class-B pushpull amplifier characters?

Question

cross over distortion?

Answer 1

A pushpull amplifier has two active divices. One drives the output positive; one drives it negative and each conducts for half a cycle. Yes, there can be crossover distortion when one device turns off and the other turns on. You can fix that pretty easily with some negative feedback. Try here ...

http://en.wikipedia.org/wiki/Electronic_amplifier#Class_B_and_AB

Answer 2

OK!
In a Push Pull (PP)amplifier the output transformer's primary is center tapped for the power supply connection and the two outer halves are connected to the output tubes. The PP circuit allows the output tubes to conduct alternate cycles of the input waveform. A PP stage can be biased Class A, where current flows in both tubes for the entire input cycle (in opposite directions). Class AB is where current flows alternately in both halves, but less than a full cycle in each. And Class B is where current flows only half the time in each tube.

In Class B does not, in practice, actually move smoothly from one half of the waveform to the other half. Therefore, Class B amplifiers generally introduce crossover distortion, whereas Class A amps have no crossover distortion. The advantage of Class B is efficiency, since it draws current only as needed.

Answer 3

To amplify both the positive and negative portions of the input signal with transistors that are operating class B requires a minimum of two transistors. But in general the class of operation A, AB, B, or C signifies what portion of the input signal that an individual transistor amplifier conducts on rather than signifying the number of transistors. Originally a single stage vacuum tube amplifier operating class B was defined as an amplifier that conducts on slightly less than 180 degrees of the input signal when it`s input is a sine wave. The reasoning behind using class B operation is to avoid wasting dc power when there is no ac input to an amplifier. Simultaneous means at the same time. But for a true class B push pull operation one transistor conducts during all but the first .7V of the positive portion of the input signal and the other transistor conducts on all but the first .7V of the negative portion of the input signal. The two transistor`s collector outputs are combined to form an amplified version of the input signal with a tad of distortion near 0 Volts cross over on the ac output signal. Both transistors never conduct simultaneously in a true class B operation. There is always a short time lapse between the time that one transistor cuts off and the time that the other transistor starts conducting if both transistors are operating true class B. This may sound trivial but It might not be correct to say that one transistor is responsible for the positive half cycle of the wave form and the other transistor is responsible for the negative half cycle of the wave form. Audio produces complex wave forms and therefore I think the positive and negative portions of an audio signal might not necessarily be a half cycle. It might be more correct to use portion instead of half cycle. However I would not argue this point too stringently.

Answer 4

I agree with both above. In theory a Class-B amp would draw no current until driven by a signal. But to lower cross over distortion, an idling current is allowed through the tubes. This smooths the transition from one tube to the other.