578
21 Diodes
Rectifiers generally have slow switching speeds. A significant time delay arises from the necessary
charge-carrier recombination when the diode switches from low (forward) to high (reverse) impedance.
This poses typically no problem for line-frequency (50–60 Hz) applications. For fast applications,
however, the minority carrier lifetime needs to be reduced, e.g. by doping Si with Au (see Sect. 10.9).
21.5.2 Frequency Mixing
The nonlinear characteristic of the diode allows the mixing of frequencies, e.g. for second- (or higher-)
harmonic generation, upconversion or demodulating of radio-frequency (RF) signals. A single balanced
mixer is shown in Fig. 21.64a, b. The RF signal consists of a RF carrier frequency f 0 modulated with
an intermediate frequency (IF) signal f IF (t). It is mixed with a local oscillator (LO) that has a constant
frequency f LO outside the RF modulation bandwidth f 0 ± f IF . The IF signal can be detected from the
setup in Fig. 21.64a if filtered through a low-pass filter to avoid loss of power to the IF amplifier. The
temperature dependence of the diode characteristic (via j s and β) on mixing efficiency is typically less
than 0.5 dB for a 100-K change in temperature.
Problems of single-diode mixers are the radiation of local-oscillator power from the RF input port,
25
loss of sensitivity by absorption of input power in the local oscillator circuit, loss of input power in
the intermediate frequency amplifier, and the generation of spurious output frequencies by harmonic
mixing. Some of these problems can be solved by circuit techniques, but these circuits often introduce
new problems. Most mixers therefore use multiple-diode techniques to provide a better solution of
these problems. In Fig. 21.64c, the circuit diagram of a double balance mixer is shown. Even-order
harmonics of both the LO and the signal frequency are rejected. This mixer does not require a low-pass
filter to isolate the IF circuit. The three ports are isolated from each other by the symmetry of the
circuit. These mixers usually cover a broader frequency band than the others. Ratios as high as 1000:1
are available. Microwave equivalents (working at f 1 GHz) of such mixer circuits are available.
Bandwidth ratios as high as 40:1 are available at microwave frequencies in MMICs (millimeter-wave
integrated circuits).
The common drawback of MMIC diodes is that they are obtained from the Schottky barriers used in
field-effect transistors, that have inferior performance compared to discrete diodes. The use of pHEMT
technology
26 for millimeter-wave applications provides diodes that differently from regular Schottky
diodes, since they consist of a Schottky barrier in series with a heterojunction. In Fig. 21.64d, a MMIC
45 GHz mixer is shown using fast GaAs-based pHEMTs.
21.5.3 Voltage Regulator
In a voltage regulator, the large variation of resistance with applied bias is used. This effect occurs in
the forward direction and close to the breakdown voltage.
In Fig. 21.65a, a simple circuit is shown. When the input voltage V in is increased, the current
increases. The preresistor R 1 = 5 k and the load resistor represent a voltage divider with V in =
I R 1 + V out . The total current I is given by the two currents through the diode and the load resistor
I = I s [exp(βV out /n) − 1] + V out /R L . Therefore, the output voltage is implicitly given by
25 That in military applications could make the mixer detectable by the enemy.
26 Pseudomorphic high electron mobility transistors, cf. Sect. 24.5.8.
21 Diodes
Rectifiers generally have slow switching speeds. A significant time delay arises from the necessary
charge-carrier recombination when the diode switches from low (forward) to high (reverse) impedance.
This poses typically no problem for line-frequency (50–60 Hz) applications. For fast applications,
however, the minority carrier lifetime needs to be reduced, e.g. by doping Si with Au (see Sect. 10.9).
21.5.2 Frequency Mixing
The nonlinear characteristic of the diode allows the mixing of frequencies, e.g. for second- (or higher-)
harmonic generation, upconversion or demodulating of radio-frequency (RF) signals. A single balanced
mixer is shown in Fig. 21.64a, b. The RF signal consists of a RF carrier frequency f 0 modulated with
an intermediate frequency (IF) signal f IF (t). It is mixed with a local oscillator (LO) that has a constant
frequency f LO outside the RF modulation bandwidth f 0 ± f IF . The IF signal can be detected from the
setup in Fig. 21.64a if filtered through a low-pass filter to avoid loss of power to the IF amplifier. The
temperature dependence of the diode characteristic (via j s and β) on mixing efficiency is typically less
than 0.5 dB for a 100-K change in temperature.
Problems of single-diode mixers are the radiation of local-oscillator power from the RF input port,
25
loss of sensitivity by absorption of input power in the local oscillator circuit, loss of input power in
the intermediate frequency amplifier, and the generation of spurious output frequencies by harmonic
mixing. Some of these problems can be solved by circuit techniques, but these circuits often introduce
new problems. Most mixers therefore use multiple-diode techniques to provide a better solution of
these problems. In Fig. 21.64c, the circuit diagram of a double balance mixer is shown. Even-order
harmonics of both the LO and the signal frequency are rejected. This mixer does not require a low-pass
filter to isolate the IF circuit. The three ports are isolated from each other by the symmetry of the
circuit. These mixers usually cover a broader frequency band than the others. Ratios as high as 1000:1
are available. Microwave equivalents (working at f 1 GHz) of such mixer circuits are available.
Bandwidth ratios as high as 40:1 are available at microwave frequencies in MMICs (millimeter-wave
integrated circuits).
The common drawback of MMIC diodes is that they are obtained from the Schottky barriers used in
field-effect transistors, that have inferior performance compared to discrete diodes. The use of pHEMT
technology
26 for millimeter-wave applications provides diodes that differently from regular Schottky
diodes, since they consist of a Schottky barrier in series with a heterojunction. In Fig. 21.64d, a MMIC
45 GHz mixer is shown using fast GaAs-based pHEMTs.
21.5.3 Voltage Regulator
In a voltage regulator, the large variation of resistance with applied bias is used. This effect occurs in
the forward direction and close to the breakdown voltage.
In Fig. 21.65a, a simple circuit is shown. When the input voltage V in is increased, the current
increases. The preresistor R 1 = 5 k and the load resistor represent a voltage divider with V in =
I R 1 + V out . The total current I is given by the two currents through the diode and the load resistor
I = I s [exp(βV out /n) − 1] + V out /R L . Therefore, the output voltage is implicitly given by
25 That in military applications could make the mixer detectable by the enemy.
26 Pseudomorphic high electron mobility transistors, cf. Sect. 24.5.8.