amplifiers. basic amplifier concepts ideally, an amplifier produces an output signal with identical...
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Amplifiers
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BASIC AMPLIFIER CONCEPTS
Ideally, an amplifier produces an output signal with identical waveshape as the input signal, but with a larger amplitude.
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Inverting Amplifiers
Inverting amplifiers have negative voltage gain, and the output waveform is an inverted version of the input waveform.
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Non-inverting Amplifiers
Non-inverting amplifiers have positive voltage gain amplify the input signals.
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Voltage-Amplifier Model
Ri: input resistance Ro: output resistanceAvo: Open loop voltage gain ( vo / vi )
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Voltage-Amplifier Model
Ri: input resistance Ro: output resistanceAvo: Open loop voltage gain ( vo / vi )
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Voltage-Amplifier Model
Ri: input resistance Ro: output resistanceAvo: Open loop voltage gain ( vo / vi )
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Current Gain
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Power Gain
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CASCADED AMPLIFIERS
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Avo=Avo1*Avo2=200*100=20000Not agree with the calculationWhy? As Ro1≠0, Ro2 ≠0
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Desirable output resistance as smallas possible.
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Operational Amplifier
1. Ideal Op-Amp and its analysis2. Practical Op-Amp and its limitations3. Application of Op-Amp
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IDEAL OPERATIONAL AMPLIFIERS
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Power Supply Connection of Op-amp
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Characteristics of Ideal Op Amp
Infinite gain for the differential input signal
Infinite input impedance
Zero output impedance
Zero gain for the common-mode input signal
Infinite bandwidth
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OP-Amp Model
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Ideal OP-Amp•Rin = ∞, so that it will not draw any power fromthe input signals•Rout = 0so that it will not degrade the signal due to the output resistance•Avd = ∞ it is to amplify the differential signals•Avcommon = 0 it is to reject any common mode input signalsBandwidth = ∞ so that it can be used for any signal spectrum
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i1
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Ideal op-amp rule1. No current ever flows into either input terminal. i1, i2 = 0
2. There is no voltage difference between the two input terminals v- = v+
We call this Summing Point Constraint
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Ideal Op-Amp
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Negative Feedback Effect• The effect of the feedback connection from the
output to the inverting input is to force the voltage at the inverting input to be equal to that at the non-inverting input.
v- = v+It is called ;• summing point constraint, or • virtual ground concept
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Illustration of the principle of summing point constraint
As i- and i+ are both zero, then, i1 = i2
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INVERTING AMPLIFIERS
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Practical Design Difficulty
Design an inverting amplifier with gain -100,R1 = 50K, then R2 = 5M , too much for real practical resistor
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Av = -100, R1 = 50K
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NON-INVERTING AMPLIFIER
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NON-INVERTING AMPLIFIER
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Voltage Follower
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Differential Amplifier
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Common Mode Rejection
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An op-amp is a differential amplifier. It is desirable to rejectany signal in common to V_ and V+ terminal. In other words, Acm should be as small as possible. The quality of rejecting the common mode signal is defined byCMMR (Common mode rejection ratio)
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Common Mode Rejection CMMRv1= 2 + 3 sin10tVv2= 2V
The common component of the two input signal is 2V.
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It is desirable for the amplifier to amplify the differenceof v1 and v2, that is 3 sin10t, and not to amplify the common component 2V.
How good the amplifier does to reject the common component is defined by the CMMR.
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OP-AMP IMPERFECTIONS IN THE LINEAR RANGEOF OPERATION
Real op amps have several categories of imperfections compared to ideal op amps.
Real op amps have finite input impedance, nonzero output impedance and finite open loop gain
Ri ≠ ∞, Avo ≠ ∞, Ro ≠ 0iin ≠ 0
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Bandwidth
Bandwidth = fH-fL
Idea op-amp, the bandwidth is infinity, so that signal at anyfrequency can be amplified by the amplifier.
Practical op-amp, the bandwidth is limited. That is, the gainis not uniform.
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The gain at frequency higher than the fBOL is diminished graduallyat a -20dB rate of decline. The unit bandwidth product is to define how good is the frequency response of the amplifier, i. e, how wide is it bandwidth.Unity bandwidth product = Avo*fBOL
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LINEAR WAVEFORM DISTORTION
If the gain of an amplifier has a different magnitude for the various frequency components of the input signal, a form of distortion known as amplitude distortion occurs. Due to bandwidth limitation.
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Phase Distortion
If the phase shift of an amplifier is not proportional to frequency, phase distortion occurs.
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NONLINEAR LIMITATIONS
The output voltage of a real op amp is limited to the range between certain limits that depend on the internal design of the op amp. When the output voltage tries to exceed these limits, clipping occurs.
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Slew-Rate Limitation
Another nonlinear limitation of actual op-amp is that the magnitude of the rate of change of the output voltage is limited.
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DC IMPERFECTIONS