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Studying structure of differential operational amplifiers and instrumentation amplifiers


Some engineers emphasize infinite gain of an ideal op amp. When analyzing an op amp, they primarily focus on virtual opens and shorts, ignoring some important concepts such as common mode rejection, bias voltage, and bias current.

1. Op-Amp Input ModelAccording to op-amp model, basic op-amp model is analyzed comprehensively: it is a differential mode signal and a common mode overlay.

Second, concept of imaginary short circuit An ideal op-amp should pay attention to virtual open and imaginary short circuit. The non-inverting and inverting inputs of op-amp are equal. The open-loop gain of an ideal op amp is infinite, but it's actually quite a bit less, in most cases by a factor of about 100 dB (100,000) times. A voltage difference of 30UV is required at common mode and phase reversing input terminals. Yes, if interference signals such as ripple and noise are added, there is little to no change at same and opposite phase terminals. Feedback is introduced to create a closed loop, and voltage difference between same and opposite phase terminals is ignored. 3. Differential mode input and common mode inputIn an application, an op amp can input a differential mode signal or a common mode signal, a common mode signal. this is due to noise, and basic vision is that in-phase mode is canceled and differential mode is boosted.

4. Input voltage range (Vin or Vcm)

The input range of an op-amp is relatively complex. Theoretically, analog input of a non-inverting and inverting terminal can be made between positive and negative rails of power supply. Top and bottom Op-amp valves are roughly symmetrical Most of time, op-amp's common-mode input voltage Vcm is 1/2 Vdd. So op amp basically operates in linear region.

5. Weak signal detection methodWhen an op-amp is used to collect a weak current signal, question often arises how to collect signal, whether to use high-frequency side current detection or low side current detection Edge current detection problem.

6. Introduction to differential amplifierBecause sensor signal is mainly output by applying a voltage difference, signal voltage difference is very small, and electromagnetic interference caused by layout and wiring will occur And common mode interference, temperature drift and others Problems. The non-inverting terminal and inverting terminal of op-amp are considered as a compartment, as long as sensor signal is fed in middle, relative noise will be much smaller. There is a pressure difference in sensor signal to avoid abnormal saturation of operational amplifier and introduce a differential amplifier. For cost reasons, most designs in industry use conventional op amps and differential amplifier is based on a subtractor model. The principle of a differential amplifier is like looking into a mirror. In physics, this is called mirror reflection. He pays attention to symmetry and balance. Only when two sides are exactly same, effect will be best. To do this, engineers need to perform impedance matching on analog front end. And because each point's reference sources are different and impedance has errors, it's often very difficult to complete an impedance match. The figure below shows a classical differential operational amplifier. Outputting noiseless voltage Uoz, KCL is used to solve non-inverting input and inverting input impedance, and the results are very different.

The following is a method for determining value of each resistor in above figure. First, according to mirror image principle, bias current is also boosted according to same factor, and ratio between four resistors can be obtained; to determine R1, you need to check There are several restrictions on op-amp, resistance must match: more than instantaneous output voltage / maximum output current, less than input bias voltage / input bias current, and attention to influence of thermal noise and so on. 7. Introduction to Instrumentation AmplifiersDifferential amplifiers can handle most analog input stages, but due to system's limited input impedance, complex matching circuits must be added. When accuracy of peripheral resistance and line impedance of PCB will arise new problems. In order to solve low input impedance problem of differential op amps, mainstream manufacturers have done a lot of optimization, and some of them use dual op amp method, as shown in figure below, to achieve instrumentation gain.

The dual op-amp has two disadvantages: it does not support unity gain and relatively weak common-mode rejection of various frequencies. Therefore, many manufacturers use three op amp method. Instrumentation amplifiers produced by many major manufacturers are also based on principle of three operational amplifiers.