tailieunhanh - Ebook The fundamentals of electrical engineering: Part 2

Part 2 book "The fundamentals of electrical engineering" includes content: Operational amplifier, time domain circuit analysis, building blocks, AC power, oscillating circuits, semiconductor devices, circuit simulation. | 6 Operational amplifier Operational amplifier The voltage controlled voltage source was introduced in the chapter . Let s recall this circuit and add two resistors to the circuit like shown in Fig. . The voltage controlled voltage source amplifies the voltage different U U1 - U2 by amplification factor A. Between U1 and U2 there is an open load R Ω . There is one resistor R1 in one of the input terminal paths and a feedback resistor R2 from one output terminal to the same input terminal. For simplicity we set U2 as the reference point 0 V and also refer the output voltage Ua to this reference point. Fig. Voltage controlled voltage source with feedback loop via resistor R2. What about the output voltage Ua Is it affected by the resistors How and why According to KCL the currents I1 and I2 sum to zero at input terminal U1. With Ohm s law KCL can be written as Using the amplification of the voltage source Ua -A U yields Due to the feedback loop the output voltage depends not only on the amplification factor A but also from the resistors R1 and R2. This voltage controlled voltage source with an infinite input resistance Re and open loop gain A where Ua A U2 - U1 A positive or negative is called ideal voltage amplifier. As Re is infinite no current will enter the input terminals and as the output is an ideal voltage source Ua is driven by the amplifier regardless of load connected to the output. Terminal U2 labeled with is called the non-inverting input and terminal U1 labeled with is called the inverting input. Fig. Circuit and model of an ideal voltage amplifier with gain A. Considering a very high amplification factor A the output voltage becomes independent from A and is just determined by the ratio of the resistors The output voltage is inverted compared to the input voltage and amplified by the ratio of the two resistors. The ideal amplifier can be used to obtain a defined amplification due to the external resistors R1 and R2. In case of A this