a very2 basic question about power amplifier class B

[marinara27]

hi all..
im running on simulation on different classes of power amplifier..im really confuse about this..in logical power amplifier amplify power right..? is it logic if power amplifier do not amplify voltage? for example, complementary quasi power amplifier?

another question is, if complementary quasi combine with common emitter circuit, is it classified as power amplifier class b ?
so confusing...

 

[betwixt]

I'm not sure about the first question but I think you are confused about amplifier classes. They are nothing to do with the circuit configuration, they are about how each cycle of the signal is carried.

Class A - The amplifier output stages conduct over the whole signal cycle
Class B - they conduct over half the signal cycle
Class C - they conduct over less than half the signal cycle.

There are other classes too but those are the basic analog ones.

Brian.

 

[marinara27]

betwixt...
what about class AB?
so sorry..im so confused..
right now im working on designing all this classes power amplifier circuit..
thanxs for your information..

 

[betwixt]

Class AB is just a combination of Class A and Class B.

In pure Class A, the output device(s) conduct over the full signal cycle. The bias is set so about half the maximum current is passed when no signal is present. When a signal is applied, it modulates the current below and above the mid point up to a maximum swing of ground and supply (fully biased on and off). Arguably it gives lowest distortion but running the output stage at an average of half power all the time is very inefficient.

In pure Class B, the output devices only conduct for one half of the signal cycle and have no bias on them at all. It would be nice if transistor were completely linear devices but they aren't. This means that during the smallest part of the signal, the transistors will still be working in their 'turn on' region of between zero and 0.6V where very little conduction takes place. It causes what is called 'cross-over distortion', a region just above and below zero where nothing happens. As the signal increases above the transistor threshold, the transistors conduct and everything works fine. It is very efficient because with no signal present, the current consumption is zero.

Class AB is a mixture of the two. The transistors are biased to conduct enough to overcome the turn-on voltage so the cross-over distortion is eliminated but not much more. At low signal level it is effectively the same as Class A. When the signal increases, it becomes the dominant source of bias and the transistor start to conduct more, this is like Class B. So AB isn't quite as efficient as Class B but it doesn't suffer from distortion and it is still greatly more efficient than Class A.

Consider this example, these are made up numbers so don't judge me on math!

Class A - current consumption a steady 1 Amp
Class AB - small signal consumption 20mA, large signal 1 Amp
Class B - small signal consumption zero, large signal 1 Amp (but distorted at small signal)

Brian.

 

[Audioguru]

You can design a power amplifier to have a high voltage gain or a low voltage gain of only one.

The output transistors can be complementary (one NPN and one PNP) or quasi-complimentary (both output transistors are NPN but the driver transistors are NPN and PNP).

The class of the amplifier is determined on how much bias current the output transistors have which determines their average current.

A common-emitter transistor is used for voltage gain in an amplifier because the output transistors are emitter-followers that do not have voltage gain.

 

[marinara27]

if i want to analysis the performance of each classes of power amplifier, what type if performance should i determine/analysis?

 

[betwixt]

Efficiency and distortion are the two main parameters for analysis. Make sure you compare them at low signal level and at high signal level as the figures get closer together as the signal increases.

Brian.

 

[marinara27]

do efficiency has relationship with power dissipation?

 

[abbeyromy]

@marinara27
For any device, more the power dissipation less the efficiency!
For all classes of amplifier you can find the efficiency and then see which is better and also find it why is that!

 

[marinara27]

abbey..how to measure efficiency using pspice?
huu..

 

[Audioguru]

You don't use simulation to measure efficiency of an amplifier. Instead you use simple arithmatic.
Power out/power in.

 

[dedalus]

You don't use simulation to measure efficiency of an amplifier. Instead you use simple arithmatic.
Power out/power in.

Power(out)/Power(in) is power gain. Power efficiency is defined as Power(out)/Power(supply), where Power(out) - average power delivered to load, Power(supply) - average power drawn from supply.

 

[Audioguru]

The power into the input of an amplifier is nearly nothing.
Of course when we talk of efficiency the input power is from the power supply.

Rod Elliot has a 20W class-A amplifier project on his website. Its heat dissipation is 110W all the time even when it has no output.
A 20W class-AB amplifier has a heat dissipation of only 4W when it has no output.

 

[marinara27]

thank you so much..
i have another question..
can we relate the power dissipation value to calculate the efficiency?
if can, how to relate it?

 

[Audioguru]

The power dissipation is power that is wasted by making heat.
Efficiency is how much power from the power supply is used in the load, not used by heating the amplifier.

If the load gets 100W and the power dissipation is 50W of heat then the power supply is supplying 150W and the efficiency is 100/150= 67%.