Op Amp is a high-gain electronic component mainly used to amplify voltage signals. It is a differential amplifier and the output depends on the voltage difference between the two inputs (positive+and negative −). The operational amplifier has the characteristics of high gain. Under ideal circumstances, the open-loop gain is very high (theoretically close to infinity). When the input impedance is high, it almost absorbs the input current and avoids interference with the front-end circuit. When the output impedance is low, it can directly drive the post-stage circuit, and it can implement double input and single output. Output=gain × (positive input - negative input).
Common Operational Amplifier Applications and Types
Common applications of operational amplifiers include voltage amplifiers, filters (low-pass, high-pass, bandpass), signal comparators (related to comparators), integrators and differentials, buffers (voltage follower), analog calculations (addition, subtraction, integration, etc.). Common circuits include inverter amplifiers, with input connected at the inverter end and reverse amplification function, and in-phase amplifiers, with input connected at the positive end and output and input in the same phase. In voltage follower circuit, positive phase input=output, providing impedance conversion without voltage amplification.
Examples of In-Phase Amplifier Circuits
Examples of In-Phase Amplifier Circuits
The in-phase amplifier circuit in the above figure is taken as an example. The closed-loop gain is determined by the feedback resistance Rf and the voltage divider Rg. The input signal and output signal of the in-phase amplifier are in the same phase.
Examples of Inverted Amplifier Circuits
Examples of Inverted Amplifier Circuits
Take the inverter amplifier circuit in the above figure as an example. Assuming that this amplifier circuit uses an ideal amplifier, the closed-loop gain is determined by the feedback resistance Rf and the input resistance Rin. The phase difference between the input signal and the output signal of the inverter amplifier is 180 degrees.
Design of adjustable linear regulated power supply with operational amplifier
The objective of the adjustable linear regulated power supply is to provide a stable and adjustable output voltage, and the output remains stable even if the input voltage or load changes. The basic structure of the adjustable linear regulated power supply includes a reference voltage source (such as TL431, zener diode or precision reference IC), an error amplifier (operational amplifier), a regulating component (usually power BJT or MOSFET), a feedback resistor voltage divider network (setting the output voltage).
Example of an adjustable linear regulated power supply circuit
Example of an adjustable linear regulated power supply circuit
Taking the adjustable linear power supply circuit in the above figure as an example, the core of this circuit is composed of LM358, regulator diode, triode and negative feedback circuit, R9 and D9 constitute a voltage stabilizing circuit. The breakdown voltage of D9 is 2.5V. Due to the high input impedance of the operational amplifier, it does not need a voltage stabilizing diode to provide much current. At this time, the IN1+of the operational amplifier is 2.5V. The operational amplifier, triode, R12 and RP3 form a negative feedback loop. The calculated voltage range should be between 2.5V and 15V. Since the actual power supply voltage of the operational amplifier is ± 12V, it is known from the data table that the output swing of the operational amplifier relative to the power rail is 1.35V to 1.61V. The maximum Vce voltage of D882 is 0.5V. The calculated maximum output range of Vout should be between 9.89V and 10.15V. Therefore, the actual output voltage range should be between 2.5 V and 10.15 V.
Attention shall be paid to the stability of reference voltage when designing the adjustable linear regulated power supply circuit. Low temperature drift and high stability reference source (such as TL431 or LM4040) shall be used. When selecting the type of operational amplifier, the output voltage range shall cover the output end (rail-to-rail), with low offset voltage and low drift characteristics. When selecting the type of power components, proper BJT or MOSFET shall be selected according to the output current to ensure its heat dissipation and safe working range. Thermal protection and stability shall also be paid attention to. For high current, the heat sink shall be used, and RC compensation shall be considered to avoid oscillation. For the setting of feedback impedance, too high R1 and R2 resistance values (within a range of several k Ω is recommended) shall be avoided to improve the stability and anti-noise capability. and the input voltage needs to be higher than the maximum output voltage+VCE (saturated voltage drop) or Vds (MOSFET), Current sampling resistance and secondary comparator can be added to realize overcurrent protection function.
This design has the characteristics of fine adjustment of output voltage, high heat generation of power transistor, low efficiency (linear characteristics), low noise and fast response, only applicable to the situation where the input voltage is higher than the output voltage, simple structure, easy integration, and protection mechanism and good heat dissipation are required for high power application.