The purpose of any amplifier is to produce an output which follows the characteristics of the input signal but is sufficiently large enough to supply the needs of the load connected to it.
We have seen that the power output of an amplifier is the product of the voltage and current, (P = VxI) applied to the load, while the power input is the product of the DC voltage and current taken from the power supply.
Although the amplification of a Class A amplifier, (where the output transistor conducts 100% of the time) can be high, the efficiency of the conversion from the DC power supply to an AC power output is generally poor at less than 50%. However if we modify the Class A amplifier circuit to operate in Class B mode, (where each transistor conducts for only 50% of the time) the collector current flows in each transistor for only 180o of the cycle. The advantage here is that the DC-to-AC conversion efficiency is much higher at about 75%, but this Class B configuration results in distortion of the output signal which can be unacceptable.
One way to produce an amplifier with the high efficiency output of the Class B configuration along with the low distortion of the Class A configuration is to create an amplifier circuit which is a combination of the previous two classes resulting in a new type of amplifier circuit called a Class AB Amplifier. Then the Class AB amplifier output stage combines the advantages of the Class A amplifier and the Class B amplifier while minimising the problems problems of low efficiency and distortion associated with them.
As we said above, the Class AB Amplifier is a combination of Classes A and B in that for small power outputs the amplifier operates as a class A amplifier but changes to a class B amplifier for larger current outputs. This action is achieved by pre-biasing the two transistors in the amplifiers output stage. Then each transistor will conduct between 180o and 360o of the time depending on the amount of current output and pre-biasing. Thus the amplifier output stage operates as a Class AB amplifier.
First lets look at a comparison of output signals for the different amplifier classes of operation.
Then the amplifier classes are always defined as follows:
For Class A operation the switching transistors Q-point is located near to the centre of the output characteristics load line of the transistor and within the linear region. This allows the transistor to conduct for the complete 360o so the output signal varies over the full cycle of the input signal.
The main advantage of Class A is that the output signal will always be an exact reproduction of the input signal reducing distortion. However it suffers from poor efficiency, because to bias the transistor in the center of the load line there must always be a suitable DC quiescent current flowing through the switching transistor even if there is no input signal to amplify.
For Class B operation, two complimentary switching transistors are used with the Q-point (that is its biasing point) of each transistor located at its cut-off point. This allows for one transistor to amplify the signal over one half of the input waveform, while the other transistor amplifies the other half. These two amplified halves are then combined together at the load to produce one full waveform cycle. This NPN-PNP complimentary pair is also known as a push-pull configuration.
Because of the cut-off biasing, the quiescent current is zero when there is no input signal, therefore no power is dissipated or wasted when the transistors are in the quiescent condition, increasing the overall efficiency of a Class B amplifier with respect to Class A.
However, as the Class B amplifier is biased so that the output current flows through each transistor for only half of the input cycle, the output waveform is therefore not an exact replica of the input waveform since the output signal is distorted. This distortion occurs at every zero-crossing of the input signal producing what is generally called cross-over distortion as the two transistors switch “ON” between themselves.
This distortion problem can be easily overcome by locating the biasing point of the transistor slightly above cut-off. By biasing the transistor slightly above its cut-off point but much below the centre Q-point of the class A amplifier, we can create a Class AB amplifier circuit. Then the basic purpose of a Class AB amplifier is to preserve the basic Class B configuration while at the same time improving its linearity by biasing each switching transistor slightly above threshold.
So how do we do this. A Class AB amplifier can be made from a standard Class B push–pull stage by biasing both switching transistors into slight conduction, even when no input signal is present. This small biasing arrangement ensures that both transistors conduct simultaneously during a very small part of the input waveform by more than 50 per cent of the input cycle, but less than 100 per cent.
The 0.6 to 0.7V (one forward diode volt drop) dead band that produces the crossover distortion effect in Class B amplifiers is greatly reduced by the use of suitable biasing. The pre-biasing of the transistor devices can be achieved in a number of different ways using either a preset voltage bias, a voltage divider network, or by using a series connected diode arrangement.
Here the biasing of the transistors is achieved by using a suitable fixed bias voltage applied the bases of TR1 and TR2. Then there is a region where both transistors are conducting and the small quiescent collector current flowing through TR1 combines with the small quiescent collector current flowing through TR2 and into the load.
When the input signal goes positive, the voltage at the base of TR1 increases producing a positive output of a similar amount which increases the collector current flowing through TR1 sourcing current to the load, RL. However, because the voltage between the two bases is fixed and constant, any increase in the conduction of TR1 will cause an equal and opposite decrease in the conduction of TR2 during the positive half cycle.
As a result, transistor TR2 eventually turns off leaving the forward biased transistor, TR1 to supply all the current gain to the load. Likewise, for the negative half of the input voltage the opposite occurs. That is, TR2 conducts sinking the load current while TR1 turns off as the input signal becomes more negative.
Then we can see that when the input voltage, Vin is zero, both transistors are slightly conducting due to their voltage biasing, but as the input voltage becomes more positive or negative, one of the two transistors conducts more either sinking of sourcing the load current. As the switching between the two transistors occurs nearly instantly and is a smooth one, the crossover distortion which affects the Class B configuration is greatly reduced. However, incorrect biasing can cause sharp crossover distortion spikes as the two transistor switch over.
The use of a fixed biasing voltage allows each transistor to conduct for more than one-half of the input cycle, (Class AB operation). However, it is not very practical to have extra batteries within the amplifiers output stage design. One very simple and easy way of producing two fixed biasing voltages to set a stable Q-point near to the transistors cut-off, is to use a resistive voltage divider network.
When a current passes through a resistor, a voltage drop is developed across the resistor as defined by Ohm’s law. So by placing two or more resistors in series across a supply voltage we can create a voltage divider network that produces a set of fixed voltages at the values of our choosing.
The basic circuit is similar to the above voltage biasing circuit in that transistors, TR1 and TR2 conduct during the opposite half cycles of the input waveform. That is, when VIN in is positive, TR1 conducts and when VIN is negative, TR2 conducts.
The four resistances R1 to R4 are connected across the supply voltage Vcc to provide the required resistive biasing. The two resistors, R1 and R4 are chosen to set the Q-point slightly above cut-off with the correct value of VBE being set at about 0.6V so that the voltage drops across the resistive network brings the base of TR1 to about 0.6V, and that of TR2 to about –0.6V.
Then the total voltage drop across biasing resistors R2 and R3 is approximately 1.2 volts, which is just below the value required to turn each transistor fully-on. By biasing the transistors just above cut-off, the value of the quiescent collector current, ICQ, should be zero. Also, since both switching transistors are effectively connected in series across the supply, the VCEQ volt drop across each transistor will be approximately one-half of Vcc.
While the resistive biasing of a Class AB amplifier works in theory, a transistors collector current is very sensitive to changes in its base biasing voltage, VBE. Also, the cut-off point of the two complimentary transistors may not be the same, so finding the correct resistor combination within the voltage divider network may be troublesome. One way to overcome this is to use an adjustable resistor to set the correct Q-point as shown.
An adjustable resistor, or potentiometer can be used to bias both transistors onto the verge of conduction. Then transistors TR1 and TR1 are biased via RB1-VR1-RB2 so that their outputs are balanced and zero quiescent current flows into the load.
The input signal which is applied via capacitors C1 and C2 is superimposed onto the biasing voltages and applied to the bases of both transistors. Note that both the signals applied to each base has the same frequency and amplitude as they originated from VIN.
The advantage of this adjustable biasing arrangement is that the basic amplifier circuit does not require the use of complimentary transistors with closely matched electrical characteristics or and exact resistor ratio within the voltage divider network as the potentiometer can be adjusted to compensate.
As resistors are passive devices that convert electrical power into heat due to its power rating, the resistive biasing of a Class AB amplifier, either fixed or adjustable, can be very sensitive to changes in temperature. Any small changes in the operating temperature of the biasing resistors (or transistors) may affect their value producing undesirable changes in the quiescent collector current of each transistor. One way to overcome this temperature related problem is to replace the resistors with diodes to use diode biasing.
While the use of biasing resistors may not solve the temperature problem, one way to compensate for any temperature related variation in the base-emitter voltage, (VBE) is to use a pair of normal forward biased diodes within the amplifiers biasing arrangement as shown.
A small constant current flows through the series circuit of R1-D1-D2-R2, producing voltage drops which are symmetrical either side of the input. With no input signal voltage applied, the point between the two diodes is zero volts. As current flows through the chain, there is a forward bias voltage drop of approximately 0.7V across the diodes which is applied to the base-emitter junctions of the switching transistors.
Therefore the voltage drop across the diodes, biases the base of transistor TR1 to about 0.7 volts, and the base of transistor TR2 to about –0.7 volts. Thus the two silicon diodes provide a constant voltage drop of approximately 1.4 volts between the two bases biasing them above cut-off.
As the temperature of the circuit rises, so too does that of the diodes as they are located next to the transistors. The voltage across the PN junction of the diode thus decreases diverting some of the transistors base current stabilising the transistors collector current. If the electrical characteristics of the diodes are closely matched to that of the transistors base-emitter junction, the current flowing in the diodes and the current in the transistors will be the same creating what is called a current mirror. The effect of this current mirror compensates for variations in temperature producing the required Class AB operation thereby eliminating any crossover distortion.
In practice, diode biasing is easily accomplished in modern day integrated circuit amplifiers as both the diode and switching transistor are fabricated onto the same chip, such as in the popular LM386 audio power amplifier IC. This means that they both have identical characteristics curves over a wide temperature change providing thermal stabilisation of the quiescent current.
The biasing of a Class AB amplifier output stage is generally adjusted to suit a particular amplifier application. The amplifiers quiescent current is adjusted to zero to minimise power consumption, as in Class B operation, or adjusted for a very small quiescent current to flow that minimises crossover distortion producing a true Class AB amplifier operation.
In the above Class AB biasing examples, the input signal is coupled directly to the switching transistors bases by using capacitors. But we can improve the output stage of a Class AB amplifier a little more by the addition of a simple common-emitter driver stage as shown.
Transistor TR3 acts as a current source that sets up the required DC biasing current flowing through the diodes. This sets the quiescent output voltage as Vcc/2. As the input signal drives the base of TR3, it acts as an amplifier stage driving the bases of TR1 and TR2 with the positive half of the input cycle driving TR1 while TR2 is off and the negative half of the input cycle driving TR2 while TR1 is off, the same as before.
Like with most electronic circuits, there are many different ways to design a power amplifiers output stage as many variations and modifications can be made to a basic amplifier output circuit. The job of a power amplifier is to deliver an appreciable level of output power (both current as well as voltage) to the connected load with a reasonable degree of efficiency. This can be achieved by operating the transistor(s) in one of of two basic operating modes, Class A or Class B.
One way of operating an amplifier with a reasonable level of efficiency is to use a symmetrical Class B output stage based on complementary NPN and PNP transistors. With a suitable level of forward biasing its possible to reduce any crossover distortion as a result of the two transistors being both cut-off for a brief period of each cycle, and as we have seen above, such a circuit is known as a Class AB amplifier.
Then putting it all together, we can now design a simple Class AB power amplifier circuit as shown, producing about one watt into 16 ohms with a frequency response of about 20Hz to 20kHz.
We have seen here that a Class AB amplifier is biased so that output current flows for less than one full-cycle of the input waveform but more than a half cycle. The implementation of Class AB amplifiers is very similar to the standard Class B configurations in that it uses two switching transistors as part of a complementary output stage with each transistor conducting on opposite half-cycles of the input waveform before being combined at the load.
Thus by allowing both switching transistors to conduct current at the same time for a very short period, the output waveform during the zero crossover period can be substantially smoothed reducing the crossover distortion associated with the Class B amplifier design. Then the conduction angle is greater than 180o but much smaller than 360o.
We have also seen that a Class AB amplifier configuration is more efficient than a Class A amplifier but slightly less efficient than that of a Class B because of the small quiescent current needed to bias the transistors just above cut-off. However, the use of incorrect biasing can cause crossover distortion spikes producing a worse condition.
Having said that, Class AB amplifiers are one of the most preferred audio power amplifier designs due to their combination of reasonably good efficiency and high-quality output as they have low crossover distortion and a high linearity similar to the Class A amplifier design.