Amplifiers

BASIC AMPLIFIER CONCEPTS Ideally, an amplifier produces an output signal with identical waveshape as the input signal, but with a larger amplitude.

Inverting Amplifiers Inverting amplifiers have negative voltage gain, and the output waveform is an inverted version of the input waveform.

Non-inverting Amplifiers Non-inverting amplifiers have positive voltage gain amplify the input signals.

Voltage-Amplifier Model R i: input resistance R o : output resistance A vo : Open loop voltage gain ( v o / v i )

Voltage-Amplifier Model R i: input resistance R o : output resistance A vo : Open loop voltage gain ( v o / v i ) 1.It will ensure v s is not degraded. 2.It enhances the power efficiency as limited power is drawn from the signal source.

Voltage-Amplifier Model R i: input resistance R o : output resistance A vo : Open loop voltage gain ( v o / v i ) A zero output resistance will maintain the gain.

Current Gain

Power Gain

CASCADED AMPLIFIERS

Avo=Avo1*Avo2=200*100=20000 Not agree with the calculation Why? As Ro1≠0, Ro2 ≠0

If R o1 =R o2 =0 Desirable output resistance as small as possible.

Operational Amplifier 1. Ideal Op-Amp and its analysis 2. Practical Op-Amp and its limitations 3. Application of Op-Amp

IDEAL OPERATIONAL AMPLIFIERS

Power Supply Connection of Op-amp

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

OP-Amp Model

Ideal OP-Amp R in = ∞, so that it will not draw any power from the input signals R out = 0 so that it will not degrade the signal due to the output resistance A vd = ∞ it is to amplify the differential signals A vcommon = 0 it is to reject any common mode input signals Bandwidth = ∞ so that it can be used for any signal spectrum

Ideal op-amp rule 1. No current ever flows into either input terminal. i 1, i 2 = 0 2. There is no voltage difference between the two input terminals v - = v + We call this Summing Point Constraint

Ideal Op-Amp A vc is the closed loop gain

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

Illustration of the principle of summing point constraint As i- and i+ are both zero, then, i 1 = i 2

INVERTING AMPLIFIERS

Practical Design Difficulty Design an inverting amplifier with gain -100, R1 = 50K, then R2 = 5M, too much for real practical resistor

Vx

Av = -100, R1 = 50K

NON-INVERTING AMPLIFIER

Summer

Voltage Follower

Differential Amplifier

Common Mode Rejection An op-amp is a differential amplifier. It is desirable to reject any signal in common to V _ and V+ terminal. In other words, A cm should be as small as possible. The quality of rejecting the common mode signal is defined by CMMR (Common mode rejection ratio) 

Common Mode Rejection CMMR v1= sin10tV v2= 2V The common component of the two input signal is 2V. It is desirable for the amplifier to amplify the difference of 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.

OP-AMP IMPERFECTIONS IN THE LINEAR RANGE OF 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 R i ≠ ∞, A vo ≠ ∞, R o ≠ 0 i in ≠ 0

Bandwidth Bandwidth = f H -f L Idea op-amp, the bandwidth is infinity, so that signal at any frequency can be amplified by the amplifier. Practical op-amp, the bandwidth is limited. That is, the gain is not uniform.

The gain at frequency higher than the f BOL is diminished gradually at 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 = A vo *f BOL

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.

Phase Distortion If the phase shift of an amplifier is not proportional to frequency, phase distortion occurs.

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.

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.

DC IMPERFECTIONS