Amplifiers do not actually increase the strength of an electronic signal but it simply the signal is copied and amplified. There are different schemes for amplifying the signal.
[ Class A amplifier | Class B Amplifier | Class AB Amplifier | Class D Amplifier | Class T Amplifier | Bypass capacitor ]
There are different classes of amplifiers.
1. Class A amplifier :
Class-A amplifiers use one or more transistors that conduct during both the positive and negative cycles of the signal. Therefore, Class A amplifiers have current flowing through the transistor for the full cycle of the ac waveform.
With class A bias, the base current is set so that collector current flows.
Hence, Increasing or decreasing base current makes collector current to increase and decrease.
In order to carry current through the transistor for the full cycle of the ac waveform, the following condition should be satisfied.
- A DC bias point that has an appreciable DC current flowing even when there is no signal present.
- It also requires that an appreciable voltage be across the transistor from collector to emitter. Therefore the DC power supply is supplying an appreciable power even when there is no output signal being produced.
When transistor is used for class-A amplifier, it dissipates an appreciable power and therefore gets heated up. The power efficiency of class-A amplifiers is relatively low, typically under 12%.
Theoretically an ideal capacitor coupled emitter follower could have efficiency up to 16%. An ideal form of an inductor coupled or transformer coupled class-A amplifier could have an efficiency of up to 50%, but a typical
This Class of amplifier has the lowest distortion but it is very inefficient and generates a lot of heat. A Class-A amplifier requires that the amplifier generate the full current no matter what the output is. If you were simply listening to FM or watching a movie, the amplifier would be consuming as much power as if you had it turned up to full volume. 100% of the input signal is used (conduction angle a = 360° ).
We can make a more efficient amplifier by employing a Double Ended or Push-Pull arrangement. The figure below shows one type of output stage. This type of new arrangement involves pair of transistors, a NPN bipolar transistor and a PNP bipolar transistor. Ideally, these two transistors have equivalent properties - e.g. the same current gains, etc - except for the difference in the signs of their voltages and currents. (In effect, a PNP transistor is a sort of electronic "mirror image" of a NPN one.)
The circuit shown on the left now has two transistors which we can control using a pair of input voltages v1 and v2, and . We can therefore alter the currents I1 and I2 independently.
However, the easiest way to use the circuit is to set the Quiescent Current, Iq to half the maximum level we expect to require for the load. Then adjust the two transistor currents `in opposition`.
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.
When supplying a current IL, to the load this means that we will have.
In linear operation this limits us to a current range 0->2Iq in each transistor, and load currents in the range -2Iq. Hence this arrangement is limited in a similar way to before.
2. Class B Amplifier :
Class B amplifier increases efficiency by using one transistor to conduct the positive portion of the waveform and another transistor to conduct the negative portion of the waveform. Due to this feature, most of the audio amplifiers have Class B type of amplifier. One more useful feature of Class B amplifier is that distortions are well below human ear detecting ability.
With class B bias, the base current is set to zero. No collector current flows and the transistor go into CUT OFF State. The collector current can only be made to increase from this point, not decrease, by increasing the bias.
In other words, Class-B amplifiers have current flowing through the transistor for only half a cycle. There is no current flowing when there is no signal present. This means there is no power loss when no signal is being amplified, except a small amount in the bias circuit.
Thus efficiency close to 80% can be achieved when signal gets amplified.
- Disadvantages Amplifier :
The main problem with the class-B amplifier is that there is a distortion in the waveform where it crosses zero. The transistor characteristic becomes nonlinear as it approaches cutoff. The transistor provides to little current as the voltage approaches zero. This causes what in known as crossover distortion. By changing the bias point so that there is a small current flow when the signal input is zero provides an overlap between the two transistors and one transistor starts to conduct before the other turns off. If the overlap is properly adjusted the extra current from the transistor that is just turning on will make up for the deficiency in the transistor that is turning off. When this offset bias is used the amplifier is known as a class-AB amplifier.
3. Class AB Amplifier :
In class B design, one of the transistors works for 50% of the full cycle while other transistor works for full 50% of the cycle. In early class B amplifiers, there was a distortion created between the time the devices were switching back and forth. Some people referred to this distortion as notch distortion because there was a notch appearance on an oscilloscope between the two waveforms.
Class A/B was created to leave the transistor conducting while the second transistor was conducting. This created an overlap between the two signals. The problem with this approach is that it creates its distortion called gumming. This means that the signal would get a little fatter.
The circuit shown above is a class-AB amplifier.
- Working Amplifier:
The NPN transistor Q1 amplifies the positive half of the waveform and the PNP transistor Q2 amplifies the negative half of the waveform. The two diodes are used in the bias circuit for temperature compensation. As the temperature of the transistors goes up, the value of vbe comes down.
If the diodes and transistors are mounted on same heat sink, they can be maintained at the same temperature.
By doing so, voltage drop will decrease at the same rate keeping the bias voltage of the transistors at the same level.
Mounting of diode and transistor on same heat sink is necessary because DC base current would increase as the temperature of the transistor will increase. This could result in unnecessary increase in collector current, which would in turn increase the power dissipated in the transistor leading to heating of transistors.
The emitter resistors also provide some negative feedback to reduce the increase in the collector current. They also help to equalize the currents through parallel transistors when the single transistors are replaced by several transistors in parallel, each with its own emitter resistor to get higher current output. The value of the resistors R1 must be calculated to provide enough current to drive the base of the transistor at the highest input voltage and still have some left over to flow through the diodes. For voltages near the power supply voltage this requires a low resistance and therefore a fairly large zero signal current through the diodes. This reduces the efficiency of the amplifier.
4. Class D Amplifier :
Class D amplifiers are also called digital amplifiers.
A Class D amplifier uses transistors that are either switched on or off action to represent positive or negative values. The transistors are either on or off (1 or 0). The advantage of such a system is that they are highly efficient and does not gets heated very quickly.
The disadvantage of such amplifier is that there can be a distortion while switching of transistors takes between positive and negative values. Nowadays, many Class D amplifiers are used in Subwoofers. They are inexpensive to build and the logic is that the switching distortion is not important in a subwoofer.
5. Class T Amplifier :
Class T uses signal processing to eliminate the switching distortion of Class D.
These types of amplifiers were built to make lower priced amplifier that offers good performance.
6. Bypass capacitor
The bypass capacitor is connected in parallel with the resistor (across the resistor). The capacitor shunts or bypasses the AC current away from the resistor.
At high frequencies, the capacitor acts as short and most of the current goes to the ground via the capacitor, bypassing the resistor Rl.