JPH0365044B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a power amplifier, and more particularly to a highly efficient power amplifier with reduced power loss.

In conventional power amplifier circuits, the power supply voltage is set constant and supplies power to the output amplification transistor, so the power supply voltage level is a high value determined by the maximum power required by the load. It's summery. Therefore, even when the signal level is small, this high power supply is being fed to the output amplification transistor, so the power loss due to this transistor is extremely large, resulting in poor power conversion efficiency and a large heatsink for heat dissipation. This prevents the miniaturization of the In audio equipment,
Since the average input signal level is generally small,
The above problem is noticeable in audio power amplifiers.

In order to solve these problems, a method has been proposed in which power efficiency is improved by changing the power supply voltage level to the amplification transistor according to the signal level. In all such methods, the collector voltage of the output transistor is changed;
Since a transistor element has an equivalent capacitance Cob between the collector and base and an internal admittance hoe between the collector and emitter, the change in the collector voltage mentioned above is fed back to the base input via Cob, and is also fed back to the emitter via hoe. There are problems such as the changes being returned to the system. In particular, the effect of Cob becomes greater in the higher frequency range, and it becomes more difficult to apply negative feedback of the amplifier in the higher frequency range, making it difficult to improve by negative feedback. Therefore,
The disadvantage is that distortion increases as the frequency range increases.

SUMMARY OF THE INVENTION An object of the present invention is to provide a power amplifier that can eliminate the above drawbacks and achieve high efficiency.

The power amplifier according to the present invention has first and second transistors each supplied with power from power sources having different voltage levels, each having an equivalent Darlington configuration,
Also, the second and third
When the output power is low, power is supplied to the load through the Darlington configuration using the first and second transistors, and when the output is high, the power is supplied to the load using the inverted Darlington configuration using the second and third transistors. It is characterized in that power is supplied to the load depending on the configuration.

The present invention will be explained below using the drawings.

FIG. 1 is a circuit diagram showing the principle of the present invention, in which a first power source 1 and a second power source 2 are provided,
The relationship between the outputs V _L and V _H of the first and second power supplies is set so that V _L <V _H. The first power supply voltage V _L is connected to the first voltage through the unidirectional diode D ₁ .
The voltage is applied to the collector which is the power supply terminal of the NPN transistor _Q1 , and its emitter follower output is supplied to the load _RL . Second power supply voltage V _H
is the second NPN transistor Q ₂ through the low resistance R ₁
The emitter output serves as the base input of transistor _Q1 , and both transistors are equivalently connected in a Darlington configuration.

Further, a third PNP transistor Q ₃ whose emitter is supplied with the second power supply voltage is provided,
The collector of the second transistor _Q2 is connected to the base and its collector is connected to the emitter of the first transistor. Therefore, the transistor
The emitter of Q ₂ and the collector of transistor Q ₃ are
The base and emitter of transistor Q ₁ are connected via a PN junction, and both transistors Q ₂ and Q ₃ have an inverted Darlington connection configuration.

In this configuration, if the level of the circuit input v _i , that is, the circuit output v ₀ is small and v ₀ <+V _L , the diode D ₁ is turned on and the first power supply voltage V _L is applied to the collector of the transistor Q ₁ . Powered. At this time, if the resistor R ₁ is set small, the third transistor Q ₃ can be kept off, so the equivalent circuit at the time of a small signal is as shown in Figure 2, and the first and second transistors Transistors Q ₁ and Q ₂ operate as Darlington-connected emitter followers.
Power consumption of the first transistor _Q1 at this time P _C1
is, P _C1 = (V _L −v ₀ )・i _Q1 ……(1). Here, i _Q1 is the current of transistor _Q1 .

Next, if the input signal v _i increases and v ₀ ≧ V _L , then
Transistor Q ₁ becomes saturated and diode D ₁
is turned off, the power supply to the collector of transistor _Q1 is cut off. Then, since the voltage across the resistor _R1 also increases, the third transistor _Q3 is turned on. Therefore, the equivalent circuit at this time is as shown in FIG. Here, D indicates a base-emitter junction diode of the transistor _Q1 . That is, the transistors Q ₂ and Q ₃ form an emitter follower circuit with an inverted Darlington connection configuration, and power is supplied to the load R _L mainly by the third transistor Q ₃ . Here, the power consumption P _C3 of the transistor Q ₃ is expressed as P _C3 = (V _H −v ₀ )·i _Q3 (2). Here, i _Q3 is the current flowing through transistor _Q3 .

Now, assuming that the hfe (current amplification factors) of transistors Q ₁ and Q ₃ in their active states are both sufficiently large, each base current can be ignored, so the load current is dominated by i _Q1 and i _Q3 . , and the load current is v ₀ /R _L , so equations (1) and (2) are P _C1 ≒ (V _L − v ₀ )・v ₀ /R _L ...(3) P _C3 ≒ (V _H −v ₀ )・v ₀ /R _L ...(4). Therefore, when v ₀ <V _L at the time of a small signal, equation (3) holds true and power consumption is determined by a low power supply voltage V _L , so efficiency can be improved by setting V _H > V _L. .

Here, if V _L and V _H are each constant voltage, then
Since the collector-emitter voltage V _CE of the transistors Q ₁ and Q ₃ is controlled only by the output v ₀ , the above-mentioned drawbacks due to changes in V _CE are eliminated. However, if we do not take into account the drawbacks of changes caused by V _CE ,
Further improvement in efficiency is expected if V _L and V _H are changed according to the signal level.

FIG. 4 is a circuit diagram of an embodiment of the present invention, and is an example in which the circuit of FIG. 1 is used in a class B push-pull amplifier. Components equivalent to those in FIG. 1 are denoted by the same reference numerals, and the negative side is shown by adding "'" to the same reference numerals as the positive side elements.

The voltage amplification stage has a differential amplifier configuration including transistors Q ₄ and Q ₅ and a current source I ₁ , and the output from the collector load R ₂ of the transistor Q ₄ is the base input of the driver transistor Q ₆ . The collector output of this transistor _Q6 is a transistor
It is supplied to the bases of Q ₂ and Q ₂ ′. Note that a bias circuit for the output transistors _Q1 to _Q3 and _Q1 ' to _Q3 ' is configured by the diode _D2 , the resistor _R3 , and the current source _I0 .

As described above, according to the present invention, it is possible to improve power efficiency with a simple configuration, and the potential of the collector of each transistor does not vary depending on the power supply.
It can eliminate the negative effects of cob, hoe, etc.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing the principle of the present invention, FIGS. 2 and 3 are equivalent circuit diagrams of the circuit in FIG. 1 in various operating states, and FIG. 4 is a circuit diagram of an embodiment of the present invention. Explanation of symbols of main parts, 1...first power supply, 2
... second power supply, Q ₁ ... first transistor,
Q ₂ ... second transistor, Q ₃ ... third transistor, D ₁ ... power supply blocking diode, R _L ...
…load.

Claims (1)

[Claims] 1 a first power source, a second power source that generates a voltage higher than the first power source voltage, a first transistor powered by the first power source, and a second power source.
a second transistor that is powered by a power source and has an input signal applied to a control terminal; and a third transistor that is powered by the second power source, and the first transistor and the second transistor are connected to each other. equivalently configured with a Darlington connection and said first
A first Darlington circuit obtains an output from an output terminal of a transistor, and the second transistor and the third transistor are configured to be equivalently inverted Darlington circuits, and the output is obtained from an output terminal of the third transistor. and a second Darlington circuit to drive a load with a circuit output derived from a connection point between the output terminal of the first transistor and the output terminal of the third transistor. means for turning off the third transistor when the circuit output level is lower than the first power supply voltage;
A power amplifier comprising means for cutting off power supply to the first transistor when the circuit output level is higher than the first power supply voltage.