Transistors and Amplifiers Overview

Amplifiers and Amplification

• The most effective device for weak-signal amplification is the field-effect transistor.
• Sensitivity is the specification that denotes the effectiveness of a weak-signal amplifier, defined as the number of microvolts of signal input that produce a certain ratio of signal output to noise output (usually 10 to 1).

Power Amplifiers

• Power amplifiers are used in wireless transmitters, broadcast transmitters, and hi-fi audio equipment.
• The most frequently-used device for power amplification is the bipolar transistor, but vacuum tubes are becoming increasingly popular, especially among musicians.
• Power output is measured in watts or kilowatts, and efficiency is the ratio of signal power output to total power input (wattage demanded of the power supply or battery).
• Efficiency is typically expressed as a percentage, ranging from 30 to 50 percent in audio applications and 50 to 70 percent in wireless communications and broadcasting transmitters.

Ideal Amplifier

• An ideal amplifier will pass the input signal through to the output undistorted but enlarged (gain set by user), with no delay.
• An ideal amplifier includes all signal-processing functions associated with such signals.

Linear Electronics

• Linear electronics is a category that includes all signal-processing functions associated with electronic devices.
• Amplification is one of the most important applications of linear electronics.
• Nonlinear circuits are also classified under linear electronics.

Amplifier

• An amplifier uses a small input signal to create a large output signal, while minimizing distortion of the waveform.
• Amplifiers are used for both electrical and optical signals.

Weak-Signal Amplifiers

• Weak-signal amplifiers are used primarily in wireless receivers, acoustic pickups, audio tape players, and compact disc players.
• They are designed to deal with exceedingly small input signals, in some cases measuring only a few nanovolts (units of 10-9 volt).
• Weak-signal amplifiers must generate minimal internal noise while increasing the signal voltage by a large factor.

Decibel Power Gain

• The decibel power gain GdB in decibels is defined by: GdB = 10 log10G = 10 log10 (Po/Pi).
• A factor of 10 appears in decibel computation involving power gain, and a factor of 20 appears when voltage and current gains are used.

Cascaded Amplifiers

• When two or more stages are connected in cascade, decibel measures are particularly convenient to use in dealing with combinations.
• The net decibel gain is the sum of the individual decibel gains for the different stages.

Decibel Voltage Gain and Loss

• The net voltage gain is determined in absolute form as the product of all the individual gains.
• A negative voltage gain should not be confused with negative linear gain.
• Negative gain corresponds to the reduction of the signal level through the circuit, while negative linear gain corresponds to an inversion of the polarity.
• Voltage gains and their equivalents can be represented in decibel form.