11

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Music for your ears

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Musique 101

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Audio Spectrum

4

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Figure 1.21 Typical magnitude response of an amplifier. |T(v)| is the magnitude of the amplifier transfer function—that is, the ratio of the output Vo(v) to the input Vi(v).

Amplifiers have limited bandwidth frequency response of Amplifiers

Constant gain between w1 & w2 (Bandwidth) ; otherwise lower gain. Amp chosen so its BW coincides with required spectrum to be amplified … otherwise signals distorted

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Figure 1.11 (a) A voltage amplifier fed with a signal vI(t) and connected to a load resistance RL. (b) Transfer characteristic of a linear voltage amplifier with voltage gain Av.

Voltage Amplifier - Transfer characteristicCan also have circuit Amps

input

output

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Figure 4.26 (a) Basic structure of the common-source amplifier. (b) Graphical construction to determine the transfer characteristic of the amplifier in (a).

MOSFET AMP common source Graphical method Using load line :: slope is 1/RD

For any value of VI = VGS

Locate corresponding ID – VDS curve Find vo

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Figure 4.26 (Continued) (c) Transfer characteristic showing operation as an amplifier biased at point Q.

MOSFET Amp. Biased at QXfer characteristicinverting

Similar to switch characteristic

Biased at point Q

Vi superimposed on VIQ

Vi small linear amp operation VO proportional to Vi

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Figure 4.27 Two load lines and corresponding bias points. Bias point Q1 does not leave sufficient room for positive signal swing at the drain (too close to VDD). Bias point Q2 is too close to the boundary of the triode region and might not allow for sufficient negative signal swing.

MOSFET as linear Amp – use saturation seg.How to select Bias point

Q1 bias point• Not enough +ve swing• Too close to Vdd

Q2 bias point• Too close to triode• Insufficient –ve swing

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Figure 1.12 An amplifier that requires two dc supplies (shown as batteries) for operation.

Some Amplifiers Require 2 suppliesEg 2 supply +ve & -ve swings

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Figure 1.13 An amplifier transfer characteristic that is linear except for output saturation.

Amplifier transfer

characteristics

Amps have limitations .. may

saturate ..signal (2)

Amp linearity desiredVout(t) = A * Vin(t)

input

output

Two power supplies used

L+ >= A*vi

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Amplifier Biasing – ensures linearity

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Figure 1.14 (a) An amplifier transfer characteristic that shows considerable nonlinearity. (b) To obtain linear operation the amplifier is biased as shown, and the signal amplitude is kept small. Observe that this amplifier is operated from a single power supply, VDD.

Nonlinear response

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Figure 1.15 A sketch of the transfer characteristic of the amplifier of Example 1.2. Note that this amplifier is inverting (i.e., with a gain that is negative).

Inverting Amplifier biasing example Top limit = 10v, lower limit = 0.3v

Output 180 degrees out of phase with input

L- = 0.3vVt = 0.690

L+ =~ 10v@ Vt = 0

For 5 V bias@ Vo = 5v Vt = 0.673

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Figure 1.17 (a) Circuit model for the voltage amplifier. (b) The voltage amplifier with input signal source and load.

Voltage Amplifier Circuit Model used for simulation, circuit analysis

Gain = Aw; input resistance = Ri, Output resistance = Ro

vo = Aw * vi * RL/(RL + Ro) ; effect of output resistance Ro vo / vi = Aw * RL/(RL + Ro) ; voltage gainvi = vs * Ri / (Ri + Rs) ; effect of Ri

vo / vs = Aw * Ri / (Ri + Rs) * RL/(RL + Ro); overall voltage gain accounting for input / output impedances

Inp. resistance Out. resistance

Inp. source, / Rs Load RL

9/1415Figure 1.18 Three-stage amplifier for Example 1.3.

More Gain ? Use Cascaded stages Ex 1.3Input stage needs high input impedance

Output stage needs low output impedanceInput resistance of a stage = load resistance of previous stage

Vi1 / Vs = 1 M / ( 1M + 100k) = 0.909 ; effect of 1st RinAv1 = Vi2 / Vi1 = 10 * 100k / (100k + 1k) = 9.9 ; gain 1st stage, A = 10Av2 = Vi3 / Vi2 = 100 * 10k / (10k + 1k) = 90.9 ; gain 2nd stage, A = 100Av3 = VL / Vi3 = 1 * 100 / ( 100 + 10) = 0.909 ; gain 3rd stage, A = 1Av = VL / Vi1 = 9.9 * 90.9 * 0.909 = 818 ; 3 stage gainVL / Vs = 818 * 0.909 = 743.6 ; from source to loadIdeal Gain = 10 * 100 = 1000

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Figure 1.21 Typical magnitude response of an amplifier. |T(v)| is the magnitude of the amplifier transfer function—that is, the ratio of the output Vo(v) to the input Vi(v).

Amplifiers have limited bandwidth frequency response of Amplifiers

Constant gain between w1 & w2 (Bandwidth) ; otherwise lower gain. Amp chosen so its BW coincides with required spectrum to be amplified … otherwise signals distorted

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Figure 1.27 Use of a capacitor to couple amplifier stages.

Capacitively Coupled Amplifier Stages

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An NMOS Common Source Amplifier

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Figure 4.26 (a) Basic structure of the common-source amplifier. (b) Graphical construction to determine the transfer characteristic of the amplifier in (a).

Amp. Circuit Xfer Characteristic

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NMOS common source Amp. Cont’d Xfer Characteristic

1919Figure 4.26 (Continued) (c) Transfer characteristic showing operation as an amplifier biased at point Q.

Biased at point Q

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Figure 4.63 Capture schematic of the CS amplifier in Example 4.14.

Pspice Amplifier Example