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Transcript
The shaded area shows the consistency 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 The product of gain A0OL and FBOL is ft, and is called the gain-bandwidth product ft is defined as the frequency where the gain of the amplifier becomes 1 3 Gain-Bandwidth Limitations Assuming infinite input impedance and zero input current Voltage across resistor R1 (2.26) Definition of open-loop gain Hence from (2.27) Thus, Therefore, the closed-loop gain and (2.27) 4 For an ideal op amp But In the limit of AOL tending to infinity, This is the same result as before Op amp Closed-loop gain is given as, Putting We have, Defining we have and which is very similar in form to the open loop gain 5 Closed-Loop Bandwidth Substituting, fBCL A0CL = fBOL A0OL= ft Gain Bandwidth product is constant But we know that and Therefore This same formula applies to a non-inverting as well as an inverting amplifier 6 Gain Bandwidth Product Note that you are trading off the high gain of the op-amp for a higher bandwidth 7 Closed-Loop Gain Versus Frequency b A0CL A0CL (dB) fBCL 1 0.999990 0 4 MHz 9.9990 20 400 kHz 40 40 kHz 0.1 0.01 99.90 Not that at ft the gain becomes 1 8 Large Signal Operation Output Voltage Swing Output voltage cannot exceed the maximum supply voltage. It also depends on the type of opamp design to some extent. Output Current Limits Clipping of the output voltage occurs when either the output voltage or the output current reaches its limit 9 Example on output voltage clipping 10
