Linear amplifiers, are amplifiers with a linear output stage, in which there exists a voltage drop across the output transistors to generate the correct output voltage. Even though most of these amplifiers use some sort of switching, they are not to be confused with switching amplifiers. The output stage of a power amplifier has perhaps the greatest influence on performance and cost. The output stage must also operate at high power levels, often at elevated temperatures, where difficult loads, high voltages, and high currents may exist. Indeed, there is often a trade-off between heat generation and sound quality.
Every real amplifier has some unavoidable limitations on its performance. Some of the main limitations which should be considered are:
>> Limited bandwidth. In particular, for each amplifier there will be an upper frequency beyond which it finds it difficult or impossible to amplify signals.
>> Noise. All electronic devices tend to add some random noise to the signals passing through them, degrading the SNR (signal-to-noise ratio).
>> Limited output voltage, current and power levels. This means that a given amplifier can't provide output signals above a particular level. In other words, there is always a finite limit to the output signal size.
>> Distortion. The actual signal pattern will be altered due to non-linearities in the amplifier.
>> Finite gain. A given amplifier can have a high gain, but this gain can't be infinite, so may not be large enough for a given purpose. That's why multiple amplifiers or stages are often used to achieve a desired overal gain.
The various limitations demands the various changes in the design of the amplifiers. That lead us to the concept of amplifier classes.
Class A, B and AB
The output transistors in a push-pull class A power amplifier remain in conduction throughout the entire cycle of the audio signal, always contributing transconductance to the output stage signal path. In contrast, the output transistors in a class B design remain on for only one-half of the signal cycle. When the output stage is sourcing current to the load, the top transistor is on. When the output stage is sinking current from the load, the bottom transistor is on. There is thus an abrupt transition from the top transistor to the bottom transistor as the output current goes through zero.
The formal definition of classes A and B is in terms of the so-called conduction angle. The conduction angle for class A is 360 degrees (meaning all of the cycle), while that for class B is 180 degrees. More accurately, the definition should really be the angle over which the transistor contributes transconductance to the output stage and signal current to the output. This precludes many so-called nonswitching amplifiers from being called class A. Such amplifiers include bias arrangements that prevent the power transistor from completely turning off when it otherwise would. Most power amplifiers are designed to have some overlap of conduction between the top and bottom output transistors. This smoothes out the crossover region as the output current goes through zero. For small output signal currents, the output transistors are in the overlap zone and the output stage effectively operates in class A. These amplifiers are called class AB amplifiers because they possess some of the characteristics and advantages of both class A amplifiers and class B amplifiers. Most push-pull vacuum tube amplifiers operate in class AB mode. Class AB output stages have a conduction angle that is greater than 180 degrees, although sometimes only slightly so.
Class A
A simple example of a Class A amplifier stage is a common emitter amplifier circuit. Class A amplifiers have the general property that the output device(s) always carry a significant current level, or they have a large quiescent current. The quiescent current is defined as the current level in the amplifier when it is producing an output of zero. The main disadvantage of Class A amplifiers is that current is flowing through the output transistor and its resistor even when there is no signal. Power is being used but no sound or other form of output activity occurs. Such amplifiers are inefficient because they waste 50% of the energy supplied to them. If an amplifier is to produce enough output power to drive a motor or high-wattage speaker, we must design the output stage of the circuit to avoid such waste. The most inefficient amplifier is single ended. More efficient amplifier can be made by employing a double ended or push-pull arrangement. On Picture 1 is shown an example of output stage in push-pull arrangement which works in Class A. This arrangement employs a pair of transistors, one is an NPN, the other is a PNP bipolar transistor.
Picture 1: Push-Pull output stage in Class A
The transistors in this circuit can be controlled using a pair of input voltages, V1 and V2. Therefore, the currents I1 and I2 can be altered independently, by wish. In practice, the easiest way to use the circuit is to set the quiescent current to half the maximum level we except to require for the load. Then adjust the two transistor currents "in oposition". It is the imbalance between the two transistor currents that will pass through the load, so this means the transistors "share" the burden of driving the output load.
Class B
Simply by changing the quiescent current or bias level in class A amplifier and then operating the system slightly differently, we can make another forms of amplifier. The simplest alternative is the Class B arrangement. To illustrate how this work, consider the circuit shown on Picture 2. This arrangement again employs a pair of transistors. However, their bases (or inputs) are now linked by a pair of diodes. The current in the diodes is mainly set by a couple of constant current stages which run a bias current, ibias, through them. If the forward voltage drop across each diode is Vd, then the voltage of the input to the base of the upper transistor is Vin + Vd, while the voltage of the input to the base of the lower transistor is Vin - Vd. Taking into account that the base-emitter junction of a bipolar transistor is essentially a diode, then the voltage drop between the base and the emitter of the transistors will also be Vd, by absolute value. That leads to the very interesting result where the emitter voltages in the circuit shown on Picture 2 will be V1 = V2 = Vin.
Picture 2: Class B output stage amplifier
This result has two implications. First, when Vin = 0, the output voltages will be zero. Since the voltages above and below the emitter resistors RE will both be zero, it follows that there will be no current at all in the output transistors. The quiescent current level is zero and the power dissipated when there is no output is also zero. So, this circuit has perfect efficiency. The second implication is that as Vin is adjusted to produce the signal, the emitter voltages will both tend to follow it. When the load is connected to the output circuit, it will draw current from one or the other transistor, but not from both. When a positive voltage is produced, the upper transistor conducts and draws the current through the load and the lower transistor is Off. On the ther hand, when a negative voltage is produced, the lower transistor conducts and draws the current through the load and the upper transistor is Off. This again means that the system is highly efficient in power terms.
When power efficiency is the main requirement, then Class B is very useful. However, for this circuit to work as explained, it requires the voltage drops across the diodes and the base-emitter junctions of the transistors to be exactly the same. In practice, this is impossible for many reasons. Firstly, no two physical devices are absolutely identical. The diodes and the transistors will have differently doped and manufactured junctions, designed for different purposes. The currents through the transistors is far higher then through the diodes. The transistors will be hotter than the diodes due to the higher power dissipation. When the applied voltage is changed, it takes a time for a PN junction to react and for the current to change. Also, the transistor can't be turned off right away and stop conducting. As a result, the transistors tend to lag behind any swift changes.
The overall result of the above effect is that the Class B arrangement tends to have difficulty whenever the signal waveform changes its polarity and the transistors turns on and off. The result is what is called crossover distortion and this have a very bad effect on small level or high speed waveforms. This problem is enhanced due to non-linearities in the transistors, meaning that the output current and voltage don't vary linearly with the input level. The effect of the crossover distortion is shown on Picture 3. It is proportionately greater in small signals.
Picture 3: Crossover distortion (as the signal swings between positive and negative)
So, the Class A is very power inefficient, while the Class B is far more efficient, but it can lead to signal distortions. The solution is to find a half-way which will take advantages of both arrangements and will minimize the problems. The most common solution is Class AB amplification.
Class AB
The Class AB arrangement can be seen to be very similar to the Class B circuit. In the example shown on Picture 4, it just has an extra pair of diodes. The change that these makes is, when there is no output, there is a potential difference of about 2 x Vd between the emitters of the transistors. As a consequence, there will be a quiescent current of about Iq = Vd/RE, flowing through both transistors when the output is zero. For small output signals (which requires output currents in the range -2Iq < IL < 2Iq), both transistors will conduct and act as a double ended Class A arrangement. For larger signals, one transistor will be off and the other will supply the current required by the load. Hence for large signals the circuit behaves like a Class B amplifier, this mixed mixed behaviour caused this arrangement to be called Class AB.
Summary
Class A amplifiers employ a high quiescent or bias current, which causes large transistor currents even when the output signal level is small. Therefore, the power efficiency of class A amplifiers is poor, but they can offer good signal performance due to avoiding problems with effects due to low current level nonlinearities causing distortion. Double ended output design is more efficient than a single ended. Class B has a very low or perhaps zero quiescent (bias) current, and hence low power dissipation and optimum power efficiency. Class B may suffer from problems when handling low level signals. That's why the class AB is often the preferred solution in practice.
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