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The Operational Amplifier continued Note: By summing point constraint, we have Vi = 0 and ii = 0. 1 The Operational Amplifier continued The voltage follower provides unity gain, however, the output impedance is changed according to the o/p impedance of the opamp, which is very useful sometimes. Thus the op-amp acts as the buffer stage preventing the o/p load fluctuations to affect the i/p voltage signal. 2 Non-inverting amplifier design: Effects of resistance choice Av = 10 = 1+R2/R1 For v0 = 10 V, output current is 1A Most op amps cannot handle such large current, so small R’s should be avoided Very large resistances tend to be unstable, and lead to coupling of unwanted signals especially at higher frequencies. Why? 3 Op-Amp Imperfections in the linear range of operations Input and Output Impedances An Ideal op amp has infinite input impedance and zero output impedance A Real op amp has finite input impedance and nonzero output impedance For IC op amps made of BJTs open-loop input impedance is about 1 MW For IC op amps made of JFETs open-loop input impedance is about 1012 W Open loop output impedance is between 1 and 100 W Closed loop impedances will be different, and can be chosen by proper resistors Characteristics of two popular Op Amps 4 Op-Amp Imperfections in The Linear Range of Operations Gain and Bandwidth Limitations Ideal op amps have infinite open-loop gain magnitude (AoL is infinite), but the gain of a real op amp is finite and a function of frequency dc open-circuit differential voltage gain is typically between 104 to 106 The bandwidth is usually limited by the designer to prevent oscillations from feedback, by a process is called frequency compensation The open loop gain function of an op-amp usually has a single dominant pole and is given as: AOL (f) A0OL fBOL AOL (f) - open-loop gain as function of frequency - dc open-loop gain - open-loop break frequency - constant up to fBOL then it rolls off at 20 dB/decade 5