JPS5826684B2 - output amplifier - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a single-ended push-pull output amplifier, and more particularly to a bias circuit I for keeping its idle current constant.

Generally, in a class B single-ended push-pull output amplifier, in order to reduce crossover distortion, a small amount of current, ie, an idle current, flows through the output stage transistor even when there is no signal.

This idle current must be maintained at a constant value regardless of any environmental changes.

This is because if this current decreases, the crossover distortion will increase, and if it increases, there is a risk that the output transistor will be destroyed.

However, the conventional circuit has the disadvantage that it is difficult to keep the idle current constant against fluctuations in the power supply voltage.

The details will be explained below using FIG. 1.

Figure 1 shows a circuit diagram of a conventional single-ended push-pull output amplifier.
is a single-ended push-pull output amplifier, 2 is a first stage differential amplifier, 3 is a second stage differential amplifier, 4 and 5 are transistors and diodes forming a current mirror circuit, 6°7 is a driver stage transistor, 8 and 9 are output stage transistors, 10 is a Zener diode, 11
15 to 15 are resistors, and 16 and 17 are power supplies.

In the conventional single-ended output amplifier shown in the figure, a current is passed through the resistor 15, and the resulting voltage drop is used as the bias voltage of the driver stage and the output stage transistors 6, 7, 8, and 9.

That is, the Zener diode 10 and the resistor 11 cause current to flow through the two transistors forming the first stage differential amplifier 2.

As a result, current flows through the resistors 12 and 13, causing a voltage drop.

This becomes a bias voltage for the second stage differential amplifier 3, and current flows through the two transistors that make up the second stage differential amplifier 3.

In this way, a current is passed through the resistor 15, and an idle current is passed through the transistors 7, 8, and 9 of the driver stage and the output stage using the resulting voltage drop as a bias voltage.

However, the current flowing through the resistor 15, that is, the current flowing through the transistors constituting the second stage differential amplifier 3, cannot be kept constant against fluctuations in the power supply voltage.

This is because when the power supply voltage fluctuates, the Zener diode 1
The current flowing through the zero voltage varies, and its Zener voltage also varies, albeit slightly.

When the Zener voltage fluctuates, the current flowing through the resistor 11 and the current flowing through the transistors forming the first-stage differential amplifier 2 also fluctuate.

In that case, the voltage drop due to the resistance 12°13, that is, 2
The bias voltage of the stage differential amplifier 3 also fluctuates, and the current flowing through the transistors that constitute it, in other words, the resistor 15.
This is because the current flowing through the circuit also fluctuates.

As a result, as the power supply voltage increases, the current flowing through the resistor 15 increases, and the idle current increases.

Conversely, if the power supply voltage decreases, the current flowing through the resistor 15 decreases, and the idle current also decreases.

Furthermore, as the power supply voltage increases, the collector losses of the transistors 6.7 and 8.9 in the driver stage and the output stage increase, and the junction temperature of these transistors increases.

As the junction surface temperature of a transistor increases, the base-emitter voltage decreases.

In other words, if the base-emitter voltage remains the same, the collector current, or idle current, will increase.

Conversely, if the power supply voltage decreases, the idle current decreases.

As explained above, the conventional single-ended push-pull output amplifier has a drawback in that the idle current cannot be kept constant against fluctuations in the power supply voltage.

SUMMARY OF THE INVENTION An object of the present invention is to provide an output amplifier which eliminates the above-mentioned drawbacks of the prior art and maintains an idle current constant despite fluctuations in power supply voltage.

In order to achieve the above object, the present invention is characterized in that a current circuit is provided to compensate for changes in idle current due to fluctuations in power supply.

The present invention will be explained in detail below with reference to the drawings.

FIGS. 2 and 3 show circuit diagrams of two embodiments of the present invention.

In FIGS. 2 and 3, the same symbols as in FIG. 1 indicate the same parts, and 1B and 19 indicate resistors.

In the circuit shown in FIG. 2, the mechanism through which the idle current flows is exactly the same as in the conventional example shown in FIG.

The only difference is that the current flowing through the resistor 14 is
This means that the current flows into the two transistors forming the second stage differential amplifier 3 and the current flows through the resistor 1B.

This allows the idle current to be kept constant regardless of fluctuations in the power supply voltage.

The reason will be explained below. As described in the conventional example, the current flowing through the differential amplifiers 2 and 31, in other words, the current flowing through the resistors 11 and 144 fluctuates due to fluctuations in the power supply voltage.

For example, when the power supply voltage increases, the current flowing through the resistor 144 increases.

However, in the embodiment of the present invention, this current is shunted to a pair of transistors and a resistor 18.

Since the current flowing through the resistor 18 is approximately proportional to the power supply voltage, it increases as the power supply voltage rises.

Therefore, if the value of this resistor 18 is appropriately selected to equalize the increase in the current flowing through the resistor 14 and the increase in the current flowing through the resistor 18, the current flowing through one set of transistors constituting the differential amplifier 3 can be expressed as a resistance. The current flowing through 15 remains constant regardless of the power supply voltage.

If the current flowing through the resistor 15 is constant, the voltage across it, that is, the bias voltage of the transistors in the first driver stage and the output stage, is also constant, so the idle current can also be maintained at a substantially constant level.

However, as described above, it is not sufficient to suppress the fluctuations in the bias voltage of the single driver stage and the output stage in response to fluctuations in the power supply voltage.

That is, as described in the conventional example, the collector loss of the driver stage and output stage transistors increases or decreases in proportion to the power supply voltage, so the junction surface temperature thereof fluctuates.

If the bias voltage is constant, the idle current will vary depending on the junction surface temperature.

Although the amount of change in the junction surface temperature itself is not a big deal, the current due to it changes exponentially, as is well known, so the fluctuation in the idle current due to this cannot be ignored.

Therefore, in order to keep the final idle current constant, it is necessary to change the bias voltage in inverse proportion to the power supply voltage.

That is, when the power supply voltage increases, the bias voltage must be decreased, and when the power supply voltage decreases, the bias voltage must be increased.

The embodiments of the present invention can also accommodate this.

That is, if the value of the resistor 18 is selected to be lower than the previously set value, the amount of change in the current flowing through the resistor 18 will be greater than the amount of change in the current flowing through the resistor 14, and the current flowing through the resistor 15, in other words, the bias voltage. changes in inverse proportion to the power supply voltage.

Therefore, if the value of this resistor 18 is appropriately selected, the idle current can be kept constant with even higher accuracy.

In the circuit of Fig. 3, the resistance 1 in the circuit of Fig. 2 is also
Resistor 19 fulfills the role of 8 and has a similar effect.

As described above, according to the present invention, the idle current of a single-ended output amplifier can be kept constant regardless of fluctuations in the power supply voltage, and a practical output amplifier can be provided by eliminating the drawbacks of the conventional technology. It is something.

[Brief explanation of drawings]

FIG. 1 shows a circuit diagram of a conventional single-ended push-pull output amplifier, and FIGS. 2 and 3 are circuit diagrams showing two embodiments of a single-ended push-pull output amplifier according to the present invention. 2... First stage differential amplifier, 3... Second stage differential amplifier, 4... Transistor, 5...
...Diode, 6゜7...Driver stage transistor, 8,9...Output stage transistor, 10
...Zener diode, 11-15...
...Resistance, 16.17...Power supply, 18°19.
·····resistance.

Claims (1)

[Claims] 1. A second differential amplifier circuit and a push-pull output circuit each having a first and a current mirror circuit connected in series as loads; In the amplifier configured to bias the push-pull output circuit by a voltage drop element connected between the first or second differential amplifier circuit and the current mirror circuit, A resistor is connected between the common connection part of the two constituent amplifying elements and the output power supply of the differential amplifier circuit, and the resistance value of the resistor is adjusted against fluctuations in the power supply voltage of the amplifier. An output amplifier characterized in that the current flowing through the amplification element and the voltage across the voltage drop element caused by the current, that is, the bias current of the push-pull output circuit are kept constant.