Discrete component OCL power amplifier circuit schematic

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OCL, short for English Output CaPCItodess, means no output capacitors. The OCL power amplifier circuit generally uses two sets of positive and negative symmetrical power supplies. The internal circuit of the circuit is directly coupled to the load speaker. There is no input or output transformer in the middle. (The power amplifier circuit that people will not use the input and output transformers is called single-ended push-pull. Circuit), also does not require an output capacitor, which has the advantage of passband bandwidth and minimum signal distortion.
(1) Structure of the OCL power amplifier The power amplifier block diagram is shown on the right. The OCL power amplifying circuit is divided into three stages: an input stage, an excitation stage, and a power output stage. In addition, a negative feedback network and various compensation circuits are provided for the operation of the stable circuit, and some are also provided with a protection circuit.
The figure below is an actual power amplifier circuit that is now used in some low-end power amplifier machines. The circuit is used below to recognize the various components of the power amplifier.
1) The input stage input stage mainly plays a buffering role. The input stage mostly uses a differential pair tube amplifying circuit (also using an operational amplifier circuit), usually introducing a certain amount of negative feedback, increasing the stability of the entire power amplifier circuit and reducing noise. The differential amplifier consists of two amplifier circuits with the same characteristics, and the parameters of the left and right tubes are almost identical. This circuit has high stability, can suppress "zero drift", and ensure the stability of the midpoint voltage of the output stage. In some machines, the differential tube emitter uses a constant current source circuit. A common constant current source consisting of a diode and a triode and a mirror constant current source composed of two triodes are common. The input stage uses a small power tube, which works in the Class A state and has a small quiescent current.
2) Excitation stage The role of the excitation stage is to provide sufficient excitation current and stable static bias to the power output stage. The gain of the entire power amplifier is mainly provided by this stage. The excitation stage of most power amplifiers uses a single-tube amplifier circuit, and a few machines use a differential-pair tube amplifier circuit. This stage often uses a constant current source load, which not only can obtain high power supply rejection characteristics, but also has the advantages of stable working condition, good linearity, and low distortion. The excitation level is also a small power tube that works in the Class A state.
In addition, the excitation stage also provides a stable bias voltage for the latter stage (power output stage). There are many types of bias voltage circuits for the power output stage. The simplest bias circuit is composed of the collector load resistance of the excitation tube, and its thermal stability and voltage regulation are relatively poor. Some power amplifiers use a constant voltage bias circuit, that is, a voltage regulator connected by a plurality of diodes in series. The clamp circuit keeps the bias voltage of the power output stage stable; more is the constant voltage bias circuit with temperature compensation. The bias circuit consists of a triode and several resistors.
In the figure below, the bias circuit of the power output stage is connected in series with the collector load of the excitation tube Q3. R5 can be seen as the collector load resistance of Q3, and R4 and D1 are connected in series in the collector load circuit and can be considered as part of the collector load. Q3 collector current flows through R4, Dl and R5. A certain voltage drop is generated at both ends of R4 and Dl (the voltage level determines the working state of the output stage, generally about 2.1V. At this time, the output stage works in the class A and B; if it reaches 2.8V, the output stage works in A. Class state), this voltage is applied to the base of Q4, Q5 to provide bias voltage for the two tubes. At this time, Q6 and Q7 which are connected in combination with Q4 and Q5 also obtain a bias voltage and enter a linear amplification state.
3) Power output stage The power output stage is referred to as the output stage, which mainly acts as a current amplification function to provide sufficient excitation current to the speaker to ensure correct speaker playback. Therefore, it is also called current amplification stage. The output stage can also be subdivided into two levels, the push level and the last level.
The output stage uses a single-ended push-pull amplifier circuit in the form of complementary or quasi-complementary outputs. The output stage consists of two sets of composite tubes (called upper arm and lower arm) of different polarities. Using their opposite polarity characteristics, the positive and negative half-cycle signals can be automatically amplified separately, that is, they have complementary characteristics; and because one arm is always turned on during the operation, the other arm is turned off, and the push-pull is operated. The state is therefore also referred to as a complementary symmetric push-pull amplifier circuit.

Generally, the front stage of the power amplifier (here, the input stage and the excitation stage) are voltage amplification stages, and the output current is not large. In order to drive the power output tube with a small current to obtain sufficient output power, the general power output stage adopts a semiconductor triode composite connection method, that is, a composite tube is used. A composite pipe is a power pipe composed of two or more triodes connected in a certain manner. The high-power triode in the output-stage composite pipe is called a power tube (also called a power amplifier tube or an output tube), and another low-power (also useful for a medium-power tube) triode that is combined with it is called a push tube (or a drive tube). The push tube and the power amplifier tube respectively constitute the push level and final stage circuits. The general power amplifier has two power amplifier tubes for each channel, and some high-power power amplifiers also use a power amplifier tube parallel method in order to increase the output power, so that each channel has four or more power amplifier tubes. In some low-end machines, two power amplifier tubes use transistor transistors of the same polarity, that is, both tubes are NPN-type (or PNP-type) tubes, which need to be respectively different from two polarities (one is an NPN tube, and the other is A small power triode for a PNP tube is used in combination with a composite tube. Such a complementary output circuit is often referred to as a "quasi-complementary" push-pull amplification circuit.
The medium and high-end power amplifiers use a dedicated audio pair tube (an NPN tube, a PNP tube, and the characteristics are very close) as the output tube of the complementary circuit to achieve a higher technical level.
In the power output stage, the working states of the drive tube and the power amplifier tube are classified into Class A, Class B, and Class A and Class B. Generally speaking, class A power amplifiers, class B power amplifiers, and class A and class B power amplifiers classify power amplifiers according to the working state of the power output stage. The working state of each tube of the output stage is determined by the operating voltage provided by the bias circuit. Mastering the working state is of great significance for the maintenance of the power amplifier. The following briefly introduces the characteristics of these three types of power amplifiers.
In class A power amplifier, the total quiescent current of the output tube is large (usually 1A~2A), and its working point can ensure that the output tube is in the conduction state in the positive and negative half of the signal within a certain input signal amplitude. When there is no signal input, there is still a considerable quiescent current, and crossover distortion and switching distortion are not generated, so the playback effect is better. However, Class A amplifiers have low efficiency, and the power amplifier tubes are very hot (except for the use of large heat sinks, and some require forced air cooling by fans). In the class A power amplifier, the drive tube works in the class A state, the quiescent current is large (tens of milliamperes), and the heat is also large. Therefore, the middle power tube is often used as the drive tube, and is fixed on the heat sink.
Class B power amplifier refers to the static bias (no signal input state), the base of the power amplifier tube is not biased, and the power amplifier tube is only turned on under the action of a strong input signal (the absolute value of the voltage is greater than 0.6V). The class B power amplifier circuit adopts the push-pull output mode, and uses two power amplifier tubes with the same characteristics. The upper arm power amplifier tube works in the positive half cycle, and the F arm power amplifier tube works in the negative half cycle, that is, one push and one pull work in turn. In the input signal voltage between +0.6V and -0.6V, neither the upper arm power amplifier tube nor the lower arm power amplifier tube can be turned on. Therefore, distortion will occur at the intersection of the upper half and the lower half of the signal. In order to cross the distortion, when the push-pull transistor is alternately turned on and off, due to the carrier accumulation effect, its operation cannot completely reproduce the change of the input signal, but an additional pulse appears in the output signal, called switching distortion. . That is to say, Class B power amplifiers have the disadvantages of crossover distortion and switching off, but high efficiency and low energy consumption are significant advantages.
Class A and Class B power amplifiers are actually the combination of Class A and Class B, so that the tubes of the output stage enter the working state of Class A and Class B, and there is a certain static bias current. When there is no input signal, the quiescent current is small, and the power amplifier tube is in an approximately cut-off state; as long as the signal voltage of a very small amplitude is administrated during operation, the power amplifier tube can immediately enter the normal amplification state. In this type of power amplifier, the quiescent current of the output tube is mostly designed to be several tens of milliamperes, and it is also designed to be larger, such as about 200 mA (often referred to as high-biased type A). Class A and Class B power amplifier circuits solve the contradiction between distortion and efficiency. Therefore, it is the largest number of power amplifiers.