Balancing resistors have the following main roles in electronic circuits: to make the line symmetrical: balancing resistors can make the input side of the circuit symmetrical, thus reducing the error caused by asymmetry.

Offset the input leakage current: balancing resistors help to offset the input leakage current, reducing the error caused by leakage current.

Improvement of output state reliability: The use of balancing resistors improves the reliability of the output and reduces state instability due to unbalance.

Improvement of Input Response Speed: Balancing resistors can improve the response speed of the input when used at high frequencies.

Eliminate the effect of static base current on the output voltage: Balancing resistors can eliminate the effect of static base current on the output voltage and ensure the normal operation of the circuit.

Examples in Practical Applications

The role of balancing resistors is especially important in operational amplifier circuits. Operational amplifiers usually operate in closed loop (negative feedback), and the introduction of a balancing resistor ensures that the DC resistance to ground of the two inputs is equal, thus reducing the out-of-phase voltage and error caused by imbalance. For example, in in-phase proportional arithmetic circuits, balancing resistors can help maintain “false short” and “false break” conditions to ensure proper operation of the circuit.

Balancing Resistors and Other Components

Balancing resistors are often used in conjunction with feedback resistors to affect the performance of operational amplifiers. By choosing the values of these resistors appropriately, the response speed and stability of the circuit can be optimized. In addition, the use of balancing resistors can help eliminate the effect of static base current on the output voltage, ensuring the accuracy and reliability of the circuit.

I. The Role of Balancing Resistors

During signal transmission, the effects of common-mode interference signals, such as power supply clutter and ground interference, may occur. In order to offset the impact of these interference signals, the need to ensure the balance of the signal, even if the signal appears in the common mode interference signal, but also be able to eliminate it.

Balancing resistors are set up to ensure the balance of the signal, and can eliminate the interfering signals on the signal transmission line by directing them to ground. The role of the balance resistor is to produce a fixed potential in the differential mode signal, so that the signal has a common reference line, which is conducive to the amplifier amplification of the differential mode signal.

Calculation method

The size of the balancing resistor needs to be calculated according to the actual use. Generally used in parallel with the two ends of the way, that is, the balancing resistor is connected in series between the output of the signal source and the input of the amplifier, and will be connected to the ground at both ends. The specific calculation methods are as follows:

1. determine the resistance value of the balancing resistor according to the type and amplitude of the differential mode signal, e.g., for common differential mode signals, a resistance value of 10-100 ohms can be used.

2. according to the impedance size of the amplifier to determine the power size of the balancing resistor, the power size should not be less than the maximum input power of the amplifier's input port.

3. Adjust the resistance value of the balancing resistor according to the length of the wiring and other parameters in order to obtain the best balance of the signal.

Balancing resistor function

1. Make the line symmetrical. 

2. To offset the input leakage current.

3、 Improve the reliability of output terminal.

4. Improve the response speed of the input when using high frequency.

Balancing resistor, op-amp for open loop without feedback

op-amp design with balancing resistor, the purple line is the output state, because I haven't installed the simulation software, I first use the color line instead of the output waveform. The hanging resistor indicates the output internal resistance of the previous stage.

When the op-amp is powered up for the first time, since the internal resistance of the input is close to infinity, the state of the output will be very random, it may come out high or low. This is not necessarily the case with a realistically designed circuit, because the previous circuit will more or less give some conditions that will keep the op amp in a relatively fixed state. For example, when the input is a bridge with a transducer, etc., unconnected or disconnected in a dangling position, the output will be more random.

The most basic effect of adding a positively connected balancing resistor is that the in-phase end is definitely positive and the inverted end is definitely negative, which ensures that the output must be high. The opposite is true for the inverted output.

Input directly into the high and low levels

op amp input internal resistance is close to infinity, so it can be connected to the above figure form, at this time can be considered as a level comparator. Here we should mention the “false break”, because there is a false break at the input, so there will be no short circuit current.

Increased balance resistor

op amp input two ports in the high resistance state voltage is always very close, here should also mention the “false short”, because the input side of the existence of the state of the false short, so the same end and the reverse end of the voltage in the balance of the resistor will be very close to the role of the 0V, but the same end is more inclined to the positive end of the reverse side is more inclined to the negative end. So you can think of the isotropic end as +0V and the inverse end as -0V. (Note that the power supply is ±5V)

The input of the op amp is just close to infinity, and there will still be a very small current present, usually a few tens of microamps or so.

5V ÷ 100k = 50 microamps

So the value of 100k can be interpreted as just enough to offset the leakage current at the input of the op-amp so that the input signal to be amplified can remain linear. Therefore, the above figure in the input voltage is very close to 0V, leakage current is large, the voltage is lower, leakage current is small on the higher, in the former output internal resistance, this voltage can be ignored. (Too large an error value between the two resistors or failure to offset the leakage current will cause the output to wobble)

Op amp suggested connection method for forward modulation and reverse modulation

The bridge voltage should not be connected to the op amp's power supply with a bead or for any other reason.

As a designer, I personally believe that there is no need to put too much effort on complex circuits, any complex circuit is a combination of simple circuits, can know the role of some of the necessary devices is good, the reason why I see some of the circuit design is very complex, that is because see has been the result of the design, rather than the design of the initial draft, so it can not be very quickly to understand the composite function of the superposition, you need to work with the whole design of the machine function in order to layer by layer to analyze the principle.

In the process of practical use, it is also necessary to consider the size of the input impedance of the output of the signal source and the input of the amplifier, as well as the influence of the capacitance and inductance of the signal transmission line and other parameters. The resistance value and power size of the balancing resistor can be adjusted through experimental measurements.

The purpose of the balancing resistor is to ensure the balance of the differential mode signal and eliminate the influence of the common mode interference signal. The method of calculating the balancing resistor needs to consider a series of factors such as the type of differential mode signal, the input impedance of the amplifier, and the characteristics of the signal transmission line. Through calculations and experiments, the resistance value and power size of a balancing resistor suitable for practical use can be obtained.