Class B Amplifier

[lonemanoj]

What is class B amplifier and where it is used ?

[studiot]

In Class A amplifiers each output device is conducting for the entire cycle of the signal (ie both positive and negative half cycles of the sine wave)

Class A amplifiers may have one or two devices in the output stage.

 

In Class B amplifiers each output device is conducting for exactly half the cycle of the signal, alternately.

Class B amplifiers have two devices in the output stage.

So one device conducts during the positive half cycle and the other device conducts during the negative half cycle, whilst the first device is non conducting or off.

 

In Class C amplifiers each output device is conducting for less than a half cycle so there are periods of non conduction between positive and negative half cycles.

The output waveshape is an amplified copy of the input for the conducting periods.

Class C is used in radio transmitter amplifers.

 

In Class D amplifiers each output device is conducting for less than a half cycle and the wave shape is a pulse, independent of the input.

[lonemanoj]

How to find overall efficiency of Class B Amplifier ?

[studiot]

Do you have pay per letter in your posts?

[lonemanoj]

I don't understand.

[studiot]

 

How to find overall efficiency of Class B Amplifier ?

 

 

Was that all you had to say to the effort I put in on your behalf?

[lonemanoj]

You interpreted it wrong. I meant to say what's the efficiency of Class B Amplifier? For this you'll have to explain me its waveform and calculate the efficiency with and without distortion.

[studiot]

http://forum.allaboutcircuits.com/threads/class-b-push-pull-amplifier.85631/#post-615183

[Enthalpy]

Efficiency of a class B amplifier:

- On an existing one, measure the output power, the consumed power, compute the ratio.

- On a design, evaluate the otuput voltage without distortion, the current consumed everywhere, compute.

- In an abstract case, a class-B amplifier has a maximum efficiency, but real amplifiers don't come very near to this limit. See below.

 

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Note 1: less than the maximum output power reduces identically the current in the load and consumed from the supply, but because the voltage at the load diminishes while the supply voltage uses to be constant, this reduces the efficiency. So the best efficiency is at maximum power.

 

Note 2: Power and efficiency relate to a sinusoidal waveform despite sounds are no sines; an other shape would give different figures. The absence of distortion on the sine gives one output power and efficiency. Accepting some distortion permits more output power and a better efficiency; manufacturers may give figures for this second case.

 

Note 3: loudspeakers (and sonar transducers, and many more) have an extremely variable impedance which is mainly reactive. To be comparable, the measures are made with the nominal output load, for instance 4 ohm or 8 ohm, perfectly resistive.

 

Note 4: the power supply wastes power as well, so one should tell if the efficiency is from plug to load, from DC supply to load, or more bizarre.

 

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For the maximum efficiency, take a push-pull output supplied with +V and -V (an H bridge would have the same efficiency) that drives a purely resistive load R.

 

The output components (usually transistors) give at each instant of the waveform the proper voltage to the load and drop the rest - this is what class D would improve. The corresponding load current is drawn from the supply at each instant.

 

The (conventionally sine) output waveform gets V as a peak, so the rms load voltage is V/sqrt(2) and the rms load power is 0.5*V²/R.

 

The (sine) load current comes from the power supply, it's V/R at the sine's peak. Because the DC supply delivers current at a constant voltage, the drawn power depends on the mean current, which is 2/pi * V/R, and the drawn power is 2/pi * V²/R. The +V rail gives this power for one half-wave, the -V rail for the other half-wave, so over the complete wave, the drawn power is (2/pi) * (V²/R) as well.

 

The maximum efficiency of the class-B amplifier is the ratio between both, or 0.5/(2/pi) ~ 78%.

 

A real amplifier drops voltage in the output transistors even at peak output voltage and consumes current elsewhere, so figures like 60% would be very decent - for the undistorted sine wave in a resistive load, and so on. The power supply would also waste a lot; it was generally not regulated to improve this, and presently is rather a switched-mode one to save transformer size.

[lonemanoj]

Very well explained. Thanks Enthalpy for your detailed explanation. This will help me a lot in my studies