RF Electronics Chapter 10: Operational Amplifiers Page 350
2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0.
the potentiometer has a 23.9 dB attenuation, that corresponds to an input voltage of 78 V
p
.
If needed the OpAmps can be protected using RF Schottky barrier diodes connected
between the input and the power supplies, as shown in figure 10.12. The input voltage
should not exceed the supply voltages (±5 V). The MMSD301T1, RF Schottky barrier
diodes used in the figure 10.12, have a maximum forward voltage of 0.6V at 10 mA. Thus
5.6 V at the junction of Rs and Rsd, will result in the diode conducting and ensuring the
voltage there is clipped. Using the series resistor Rsd = 10 kΩ, results in the maximum
voltage at the OpAmp input then being 4.88 V, so that the OpAmp input voltage can never
exceed the supply voltage. The insertion of the resistor Rsd, requires Rd to be changed
from 219.2 Ω to 217.2 Ω in order for the whole amplifier to have a 0dB gain.
Figure 10.12. Voltage sensor circuit with input protection diodes.
The, MMSD301T1 [7] diode has a 200 mA current rating, a 30 V reverse voltage rating
and a 1.2 pF diode capacitance at V
r
= 5 V. That capacitance is not properly included in
the linear AWR DE simulation of figure 10.13, so that has to be added as a separate
capacitance in figure 10.12. Figure 10.13 shows a comparison in the frequency response
of the amplifier of figure 10.4 without the protection diodes and the amplifier of figure
10.12 which includes the protection diodes. The difference in gain is 6 dB at 2 MHz.
Figure 10.13. Comparison of amplifiers of figure 10.4 and 10.12.
Figure 10.13 shows that the input protection significantly affects the high frequency
performance. For most RF amplifiers input protection is difficult. The cost of protection
RF Electronics: Design and Simulation
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