A brief discussion on the enlargement of Class A

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A brief discussion on the enlargement of Class A [Copy link]

 

Class A amplifier has a very low power utilization rate and a very high power consumption. However, for weak signals that are smaller than the conduction threshold of the PN junction, Class A amplifier plays an irreplaceable role. The diode is not a switch, but a valve, a one-way valve. When the DC power P positive and N negative are connected to the diode, the diode is connected. If the DC power is not pure, the current passing through the diode will naturally contain these impurities. The DC of the diode switch is pure DC, and those "impurities" are the signals injected from C1 and flow out through C2. Isn't this just like the input loop (emitter junction) of BJT (NPN type)? Isn't this just like the operating principle of Class A amplifier? ! Crossover distortion is a permanent problem of voltage-controlled Class-B amplifiers. The crossover distortion region is the active region of the diode. Low-level mixing, amplitude modulation or demodulation occurs in this region. Increasing the bias voltage makes the PN junction fully conductive, and the diode enters the large signal region. The diode switch must not distort the information it carries, amplify it, or allow signal distortion. The large signal region is a linear region. Placing the signal in the linear region will not cause distortion. This is the purpose of biasing. How to bias the amplifier to make the output large enough without distortion? This involves the setting of the static operating point.

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Overdrive input, excessive output, or inappropriate operating point will of course lead to distortion. In a push-pull circuit, it is not just a distortion problem. If this distortion involves the cutoff of the tube, it means that the amplifier is no longer a pure Class A amplifier! In a Class A amplifier, except for transformer coupling, other interconnection methods are AC/DC split. In a single-tube topology, it wastes power and is not very good. The push-pull AC/DC channels are the same, and the power utilization is similar to that of transformer coupling, which is more reasonable.

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Alternation, relay and synchronization are all ways of cooperation. But I think that rotation is just a division of labor and cooperation, and synchronization must be a concerted effort and joint progress and retreat. Only the pure A model can have the possibility of full interaction, so I think that true push and pull can only be achieved by pure A!

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Differential, differential, what on earth is differential? ! Two identical current series negative feedback common emitter circuits, with Re combined into a common impedance, are the structure of a differential circuit. If you want the circuit to have an output, the two tubes must be differential. In fact, in addition to differential, there is another feature, that is, the total current of the two tubes is always constant! This means that when the circuit is running, it will not cause any fluctuations on the power line, there is no external interference, the voltage drop on Re remains unchanged, and the output amplitude will not change if the voltage drop remains unchanged. In fact, as long as the differential drive is correct, Re can be added or not, and the constant current will remain the same. Using a voltage regulator diode is the simplest way to stabilize the voltage. The voltage regulator is connected in parallel with the load. When the load changes, the current of the load and the voltage regulator changes differentially, and the power supply current hardly fluctuates. The impedance and various parasitic parameters in the power supply will not be touched, and the noise is naturally less and smaller, and the transient response is very fast. Perhaps this is the reason why the sound is good but also needs parallel voltage regulation support. Is it the truth or nonsense? I don't have a golden ear and have no say. If the pure A push-pull is made into a full bridge, the two sets of single-ended push-pull are anti-phase embedded to form a differential effect, and the power supply rejection ratio and common-mode rejection ratio are improved in theory. The disadvantage is that the DC channel reappears, but they are all dynamic loads (tubes), and the waste of electricity is much less than that of a single tube. In theory, as long as the power supply is smoothed and decoupled well, the voltage regulator can be added.

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The signal carried by the diode switch is usually negligible compared to the input stroke of the diode. For small signal applications of Class A amplification, there is no way to solve this problem of resource waste. The working area (amplification area) carrying the output also has this problem, but it can be solved. Reducing Vcc (power supply voltage) can reduce power waste and tube consumption.

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When we talk about Class A, we have to mention the sliding Class A. Most of the senior electronics enthusiasts should have played around with it, right? The sliding Class A amplifier is for radios, not audiophiles. The kind used for audio is called super Class A or new Class A. I always feel that the sliding Class A is very similar to a rifle or AGC. Regardless of the circuit structure and operation process, the common point of the three is that they all have a specific feedback form even though there is no feedback, and they are all high-in and low-out (frequency)!

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Rifle, AGC and sliding A-type all use the detection output as the control material. The detection output of the rifle is audio information, and the amplitude modulation signal becomes the burr on the audio output. The detection output of AGC and sliding A-type are both DC. This DC is used to control the configuration parameters of the high-power stage in AGC, or the working point of the AGC-specific high-power tube. In sliding A-type, it is also used to change the static working point of the triode. However, sliding A-type uses normal triodes with good linearity and fixed configuration. The system gain does not change due to the sliding of the static working point.

hk6108

hk6108 posted on 2018-5-12 19:49 Differential, differential, what exactly is differential? ! Two completely identical current series negative feedback common emitter circuits, combining Re into a common impedance, is...

Differential is also true push-pull, differential must be pure A!! The rise and fall of the collector potential of the two tubes is just an appearance, the transfer and migration of current is the essence! One moving and one static is also considered differential, but it does not conform to usage habits and market requirements. Opposites complement each other, which is the true differential. Only pure A can be opposite and complementary throughout the process! The differential DC channel is two tubes in parallel. If it is not pure A, it cannot maintain a constant current forever, nor can it collect all the current in one tube at the peak of "AC" (the A tube will never be cut off).

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Sliding, in audiophile audio, is called dynamic bias. Compared with radio users, the topology is different but the principle is similar. When the output is weak, the AC baselines of the two tubes are close to each other (both close to the DC ground). Another way is that you can say it is an evolution of the low bias current AB class. Its AC baseline (static operating point) is fixed. The difference from the AB class is that when the output passes zero, the tube maintains the minimum bias current state, positioned at the edge of the amplification area, and never cuts off.

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This circuit is Class A, yes, but if no bias is added and Ce is charged directly by the signal, this circuit becomes Class C! Using the same set of circuits (the load part should be cut off) to replace Re, and using a low-voltage DC power supply to replace Ce, it becomes Class A and Class C, working together to achieve seamless connection between high and low levels of large audio signal output. The application of Class C amplification is no longer limited to the RF field. Professionals are really creative!

maychang

hk6108 posted on 2018-5-14 21:22 Rifle, AGC and sliding Class A all use the detection output as the control material. The detection output of the rifle is audio information, and the amplitude modulation signal becomes...

The sliding Class A audio amplifier circuit in the second of the three pictures may be wrong. The transistor should be a PNP tube.

maychang

hk6108 posted on 2018-5-15 13:17 This circuit is Class A, yes, but if no bias is added, and Ce is charged directly by the signal, this circuit becomes Class C! With the same...

Replacing Ce with a low-voltage DC power supply, it becomes Class A and Class C working together, dividing the work and cooperating, so that the large signal output of the audio can achieve seamless connection between high and low levels. It seems impossible to achieve this.

hk6108

maychang posted on 2018-5-15 15:40 The second sliding Class A audio amplifier circuit in the three pictures may be wrong. The transistor should be a PNP tube.

Or the power supply may be reversed.

maychang

hk6108 posted on 2018-5-15 19:47 Or maybe the power supply is reversed.

If the power supply is reversed, then the direction of diode D3 also needs to be reversed. In addition, the direction of the electrolytic capacitor also needs to be reversed.

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The signal driving the audio amplifier is usually voltage. Class B was abandoned because of its crossover distortion. Although Class AB has a market, it is also mixed with crossover distortion and makes golden ears unhappy. The invention of Class AA has allowed the Class B model to see the light of day again and has a place to be used again. Like other amplifiers, Class AA also uses the voltage amplifier front stage to drive the current amplifier back stage. A2 is pure Class B. Its output is transmitted back to the positive and negative input terminals at the same time through the bridge. The layout is closed-loop, but there is no feedback, which is equivalent to an open loop. This is a bootstrap connection method, which can increase the input impedance and reduce the load of A1 (pure Class A).

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maychang posted on 2018-5-15 15:44 Replacing Ce with a low-voltage DC power supply means Class A and Class C work together to achieve seamless connection between high and low levels in the large signal output of audio...

After all, it is a problem with the Class C part. Switching distortion is bound to exist, which is unacceptable to audio enthusiasts. Sliding Class A also has this problem. Any PN junction switch will damage the sound quality. Although the amplifier is pure Class A, the rectification effect of the sliding diode is also bad. In addition, the capacitor voltage cannot change suddenly. For large-amplitude sudden changes in signals, the sliding mechanism cannot react quickly, which will cause transient distortion. Of course, don't worry about it in the radio, because the AM signal from the radio station is already distorted after detection.

maychang

hk6108 posted on 2018-5-24 12:37 After all, it is a problem with the Class C part. Switching distortion is bound to exist, which is unacceptable to audio enthusiasts. Sliding Class A also has this problem. Any PN junction...

The transistor polarity in this picture is correct, and the one on the 10th floor is wrong

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Class A, for small signals, is a structural problem. Without Class A, you can't survive! For audio-visual entertainment, pure Class A is still a luxury, a waste of resources, and the effect is not acceptable to the public; crossover distortion and switch humming are unacceptable to the golden ears. I don't know how much impact it has on the calculation accuracy of mathematical tools. Analog multipliers use differential circuits, and differentials are pure Class A. DSB amplitude modulation is done with multipliers. For ordinary amplitude modulation demodulation, envelope detection can be used, but SSB operation involves every moment, and various distortions are not allowed in any step of the whole process. Therefore, the RF power amplifier of the SSB transmitter is said to use pure Class A. If so, the benefits of band reduction are not worth the waste caused by the low efficiency of pure Class A (low electrical efficiency and low tube efficiency).

maychang

hk6108 posted on 2018-5-24 12:37 After all, it is a problem of the Class C part. Switching distortion is bound to exist, which cannot be accepted by audio enthusiasts. The sliding Class A also has this problem. Any PN junction...

The sliding Class A is a product of the era when battery-powered transistors were much more expensive than resistors, capacitors and diodes. In order to have a relatively high power efficiency and save a transistor, the sliding Class A circuit was designed. For example, in the circuit on the 16th floor, compared with the Class A and B push-pull circuit, it saves a transistor, but adds two electrolytic capacitors and a diode. This will only be adopted when the price of two electrolytic capacitors plus a diode is lower than that of a transistor. Nowadays, the price of triodes is quite cheap. The push-pull circuit has lower hardware cost and higher efficiency than the sliding Class A. The sliding Class A no longer has any advantages.

PowerAnts

maychang published on 2018-5-25 11:47 Sliding Class A is a product of the era when battery-powered transistors were much more expensive than resistors, capacitors, and diodes. And they wanted to have a relatively high power efficiency...

To be honest, a small transistor is 4 cents, an electrolytic is 4 cents, a resistor is 4 cents, an assembly chip is 1.5 cents, and a plug-in is 4 cents. Using 3 resistors is more than 1 cent more expensive than 1 transistor