It is common for someone to always look for something better. No exception in the field of amplifier design, designers, enthusiasts or electronics engineers never stop looking for various better concepts. There are several types of audio amplifiers that are categorized as class A, B, AB, C, D, T, G, H amplifiers and several other types that have not been mentioned here. The following article briefly discusses the characteristics and concepts of the power amplifier (PA) system.
1. Fidelity and Efficiency
Audio amplifiers are literally interpreted as amplifying and strengthening the input signal. But what actually happens is, the input signal is copied and then re-produced into a larger and stronger signal. This is where the term fidelity comes from, which means how similar the shape of the output signal from the replica is to the input signal. Sometimes the input signal in the process is then distorted for various reasons, so that the shape of the output signal becomes deformed. An amplifier system is said to have high fidelity if the system is able to produce an output signal that is exactly the same as the input signal. Only the voltage level or amplitude has been enlarged and strengthened. On the other hand, efficiency must also be considered. The efficiency in question is the efficiency of the amplifier which is expressed by the percentage of the output power compared to the input power. An amplifier system is said to have a high level of efficiency (100%) if there are no losses in the amplification process that are wasted as heat.
2. Class A Power Amplifier
An example of a class A amplifier is the basic common emitter (CE) transistor circuit. A class A amplifier is made by setting the appropriate bias current at a certain point on its load line. Such that this Q point is right in the middle of the VCE-IC curve load line of the amplifier circuit and let's call this point point A. The following image is an example of a common emitter circuit with an NPN transistor Q1.
figure 1: class A basic circuit
The load line on this amplifier is determined by the resistors Rc and Re from the formula VCC = VCE + IcRc + IeRe. If Ie = Ic then it can be simplified to VCC = VCE + Ic (Rc + Re). Next, the reader can draw the load line of this circuit from the formula. While the resistors Ra and Rb are installed to determine the bias current. The reader can determine the size of the resistors in the circuit by first determining how much current Ib cuts the Q point.
Figure 2: Load line and Q point of class A
The current Ib is usually listed on the datasheet of the transistor used. The AC signal gain can be calculated using the theory of AC signal circuit analysis. The AC circuit analysis is by short-circuiting each component of the capacitor C and imaginarily connecting VCC to ground. In this way, the circuit in figure-1 can be assembled like figure-3. Resistors Ra and Rc are connected to ground and all capacitors are short-circuited.
Figure 3: imaginary circuit of class A ac analysis
With the presence of the Ce capacitor, the Re value in the AC signal analysis becomes meaningless. Readers can search for further literature that discusses transistor gain to find out how to calculate the transistor gain value in detail. Gain is defined as Vout/Vin = rc / re , dimana rc adalah resistansi Rc paralel dengan beban RL (pada penguat akhir, RL adalah speaker 8 Ohm) dan re is the transistor gain resistance. The value of re dapat dihitung dari rumus re = hfe/hie whose data is also in the transistor datasheet. Figure-4 shows an illustration of the input signal gain and its projection into an output signal against the xy curve line, the gain formula vout = (rc/re) Vin.
Figure 4: Class A gain curve
The characteristic of class A amplifier, all output signals work in the active region. Class A amplifier is called an amplifier that has a high level of fidelity. As long as the signal is still working in the active region, the shape of the output signal will be exactly the same as the input signal. However, this class A amplifier has a low efficiency of approximately only 25% - 50%. This is none other than because the Q point is at point A, so even though there is no input signal (or when the input signal = 0 Vac) the transistor still works in the active region with a constant bias current. The transistor is always active (ON) so that most of the power supply source is wasted as heat. Because of this, class A amplifier transistors need to be added with extra cooling such as a larger heatsink.
3. Class B Power Amplifier
Excessive heat is a problem in class A amplifiers. So a class B amplifier is made with the Q point shifted to point B (in figure-5). Point B is a point on the load line where this point intersects the current line Ib = 0. Because of the location of the point, the transistor only works actively in one part of the wave phase. Therefore, class B amplifiers are always made with 2 transistors Q1 (NPN) and Q2 (PNP).
Figure 5: Q points of amplifiers A, AB and B
Because these two transistors work alternately, the class B amplifier is often called a Push-Pull amplifier. The basic circuit of a class B Power Amplifier is as shown in Figure-6. If the signal is a sine wave, then transistor Q1 is active in the first 50% cycle (positive phase 0o-180o) and then it is the turn of transistor Q2 to be active in the next 50% cycle (negative phase 180o -- 360o). Class B amplifiers are more efficient than class A, because if there is no input signal (vin = 0 volts) then the bias current Ib is also = 0 and practically makes both transistors OFF.
figure 6: basic circuit of class B amplifier
The efficiency of a class B amplifier is approximately 75%. However, this does not mean that the problem is solved, because the transistor has non-ideal properties. In reality, there is a Vbe clamping voltage of approximately 0.7 volts which causes the transistor to remain OFF even though the Ib current is several mA greater than 0. This causes the cross-over problem during the transition from transistor Q1 to transistor Q2 which alternately become active. Figure-7 shows this cross-over problem which is caused by the dead zone of transistors Q1 and Q2 during the transition. In the final amplifier, one way to overcome the cross-over problem is to add a cross-over filter (passive filter L and C) to the speaker input.
Figure 7: Class B gain curve
4. Class AB Power Amplifier
Another way to overcome cross-over is to shift the Q point on the load line slightly from point B to point AB (figure-5). This is intended so that when the signal transitions from the positive phase to the negative phase and vice versa, there is an overlap between transistors Q1 and Q2. At that time, transistor Q1 is still active while transistor Q2 starts to be active and so on in the opposite phase. Class AB amplifiers are a compromise between efficiency (around 50% - 75%) and maintaining the fidelity of the output signal.
Figure 8: overlapping output signals of class AB amplifier
There are several techniques that are often used to shift the Q point slightly above the cut-off region. One example is as shown in Figure 9 below. Resistor R2 here functions to provide a clamping voltage between the base of transistors Q1 and Q2. Readers can determine the value of R2 to provide a certain bias current for both transistors. The clamping voltage on R2 is calculated from the voltage divider R1, R2 and R3 with the formula VR2 = (2VCC) R2 / (R1 + R2 + R3). Then determine the base current and see its relationship with the current Ic and Ie so that its relationship with the clamping voltage R2 can be calculated from the formula VR2 = 2x0.7 + Ie (Re1 + Re2). Class AB amplifiers apparently have problems with this technique, because there will be signal fattening on both transistors when they are active during the transition. This problem is called gumming.
Figure 9: basic circuit of class AB amplifier
To avoid this gumming problem, it turns out that the engineer (who may not be an engineer) did not lose his mind. So a technique was created that only activates one of the transistors during the transition. The method is to make one of the transistors work in class AB and the other one works in class B. This technique can be done by giving a constant bias to one of the transistors that works in class AB (usually always PNP). The method is to wedge the base of the transistor using a series of diodes or an arrangement of one active transistor. So sometimes an amplifier like this is also called a class AB plus B amplifier or can be claimed as class AB only or class B because it is basically a class B Power Amplifier. This designation depends on how your amplifier product is advertised. Because class AB amplifiers already have a better connotation than classes A and B. But what is important is that with these techniques the goal of getting better efficiency and fidelity can be met.
5. Class C Power Amplifier
If a class B amplifier needs 2 transistors to work properly, then there is an amplifier called class C which only needs 1 transistor. There are some applications that only need 1 positive phase. Examples are pilot frequency detectors and amplifiers, RF tuner amplifier circuits and so on. Class C amplifier transistors work actively only on the positive phase, even if necessary narrow enough only at the peaks are amplified. The rest of the signal can be replicated by the L and C resonance circuits. A typical class C amplifier circuit is as in the following circuit.
figure 10: basic class C amplifier circuit
This circuit also does not need to be biased, because the transistor is intentionally made to work in the saturation region. The LC circuit in the circuit will resonate and play an important role in replicating the input signal into an output signal with the same frequency. This circuit, if given feedback, can become an RF oscillator circuit that is often used in transmitters. Class C amplifiers have high efficiency, even up to 100%, but their fidelity is indeed lower. But actually high fidelity is not the goal of this type of amplifier.
6. Class D Power Amplifier
Class D amplifiers use PWM (pulse width modulation) technique, where the width of the pulse is proportional to the amplitude of the input signal. At the final stage, the PWM signal drives the switching transistor ON and OFF according to the pulse width. The switching transistor used is usually a FET type transistor. The concept of a class D amplifier is shown in figure-11. The sampling technique in a class D amplifier system requires a triangle wave generator and a comparator to produce a PWM signal that is proportional to the amplitude of the input signal. The PWM signal pattern resulting from this sampling technique is as shown in figure-12. Finally, a filter is needed to increase fidelity.
Figure 11: Class D amplifier concept
Figure 12: Illustration of PWM modulation of class D amplifier
Some PA manufacturers claim their class D amplifiers as digital amplifiers. Coincidentally, the notation D can be interpreted as Digital. Actually, it is not exactly like that, because the digital process should contain a process of manipulating a series of bits which ultimately has a digital to analog conversion process (DAC) or to PWM. Even if you want to call it digital, a class D amplifier is a 1-bit digital amplifier (only on or off).
7. Class E Power Amplifier
The class E amplifier was first published by father and son Nathan D and Alan D Sokal in 1972. With a structure similar to a class C amplifier, a class E amplifier requires an L/C resonant circuit with transistors that only work for less than half the duty cycle. The difference is that class C transistors work in the active region (linear). While in a class E amplifier, the transistor works as a switching transistor like a class D amplifier. Usually the transistor used is a FET type transistor. Because it uses a FET type transistor (MOSFET/CMOS), this amplifier is efficient and suitable for applications that require a large current drive but with a very small input current. Even with logic current and voltage levels, the switching transistor can work. Because it is known to be efficient and can be made in one IC chip and with relatively small heat dissipation, class E amplifiers are widely applied in mobile transmission equipment such as mobile phones. Here the antenna is part of the resonant circuit.
8. Class T Power Amplifier
Class T amplifiers can be called digital amplifiers. Tripath Technology creates digital amplifier designs with a method they call Digital Power Processing (DPP). Perhaps inspired by class D PA, the final circuit uses the concept of PWM modulation with switching transistors and filters. In class D amplifiers, the process behind it is an analog process. While in class T amplifiers, the previous process is the manipulation of digital bits. Inside there is an audio processor with a feedback process that is also digital for timing delay and phase correction.
9. Class G Power Amplifier
Class G is classified as an analog amplifier whose purpose is to improve the efficiency of class B/AB amplifiers. In class B/AB, the supply voltage is only one pair which is often denoted as +VCC and --VEE for example +12V and --12V (or written as +/-12volt). In class G amplifiers, the supply voltage is made in stages. Especially for applications that require power with high voltage, to be efficient, the supply voltage is 2 or 3 different pairs. For example, there is a supply voltage of +/-70 volts, +/-50 volts and +/-20 volts. The concept of the class G PA circuit is as in figure-13. For example, for soft and low sounds, the active supply voltage pair is +/-20 volts. Then if it is necessary to drive a loud sound, the supply voltage can be switched to the maximum supply voltage pair of +/-70 volts.
Figure 13: Class G amplifier concept with cascaded supply voltages
10. Class H Power Amplifier
The concept of a class H amplifier is the same as a class G amplifier with a supply voltage that can be changed as needed. It's just that in a class H amplifier, the high and low supply voltages are designed to be more linear, not limited to only 2 or 3 stages. The supply voltage follows the output voltage and is only a few volts higher. This class H amplifier is quite complex, but it will be very efficient.
Closing
High fidelity and high efficiency are two main goals in every amplifier circuit design. There are several other concepts and methods of amplifiers that have not been mentioned in this article. Usually these concepts and methods are named according to the Latin alphabet. Maybe at some point we will get to know the class Z amplifier.
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Compiled by Aswan Hamonangan