the RF without the resistor! Some day we'll add an analysis to back
up this bold statement... For very high speed, you can eliminate
the gate choke resistor by using a low impedance bias network (quarterwave
stub terminated in a capacitor for example).
The second method to achieve
positive bias control 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 an appropriate capacitor(s).
You will still need DC blocks on the RF ports.
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.