CHAPTER 20 OPERATIONAL AMPLIFIERS OPAMPS Introduction to operational
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CHAPTER 20 OPERATIONAL AMPLIFIERS (OP-AMPS)
Introduction to operational amplifiers n Symbol and Terminals
n. Op-Amp Characteristics
The Ideal Op-Amp n The ideal op-amp has ¨ Infinite voltage gain ¨ Infinite bandwidth ¨ Infinite input impedance(open) ¨ Zero output impedance
The Practical Op-Amp n The practical op-amp has: ¨ high voltage gain ¨ wide bandwidth ¨ high input impedance(open) ¨ low output impedance
Comparison between ideal and non -ideal op-Amp n n n n n Ideal Op-AMP Infinite Gain Infinite Bandwidth Infinite Input Resistance Zero Output Resistance Zero Noise No Temperature Sensitivity Zero Power Consumption Perfectly Linear … … n n n n n Non-Ideal Op-AMP 100, 000 0 -MHz 100 M Ohm 50 Ohm Microvolt micro. V/C m. Watt 0. 1% nonlinearity …
A simple op-amp arrangement
The differential amplifier 2 sets of Common Emitter Amplifier (mirror)
Operational Mode n Single-Ended Input n Differential Input n Common Mode Input
n Single-Ended Input
Single-Ended Input ¨ Either input is shorted to ground ¨ Output of non-zero input ¨ Amplified ¨ different phase from input ¨ Output of zero input ¨ Mirror of output of non-zero input ¨ But same phase as input
n Differential Input
The differential amplifier (con’t) Differential Input
Differential Input n Both inputs ¨ Same Magnitude and frequency ¨ But different phase n To analyze output ¨ Use superposition n n Consider each input at a time Output Voltage ¨ Each output is summed of the results of both inputs
n Common Mode Input
The differential amplifier (continue) Common-Mode Input
Common Mode Input n The most widely used n Inputs ¨ Same Magnitude and frequency ¨ Same phase n To analyze output ¨ Use superposition n Consider each input at a time n Output Voltage ¨ Zero output n Result from input 1 is canceled by the result from input 2
Common-Mode Rejection Ratio n The measure of an amplifier’s ability to reject common-mode signals is a parameter called the common-mode rejection ratio (CMRR) is the differential gain is the common-mode gain
OP-AMP PARAMETERS n Input offset voltage (Vos) is the differential dc voltage required between the inputs to force the differential output to zero volt
OP-AMP PARAMETERS n Input Offset Current is the difference of the input bias currents, expressed as an absolute value
OP-AMP PARAMETERS n Input bias current is the average of both input currents and is calculated as follow:
OP-AMP PARAMETERS n Input impedance ¨ The differential input impedance ¨ Common-mode input impedance
OP-AMP PARAMETERS n The output impedance is the resistance viewed from the output terminal of the opamp
OP-AMP PARAMETERS The common-mode input voltage range is the range of input voltages which will not cause clipping or other output distortion n Open-Loop Voltage Gain, Aol n Open-loop voltage gain is the internal voltage gain of the device and represents the ratio of output voltage to input voltage when there are no external components, and it can range up to 200, 000 and is not a well controlled parameters
Slew rate n n The maximum rate of change of the output voltage in response to a step input voltage is the slew rate of an op-amp The slew rate is dependent upon the frequency response of the amplifier stages within the op-amp Slew rate =∆Vout/∆t
NEGATIVE FEEDBACK n n When negative feedback is present, the noninverting and inverting inputs are nearly identical Without negative feedback, an extremely small difference in the two input voltage drives the op -amp to its output limits and it becomes nonlinear
OP-AMP configurations with negative feedback n Closed-Loop Voltage Gain, Acl , is the voltage gain of an op-amp with negative feedback
Op-Amp Applications n Comparator n Non-inverting Amp. n Inverting Amp. n Summing Amp.
Comparator n Output depends on both input voltages ¨ if V+ > Vn Vout = Vs+ -> +5 V ¨ Else if V+ < Vn Vout = Vs- -> -5 V
Noninverting amplifier
Inverting Amplifier
Summing Amp.
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