What role does the negative feedback resistor provided by electronic
enthusiasts play in the op amp circuit? The method of connecting RF resistors in
parallel with equivalent capacitors. High-power resistors used in RF and
microwave frequency bands are mostly used in Wilkinson power dividers or
combiners. For best performance, the 100 ohm isolation resistor used in the
Wilkinson power splitter must have a small equivalent capacitance to reduce the
impact on insertion loss. Additionally, if isolation resistors are used in a
Wilkinson combiner, they need to absorb half of the input power at each input
port. What is the role of negative feedback resistor in op amp circuit?
When the internal resistance of the signal source is large, adding a
feedback resistor with the same resistance as the internal resistance of the
signal source can reduce the output offset voltage and improve the following
accuracy.
The ideal closed-loop gain of both voltage followers is equal to unity.
In a voltage follower, the influence of common mode rejection ratio will be
enhanced. In addition, not connecting a resistor between the non-inverting
terminal and the signal source is beneficial to reducing the steady-state
error.
However, when the matching resistance is zero, the feedback resistance is
required to be zero. When blockage occurs, the current in the feedback loop is
large, which is not conducive to the protection of the input stage. Therefore,
care should be taken during use.
A follower with a feedback resistor has a certain current limiting
protection effect on the circuit when the circuit is blocked. This is its
advantage. But the steady state error has increased a bit.
【Note】What is blockage?
The voltage follower is originally a non-inverting operational amplifier.
One of the common characteristics of non-inverting operational amplifiers is
that a common mode voltage is added to the non-inverting and inverting
terminals.
Methods of connecting radio frequency resistors in parallel with equivalent
capacitors
High-power resistors used in RF and microwave frequency bands are mostly
used in Wilkinson power dividers or combiners. For best performance, the 100 ohm
isolation resistor used in the Wilkinson power splitter must have a small
equivalent capacitance to reduce the impact on insertion loss. Additionally, if
isolation resistors are used in a Wilkinson combiner, they need to absorb half
of the input power at each input port.
Separate resistors are often used in designing high power attenuators. At
low frequencies, this is feasible; however, at high frequencies, the parasitics
of the separate resistors can cause the attenuator to behave worse than
expected.
High power resistors come in different shapes and sizes. The most common
types of applications are: surface mount resistors, resistors with leads (with
or without insulating housing), resistors with leads and insulating housing, and
mounted on conductive flanges. The shapes of various high power resistors are as
follows:
Specifications of high power resistors
The main parameters of high-power resistors include: resistance value,
maximum power capacity (mostly at 100 degrees temperature), power-temperature
curve and mechanical dimensions. In addition, the maximum or typical equivalent
capacitance value also needs to be provided in some cases.
These two parameters, resistance and maximum power, are generally
relatively well-defined and useful to designers. Relatively speaking, the
parameter equivalent capacitance is relatively vague. In most cases,
manufacturers do not provide the frequency at which the equivalent capacitance
is measured and which test method is used.
There are many types of equivalent capacitance. The parallel equivalent
capacitance refers to the capacitance formed by the radio frequency scattering
field between the resistive film and the ground plane (Figure 2). Other
equivalent capacitances, such as those between the input and output pads, are
generally not considered as important because they have less impact on practical
applications, especially at low frequencies.
So far, there is no standard for testing the equivalent capacitance of high
power resistance above 1MHz. According to the MIL-STD202G standard, the
recommended test frequencies for equivalent capacitance are: 60Hz, 120Hz, 1KHz,
10KHz and 1MHz.
Some people have pointed out that the equivalent capacitance measured at
1MHz must meet the MIL standard, but this capacitance information is of no use
to designers who want it to work at 2.7GHz. The same situation applies to base
station products operating in the GHz frequency range.
Test methods and extraction of equivalent capacitance
When high-power resistors are used in radio frequency and microwave
frequency bands, they have the property of damaging transmission lines. Figure 3
is a lumped element model of a resistor and its high-frequency equivalent
circuit model. The parallel capacitance in Figure 3 can be extracted by testing
the S parameters. However, the type of test equipment, calibration technique,
and dielectric constant of the material will all affect the test results.
In order to demonstrate the extraction process of equivalent capacitance
more vividly, we used a 500W 50-ohm resistor with leads to create a sample as
shown in Figure 4. The establishment of the reference plane in Figure 3 is
achieved through the calibration and settings of the test instrument.
Test data and modeling data
In order to verify the parameter extraction process, we used
MicrowaveOffice to build an EM resistance model. As shown in Figure 5, the S
parameters obtained by EM analysis and the S parameters obtained by testing are
marked together on an original Smith diagram. As can be seen from the figure,
below 2.7GHz, the tested data and modeled data have a good correlation.
Calculation of equivalent capacitance
In order to obtain the parallel equivalent capacitance of the resistor, we
need to use the value b, whose value at any frequency can be obtained directly
from the diagram we gave as an example.
For example, as follows, we can find from the green line (test data): at
2.3GHz, b=1.083. Plugging this into the formula below gives us a value of 1.5pF
for the equivalent capacitance.
If we follow the red line (modeled data) to get b, and then recalculate the
equivalent capacitance at 2.3GHz, it is 1.55pF. These two data again show a good
correlation between the test data and the modeled data.
In addition to the calculation of a single frequency point, we show the
frequency sweep test results of parallel capacitors in Figure 6. The results
also show that the S parameters obtained by the two methods are highly
correlated.
High power resistors are widely used in power distribution circuits. The
parallel equivalent capacitance (parallel end to ground) due to RF scattered
fields between the resistive film and ground becomes an important design
parameter. Typical values of equivalent capacitance are generally given measured
at a frequency of 1MHz. However, in the GHz frequency band, a high-frequency
equivalent capacitance value will be more meaningful for design.
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