voltage amplifier[′vōl·tij ′am·plə‚fī·ər]
An electronic circuit whose function is to accept an input voltage and produce a magnified, accurate replica of this voltage as an output voltage. The voltage gain of the amplifier is the amplitude ratio of the output voltage to the input voltage. Often, electronic amplifiers designed to operate in different environments are categorized by criteria other than their voltage gain, even though they are voltage amplifiers in fact. Many specialized circuits are designed to provide voltage amplification. See Cascode amplifier, Video amplifier
Voltage amplifiers are distinguished from other categories of amplifiers whose ability to amplify voltages, or lack thereof, is of secondary importance. Amplifiers in other categories usually are designed to deliver power gain (power amplifiers, including push-pull amplifiers) or to isolate one part of a circuit from another (buffers and emitter followers). Power amplifiers may or may not have voltage gain, while buffers and emitter followers generally produce power gain without a corresponding voltage gain. See Buffers (electronics), Emitter follower, Power amplifier, Push-pull amplifier
Transistor amplifiers, such as the junction field-effect transistor (JFET) or the bipolar junction transistor (BJT) amplifier, will not operate properly without proper gate (JFET) or base (BJT) bias voltages applied in series with the signal voltage. These bias circuits can be modeled as ideal voltage sources. The bias and signal voltages are chosen so that the total input voltage—bias plus signal—will not cut off or saturate the amplifier for any value in the range of the input signal voltage. In addition to a bias voltage source, well-designed bipolar transistor amplifiers require negative feedback at dc to protect the transistor from thermal runaway. See Bias (electronics)
To obtain high gain, cascades of single amplifier circuits are used, usually with a coupling network, actually a simple filter, inserted between the stages of amplification. One such filter is a high-pass network formed by a coupling capacitor, the output resistances of the driving stage, and the input resistance of the driven stage. Since dc voltages are blocked by the capacitor, this ac coupling permits independently setting dc bias voltages for each amplifier stage in the cascade. The coupling network also rejects signals with ac frequency components below a cutoff. The capacitor must be sufficiently large not to attenuate any of the frequencies that are to be amplified. If dc is to be amplified, a direct-coupled amplifier is required, and the design is somewhat more complicated since dc bias voltages on each transistor now cannot be set independently. See Direct-coupled amplifier
The amplifiers discussed above are called single-ended amplifiers, since their input and output voltages are referred to a common reference point which by convention is called ground. These single-ended circuits, while satisfactory for most noncritical applications, have several weaknesses which degrade their performance in high-gain, weak-signal applications. Their unbalanced construction and their use of a common ground point for return currents makes them susceptible to noise pickup.
To minimize noise on sensitive signal lines, special balanced differential amplifier circuits are often used in critical amplifier applications. Differential amplifiers are designed to have equal impedances to ground for each side of the signal line and to have an output voltage proportional to the difference of the voltages from each signal line to ground. This symmetry cancels common-mode noise voltages, voltages which tend to appear on each of the signal lines as equal voltages to ground. Proper circuit design, with attention to the symmetry of the input circuit construction, can ensure that the majority of undesired noise pickup will be common-mode noise and, hence, will be attenuated by the differential amplifier. See Differential amplifier, Instrumentation amplifier
In cases where a voltage amplifier is required for some special purpose, operational amplifiers are often used to fill the need. The operational amplifier is an integrated circuit containing a cascade of differential amplifier stages, usually followed by a push-pull amplifier acting as a buffer. The differential voltage gain of the operational amplifier is very high, about 100,000 at low frequencies, while its input impedance is in the megohm range and its output impedance is usually under 100 ohms. The amplifier is designed to be used in a negative-feedback configuration, where the desired gain is controlled by a resistive voltage divider feeding a fraction of the output voltage to the inverting input of the operational amplifier.
With needed amplification built into many integrated circuits and with the availability of operational amplifiers for special-purpose amplification needs, there is seldom a need to design and build a voltage amplifier from discrete components. See Amplifier, Operational amplifier