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References:

A.R. Hambley, Electronics, © Prentice Hall, 2/e, 2000

A. Sedra and K.C. Smith, Microelectronic Circuits, © Oxford University Press, 4/e, 1999

Introduction to Amplifiers

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The Electronic Amplifier

Figure 1.16 Input waveform and corresponding output waveforms.

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Amplifier’s Waveforms

Fig. 1.13 An amplifier transfer characteristic that is linear except for output saturation. 4

Amplifier’s Transfer Function (TF): Ideal Case

Fig. 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.

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Amplifier’s TF: Non-ideal Case

Figure 1.22 The power supply delivers power to the amplifier from several constant voltage sources.

Where does amplification come from ?

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Figure 1.23 Illustration of power balance.

No free lunch !!!

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Amplifier models

Voltage amplifier Current amplifier Transconductance amplifier Transresistance amplifier

For a given amplifier, a particular model may be preferable.However, any of the four can be used to model the amplifier

Figure 1.17 Model of an electronic amplifier, including input resistance Ri and output resistance Ro.

Voltage Amplifier Model

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Figure 1.25 Current-amplifier model.

Current Amplifier Model

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Figure 1.28 Transconductance-amplifier model.

Transconductance Amplifier Model

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Figure 1.30 Transresistance-amplifier model.

Transresistance Amplifier Model

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Figure 1.32 If we want to sense the open-circuit voltage of a source, the amplifier should have a high input resistance, as in (a). To sense short-circuit current, low input resistance is called for, as in (b).

Voltage and Current Sources.Practical Considerations: Rin

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Figure 1.33 If the amplifier output impedance Ro is much less than the (lowest) load resistance, the load voltage is nearly independent of the number of switches closed.

Practical Considerations: Ro

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Figure 1.34 To avoid reflections, the amplifier input resistance Ri should

equal the characteristic resistance Zo of the transmission line.

Effect of interconnections

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Keeping AC (signal) and DC (power supply) away from each other

vs

Rsloadamplifier

DC

Figure 1.39 Gain versus frequency for a typical amplifier showing the upper and

lower half-power (3-dB) frequencies (fH and fL ) and the half-power bandwidth B.

A Typical Amplifier’s Frequency Response

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Decibel Notation

Power Gain Voltage Gain Current Gain

Figure 1.19 Cascade connection of two amplifiers.

Multistage Amplifiers

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How do we model a multiple stage amplifier?

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Figure 1.37 Capacitive coupling prevents a dc input component from affecting the first stage, dc voltages in the first stage from reaching the second stage, and dc voltages in the second stage from reaching the load.

Coupling multistage amplifiers

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Figure 1.38 Capacitance in parallel with the signal path and inductance in series with the signal path reduce gain in the high-frequency region.

Parasitic effects

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Figure 1.36 Gain versus frequency.

Amplifier’s frequency response

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Figure 1.40 Gain magnitude versus frequency for a typical bandpass amplifier.

A band pass amplifier

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Figure 1.41 Input pulse and typical ac-coupled broadband amplifier output.

Effect of having a band limited frequency response

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Figure 1.42 Rise time of the output pulse. (Note: No tilt is shown. When tilt is present, some judgement

is necessary to estimate the amplitude Vf).

Zooming-in the output pulse

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Figure 1.43 Differential amplifier with input sources.

The Differential Amplifier

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Why do we need differential amplifiers ?

Figure 1.47 Setup for measuring differential gain. Ad = vo/vid.

Differential Gain

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Figure 1.46 Setup for measurement of common-mode gain.

Common-mode Gain

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