tailieunhanh - Introduction to Electronics - Part 8

Nonideal Operational Amplifiers Ngoài ra các bộ khuếch đại điện áp hoạt động, có các bộ khuếch đại hoạt động hiện tại và các bộ khuếch đại transconductance hoạt động (OTAs). Thảo luận này được giới hạn trong các bộ khuếch đại điện áp. | Nonideal Operational Amplifiers Introduction to Electronics 193 Nonideal Operational Amplifiers In addition to operational voltage amplifiers there are operational current amplifiers and operational transconductance amplifiers OTAs . This discussion is limited to voltage amplifiers. Linear Imperfections Input and Output Impedance Ideally Rin and Rout 0. Realistically Rin ranges from 1 MQ in BJT op amps to 1 TQ in FET op amps. Rout ranges from less than 100 Q in general purpose op amps to several kQ in low power op amps. Gain and Bandwidth Ideally Av and BW . Realistically Av ranges from 80 dB 104 to 140 dB 107 . Many internally-compensated op amps have their BW restricted to prevent oscillation producing the Bode magnitude plot shown Fig. 275. Typical op amp Bode magnitude response. The transfer function then has a single-pole low-pass form A A s A s 1 291 2nfb And gain-bandwidth product is constant f Afb Af 292 Nonideal Operational Amplifiers Introduction to Electronics 194 Nonlinear Imperfections Output Voltage Swing BJT op amp outputs can swing to within 2 VBE of VSUPPLY . FET op amp outputs an swing to within a few mV of VSUPPLY . Output Current Limits Of course currents must be limited to a safe value. Some op amps have internal current limit protection. General purpose op amps have output currents in the range of tens of mA. For examples the LM741 has an output current rating of 25 mA while the LM324 can source 30 mA and sink 20 mA. Slew-Rate Limiting This is the maximum rate at which vO can change dVo dt SR. It is caused by a current source driving the compensation capacitor. As an example the LM741 has a SR of V is. Fig. 276. Illustration of op amp slew-rate limiting. Nonideal Operational Amplifiers Introduction to Electronics 195 Full-Power Bandwidth This is defined as the highest frequency for which an undistorted sinusoidal output is obtainable at maximum output voltage v0 t Vom sinft dVo dt ftVOM 2nfVOM 293 max Solving for f and giving it a special