Operational amplifiers are very common electronic parts, so here we will not consider the special requirements of amplification and noise factor in the selection, but only discuss the design and use of common operational amplifiers and express our personal views.
There are some basic design units in any complex circuit, so we will start with the role of the balancing resistor here, and will not rule out the possibility of adding other contents later on according to questions and queries.
The role of the balancing resistor
1、 Make the line symmetrical.
- Counteracts input leakage currents.
3、 To improve the reliability of the output state.
4、 Improving the response speed of the input when used at high frequencies.
Balanced resistor with op-amp as open loop with no feedback
The above diagram shows the op-amp design with balanced resistors. The purple line is the output state, as I haven’t installed the simulation software yet, I’ll go ahead and replace the output waveform with a colour line. The overhanging resistors indicate the output internal resistance of the previous stage.
When the op-amp is first powered up, the internal resistance of the input is close to infinity, so the output state will be very random, and may be high or low. This is not necessarily the case in a well-designed circuit, as the previous stage will give some conditions to keep the op-amp in a relatively fixed state. For example, when the input is a bridge with a sensor, etc., unconnected or disconnected in suspension, the output will be more random.
As shown in the diagram after increasing the positive connection of the balance resistor, the most basic effect is that the in-phase end must be positive, the anti-phase end must be negative, can ensure that the output must be high potential. The reverse connection has the opposite output state.
Direct input high and low levels at the inputs
The internal resistance of the op-amp input is close to infinity, so it can be connected in the form of the above diagram, which can be considered as a level comparator. Here to mention the “false break”, because the input exists in a false break state, so will not form a short-circuit current.
Added balancing resistors
The voltage at the two ports of the op-amp input is always close to each other in the high resistance state. Here we also have to mention the “false short”, because of the false short state at the input, the voltage at the same direction and the reverse side will be close to 0V under the action of the balancing resistor, only the same direction is more positive and the reverse side is more negative. So you can think of the isotropic terminal as +0V and the reverse terminal as -0V.(Note that the power supply is ±5V)
The op-amp input is only close to infinity, there will still be a very small current present, usually a few dozen microamps or so.
5V ÷ 100k = 50 microamps
So the value of 100k can be interpreted as exactly offsetting the leakage current at the input of the op-amp, so that the input signal to be amplified can remain linear. So the voltage at the input in the above diagram is very close to 0V, the leakage current is large, the voltage is lower, the leakage current is small is higher, in the pre-stage output internal resistance, this voltage can be ignored. (The error value between the two resistors is too large or cannot offset the leakage current will cause the output to wobble)
Op-amp recommended connection method for forward modulation and reverse modulation
Note in the diagram above that the bridge voltage should not be connected to the op-amp’s power supply with a magnetic 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, it is good to know the role of some necessary devices, the reason why you see some circuit design is very complex, it is because what you see is already the result of the design, rather than the initial draft of the design, so you will not be able to quickly understand the superimposed composite function, you need to work with the whole design function before the principle can be analysed layer by layer.
Generally symmetrical appearance will be called balanced, otherwise this resistor can be called pull-up, pull-down resistor, or forward and reverse bias resistor. In the op-amp design will often use common mode this way, that is, the same direction end and the reverse end of the design circuit is completely symmetrical, and even wiring device arrangement to maintain consistent and symmetrical, the biggest advantage of doing so is that the common mode rejection efficiency will be very high, can improve the stability of the op-amp.
