Ok, in this post I will teach you regarding Class-B amplifiers and how it works. So if we want to make the full power efficiency of that old Class A amplifier a whole lot better by cutting down on the power that just gets wasted as heat, we can totally redesign the power amplifier circuit.
Instead of just sticking with one transistor, we can throw in two transistors in the output stage.
This clever little setup is what we usually call a Class B Amplifier and it is also known as a push-pull amplifier configuration.
Now here’s how push-pull amplifiers work: they use a pair of “complementary” or matching transistors.
One of them is an NPN-type and the other is a PNP-type.
Both of these power transistors get the same input signal at the same time, but here’s the kicker—they’re out of phase with each other.
This means that one transistor is responsible for amplifying one half (or 180 degrees) of the input waveform cycle while the other transistor takes care of amplifying the other half (the remaining 180 degrees) of that same input waveform cycle.
Once they have done their thing, those two halves get combined back together at the output terminal.
Because of this cool arrangement, the conduction angle for this type of amplifier circuit only reaches 180 degrees which is basically 50% of the input signal.
The way these transistors alternate between pushing and pulling during those half cycles is what gives this circuit its fun name—“push-pull”.
But in the grand scheme of things, it is more widely recognized as the Class B Amplifier, just like you see illustrated below.
Class B Push-pull Transformer Amplifier Circuit (Current and Voltage Flow)
So the circuit we are looking at here is your typical Class B Amplifier setup.
It features this balanced center-tapped input transformer that does a neat trick: it takes the incoming waveform signal and splits it right down the middle into two equal halves.
These halves are 180 degrees out of phase with each other which is pretty cool.
Then there is another center-tapped transformer at the output that brings those two signals back together to pump up the power going to the load.
For this kind of transformer push-pull amplifier, we are using NPN transistors and guess what?
Their emitter terminals are all connected together.
Load Current Sharing
Now when it comes to how the load current flows, it is shared between the two power transistors.
As one transistor’s current goes down, the other ones goes up throughout the signal cycle.
This clever arrangement means that the output voltage and current can drop to zero at times.
The end result?
Both halves of the output waveform swing from zero all the way up to twice the quiescent current.
This setup really cuts down on dissipation and boosts the amplifier’s efficiency to about 70% which is pretty impressive!
Quiescent Collector Current
Now let us say there is no input signal hanging around.
In that case each transistor is carrying its normal quiescent collector current.
The amount of this current is set by the base bias which sits right at the cut-off point.
If our transformer is perfectly center-tapped (which we want) then those two collector currents will flow in opposite directions—this is like our ideal scenario—so there will not be any magnetization happening in the transformer core.
This helps keep distortion to a minimum.
Input Signal Dynamics
When an input signal does show up across the secondary of our driver transformer T1, things get interesting!
The base inputs for the transistors are in “anti-phase” with each other.
What this means is that if TR1’s base goes positive and kicks that transistor into heavy conduction mode, its collector current will ramp up.
At the same time TR2’s base current will dip negative, pushing it further into cut-off and causing its collector current to drop by an equal amount.
It is like a dance where one transistor handles the negative halves while the other takes care of the positive halves creating that push-pull effect we love.
Alternating Currents and Output Waveform
Different to what happens with DC conditions, these alternating currents actually add together!
This results in those two output half-cycles merging nicely to recreate a sine wave in the primary winding of the output transformer which then shows up across the load.
Class B Amplifier Biasing
Now here is a important point about Class B Amplifier operation: there is a zero DC bias going on because our transistors are biased right at cut-off.
This means that each transistor only kicks into action when the input signal exceeds the Base-emitter voltage.
So when there’s no input signal?
You guessed it—there is zero output and no power being used at all!
That means that for a Class B amplifier, its actual Q-point hangs out on the Vce part of the load line as illustrated below.
Characteristics Curves of a Class B Amplifier Circuit Output
Advantages of Class B Amplifiers
So now let us talk about Class B Amplifiers and why they have a pretty sweet advantage over their Class A counterparts.
The big deal here is that when these Class B amplifiers are in their quiescent state—meaning there is no input signal buzzing around—there is an actually no current flowing through the transistors.
This is great because it means that there is no power being wasted in the output transistors or the transformer when there’s no signal coming in.
In contrast Class A amplifiers are a bit of a heat factory since they need a significant base bias even when there’s no input signal, which leads to a lot of heat dissipation.
Efficiency of Class B Amplifiers
Because of this nifty quiescent state the overall conversion efficiency (η) of the Class B amplifier is way better than that of an equivalent Class A amplifier.
We’re talking about efficiencies that can hit as high as 70%!
That is why you will find that nearly all modern push-pull amplifiers are designed to operate in this Class B mode.
Transformerless Push-Pull Amplifier
Now onto one of the downsides of the standard Class B amplifier circuit we just discussed: it relies on those balanced center-tapped transformers which can make it pretty pricey to put together.
But do not worry, there is another type called the Complementary-Symmetry Class B Amplifier that skips the transformers altogether.
Yep you heard that right—it is transformerless!
Instead of using transformers, this design employs complementary or matching pairs of power transistors.
Benefits of Going Transformerless
Since transformers are not the part of the equation, this transformerless amplifier circuit ends up being much more compact while still delivering the same output power.
Plus without those transformers hanging around, you do not have to deal with any stray magnetic effects or transformer distortion messing with the quality of your output signal.
Below you will find an example of what a “transformerless” Class B amplifier circuit looks like.
Class B amplifier circuit with Transformerless Output Stage
so the Class B amplifier setup we are talking about makes use of complementary transistors for handling each half of the waveform.
These Class B amplifiers give you a much higher gain compared to Class A setups but one of the big downsides of Class B push-pull amplifiers is that they run into a problem we call crossover distortion.
Now if you recall from our transistor lessons, a bipolar transistor needs around 0.7 volts (measured from base to emitter) to even start conducting.
In a regular Class B amplifier, the output transistors are not “pre-biased” to be in an “ON” state from the get-go.
What this means is that any part of the output waveform that dips below this 0.7 volt mark will nnot be reproduced very well.
The switch from one transistor to the other is not clean.
Even if you’re using perfectly matched pairs of transistors, they do not stop or start conducting right at the zero crossover point.
So the output transistors for each half of the waveform (positive and negative) will each have a 0.7 volt range where they’re not conducting.
The result? Both transistors end up being “OFF” at the same time.
One easy way to get rid of crossover distortion in a Class B amplifier is to toss in two small voltage sources.
This biases both transistors to a point just a bit above their cut-off point.
And that gives us what we commonly call a Class AB Amplifier circuit.
But since it is not really practical to add extra voltage sources to the amplifier circuit, we usually use PN-junctions to give us that extra bias in the form of silicon diodes.
Understanding Class AB Amplifier Circuit
Now let us talk about the Class AB Amplifier.
We know that we need the base-emitter voltage to be greater than 0.7v for a silicon bipolar transistor to even start doing its thing.
So if we swap out those two voltage divider biasing resistors hooked up to the base terminals of the transistors with two silicon Diodes, the biasing voltage applied to the transistors would then be the same as the forward voltage drop of those diodes.
These two diodes are usually called Biasing Diodes or Compensating Diodes, and they are picked so that they work using the same characteristics of the matching transistors.
The circuit uses diode biasing as shown in the following diagram:
The Class AB Amplifier circuit is basically trying to find a middle ground between the Class A and Class B setups, right?
Basically there is this tiny diode biasing voltage that makes both of the transistors kind of turn on just a little bit, even when there’s no signal coming in.
Then when a real input signal waveform shows up, the transistors do their work like normal in the active region.
And that gets rid of that annoying crossover distortion you get with just plain Class B amplifier designs.
It is like a tiny head start to make things smoother.
So even when there’s no input signal, a little bit of collector current still trickles through but not nearly as much as you’d see in a Class A amplifier setup.
That means the transistor is “ON” for more than half of the waveform cycle but definitely not the whole thing.
We are talking a conduction angle somewhere between 180 degrees and 360 degrees.
Depending on how much extra biasing you use, that is like saying it’s “ON” for 50% to 100% of the input signal.
And you can pump up the amount of diode biasing voltage at the base terminal of the transistor by stacking more diodes in a row.
For those beefy jobs that need lots of power, like audio power amplifiers and PA systems, Class B amplifiers totally win over Class A designs.
Now just like with Class A amplifier circuits, if you want to seriously crank up the current gain (Ai) of a Class B push-pull amplifier, a cool trick is to swap out those single transistors for Darlington transistors pairs in the output section.
This will give it a real boost to the audio output!
References:
Confusion between Class B and Class AB Amplifier
Designing a two stage push-pull class B audio amplifier
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