class B amplifier

class B amplifier

[‚klas ′bē ′am·plə‚fī·ər]
An amplifier in which the grid bias is approximately equal to the cutoff value, so that anode current is approximately zero when no exciting grid voltage is applied, and flows for approximately half of each cycle when an alternating grid voltage is applied.
A transistor amplifier in which each transistor is in its active region for approximately half the signal cycle.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

amplifier classes

Analog amplifiers are cataloged by how much current flows during each wave cycle. Measured in degrees, 360º means current flows 100% of the time. The more current, the more inefficient and the more heat generated.


Class A
The amplifier conducts current throughout the entire cycle (360º). The Class A design is the most inefficient and is used in low-power applications as well as in very high-end stereo. Such devices may be as little as 15% efficient, with 85% of the energy wasted as heat.

Class B
The current flows only 180º for half the cycle, or two transistors can be used in a push-pull fashion, each one operating for 180º. More efficient than Class A, it is typically used in low-end products.

Class AB
Combines Class A and B and current flows for 180º to 200º. Class AB designs are the most widely used for audio applications. Class AB amplifiers are typically about 50% efficient.

Class C
Operating for less than half of one wave cycle (100º to 150º), Class C amplifiers are the most efficient, but not used for audio applications because of their excessive distortion.

Class G
A variation of the Class AB design that improves efficiency by switching to different fixed voltages as the signal approaches them.

Class H
An enhancement of the Class G amplifier in which the power supply voltage is modulated and always slightly higher than the input signal.

Current Flowing
The red indicates how much of the time current is flowing through one wave cycle.


Class D
Class D is a digital-like amplifier that works by turning a transistor fully on or off, but the "D" technically does not stand for digital. See Class D amplifier.

Class T
A variation of the Class D technique from Tripath. Class T modulates the pulses based on the individual characteristics of the output transistors.
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References in periodicals archive ?
At low power, the carrier amplifier sees a 2[R.sub.opt] load ([R.sub.opt] is the optimum load impedance for a class B amplifier) and operates as a class B amplifier.
The RF performances of the Doherty amplifier-I (a combination of a class B carrier amplifier and a bias-tuned class C peaking amplifier) have been compared with those of a class B amplifier alone.
TABLE I EXPERIMENTAL CONDITIONS INCLUDING OPTIMIZED BIASES AND CLASSES OF OPERATION OF THE AMPLIFIERS Comparison I Doherty Amplifier I Class B Amplifier Carrier amplifier - class B [] = 3.6 V ([] = 3.6 V) Peaking amplifier - class C ([] = 1.5 V) Comparison II Doherty Amplifier II Class AB Amplifier Carrier amplifier - class AB [] = 4.17 V, ([] - 4.17 V) [I.sub.dsQ] = 70 mA Peaking amplifier - class C ([] = 2.8 V)
Third-order inter-modulation distortion (IMD3), fifth-order intermodulation distortion (IMD5) and PAE for the Doherty-I and class B amplifiers, and for the Doherty-II and class AB amplifiers versus average output power are compared in Figures 9 and 10, respectively.
The adjacent channel leakage ratio's (ACLR) at 885 kHz offset and PAEs versus average output power for the Doherty-I and class B amplifiers, and of the Doherty-II and class AB amplifiers, are compared using a CDMA signal (forward-link, chip rate of 1.2288 Mcps and peak-to-average ratio of 11 dB).
The simple class B amplifier shown in Figure 1 is modeled by a symbolically defined device (SDD), which enables its input/output relationships to be to specified.
If a varying power waveform is applied to the class B amplifier, the device only amplifies through half of the cycle.
For class B amplifiers, where the DC power supply depends on the RF output, the results are better than class A amplifiers, reaching a theoretical maximum at [pi]/4, or 78 percent.
Looking at these drain source currents for the first time, it is somewhat surprising to see how similar the waveforms are, given that the class B amplifier only conducts current during half of the RF input cycle while the class A amplifier conducts during the entire cycle.
Because both Class AB and Class B amplifiers are much more efficient than Class A, they can be smaller and lighter weight, but their outputs must be protected from excessive power dissipation.