21.5 Applications and Special Diode Devices
579
(a)
(b)
(c)
(d)
Fig. 21.64 a Schematic circuit of single balanced mixer with input (RF: radio frequency, LO: local oscillator) and
output (IF: intermediate frequency). b Optical plan-view image (300 × 125 µm 2 ) of a high-speed single balanced mixer
with two GaAs Schottky diodes with opposite poling. The device properties are R s = 5 , for I = 1 µA a forward and
reverse voltage of 0.7 and 6 V, respectively; the capacitance of each diode is 8 fF. Reprinted with permission from [1792].
c Schematic circuit of a double balanced mixer. d Optical image (1.65 mm 2 ) of 40–45 GHz MMIC (Gilbert cell) mixer
on GaAs basis using pHEMTs. Reprinted with permission from [1793]
V out
1 +
R 1
R L
= V in − R 1 I s
exp
βV out
n
− 1
.
(21.173)
A large current change is related to a fairly small change of the voltage across the diode, which at the
same time is the output voltage. Therefore, a change in the input voltage causes only a small change
in the output voltage.
We assume a diode with n = 1 and I S = 10
−14 A with the characteristic shown in Fig. 21.65a.
The numerical example in Fig. 21.65c is calculated for R L = 2 k and 4 k, respectively. The output
voltage varies by about 0.02 V if the input varies between 5 and 9 V. In Fig. 21.65d, the differential
voltage change α =
V in
V out
∂ V out
∂ V in
is shown.
In this way, voltage peaks can be filtered from the input voltage. If two antiparallel diodes are used,
this principle works for both polarities. Instead of a diode in the forward direction, the very steep slope
of the diode I –V characteristic at the breakdown can be used. Just before breakdown, the diode has
a high resistance and the voltage drops at the load resistor. If the input voltage increases a little, the
diode becomes conductive and shorts the additional voltage (the maximum allowed breakdown current
needs to be observed!). Due to its small saturation current, typically Si diodes are used. The breakdown
voltage can be designed via the diode parameters. Such diodes with defined breakdown voltage are
called Z- or Zener diodes (see next section).
If the breakdown is due to tunneling (avalanche multiplication), the breakdown voltage decreases
(increases) with temperature. If two diodes with positive and negative temperature coefficient are put
in series, a very good temperature stability of the breakdown voltage of 0.002%/K can be achieved.
Such diodes can be used to realize a reference voltage.
579
(a)
(b)
(c)
(d)
Fig. 21.64 a Schematic circuit of single balanced mixer with input (RF: radio frequency, LO: local oscillator) and
output (IF: intermediate frequency). b Optical plan-view image (300 × 125 µm 2 ) of a high-speed single balanced mixer
with two GaAs Schottky diodes with opposite poling. The device properties are R s = 5 , for I = 1 µA a forward and
reverse voltage of 0.7 and 6 V, respectively; the capacitance of each diode is 8 fF. Reprinted with permission from [1792].
c Schematic circuit of a double balanced mixer. d Optical image (1.65 mm 2 ) of 40–45 GHz MMIC (Gilbert cell) mixer
on GaAs basis using pHEMTs. Reprinted with permission from [1793]
V out
1 +
R 1
R L
= V in − R 1 I s
exp
βV out
n
− 1
.
(21.173)
A large current change is related to a fairly small change of the voltage across the diode, which at the
same time is the output voltage. Therefore, a change in the input voltage causes only a small change
in the output voltage.
We assume a diode with n = 1 and I S = 10
−14 A with the characteristic shown in Fig. 21.65a.
The numerical example in Fig. 21.65c is calculated for R L = 2 k and 4 k, respectively. The output
voltage varies by about 0.02 V if the input varies between 5 and 9 V. In Fig. 21.65d, the differential
voltage change α =
V in
V out
∂ V out
∂ V in
is shown.
In this way, voltage peaks can be filtered from the input voltage. If two antiparallel diodes are used,
this principle works for both polarities. Instead of a diode in the forward direction, the very steep slope
of the diode I –V characteristic at the breakdown can be used. Just before breakdown, the diode has
a high resistance and the voltage drops at the load resistor. If the input voltage increases a little, the
diode becomes conductive and shorts the additional voltage (the maximum allowed breakdown current
needs to be observed!). Due to its small saturation current, typically Si diodes are used. The breakdown
voltage can be designed via the diode parameters. Such diodes with defined breakdown voltage are
called Z- or Zener diodes (see next section).
If the breakdown is due to tunneling (avalanche multiplication), the breakdown voltage decreases
(increases) with temperature. If two diodes with positive and negative temperature coefficient are put
in series, a very good temperature stability of the breakdown voltage of 0.002%/K can be achieved.
Such diodes can be used to realize a reference voltage.