JPH033962B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention detects the output of a transistor linear amplifier and compensates for the change in the base bias power supply of the transistor, thereby reducing the amplifier operating point caused by the junction temperature change due to the internal loss of the power transistor. The present invention relates to an operating point compensation circuit for a transistor linear amplifier that compensates for fluctuations and improves intermodulation distortion and input/output linearity of the amplifier.

Conventionally, in this type of transistor amplifier, a thermal sensor is attached to a heat sink for the purpose of preventing thermal runaway of the transistor. This thermal sensor is installed on the same heatsink as the transistor, but it can only sense changes in ambient temperature and heat generated from the transistor with a delay time of one minute or more, so sudden changes in junction temperature can occur. It is impossible to detect. Therefore, the internal loss of the transistor due to a change in the output level of the amplifier results in a change in the junction temperature, and it is impossible to compensate for the fluctuation in the operating point of the amplifier caused by this.

In a transistor type AB class linear amplifier,
If the output power P ₀ changes, the internal loss P _D of the transistor also changes. Due to the change in internal loss, when the transistor is on an infinite heatsink,
Even if the heatsink temperature is constant, the transistor junction temperature (T _J ) varies due to the transistor case and internal thermal resistance.

Generally, a change in the junction temperature of a power transistor has a transient response due to the thermal resistance and heat capacity of the transistor, and reaches a steady value in several seconds to more than ten seconds.

In a class AB transistor amplifier, the collector current operating point is determined by applying a bias voltage V _BE between the base and emitter. However, when a junction temperature change occurs, this collector current operating point changes due to the temperature dependence of the transistor operation. I end up moving.

The higher the output of the amplifier, the larger the power transistor, and therefore the larger the internal loss variation and the larger the operating point variation.

Conventional circuit heatsinks and temperature sensors attached to the transistor case cannot detect sudden temperature changes inside the transistor, and AB
In a class push-pull amplifier, there is only one optimal operating point, so the intermodulation distortion (IMD) of the amplifier changes depending on the output level, as shown in Figure 1.
In other words, point a is the point set as the optimum operating point according to the junction temperature at rated output.
At point b, the IMD deteriorates because the amplifier is in the saturation direction. Also, at point c, the internal loss is smaller than at point a, so the operating point moves.
IMD gets worse. At point d, the collector current AC amplitude becomes less than the operating point current, so the amplifier operates in class A and improves IMD. Similarly, the input/output linearity of the amplifier becomes non-linear as shown in FIG.

The present invention has been made to improve these phenomena.

As mentioned earlier, in a class AB transistor amplifier, a change in output level causes a change in internal loss, causing a causal relationship in which the operating point of the amplifier shifts. In the case of silicon transistors, the dependence of the junction temperature change on the operating point is:
It is known that the base bias voltage is approximately 2 mV/°C. Therefore, by detecting the output level of the amplifier and passing it through a compensation circuit, we can calculate the internal loss at that output level and the optimal base bias operating point voltage accordingly, and apply it to the base to adjust the operation of the amplifier. By keeping the point constant, the amplifier's collector current operating point remains constant even if the output level changes.

Next, the compensation circuit according to the present invention will be explained in detail using FIG. 1 is a transistor type AB class linear amplifier; 2 is a compensation voltage detection circuit using transistor internal loss; a detection circuit 2-1 detects a change in the output of the linear amplifier; and a voltage detection circuit 2-1 detects the voltage detected by the detection circuit 2-1. and a diode polygonal line approximation circuit 2-3 that performs quadratic approximation correction on the detection output of the detection circuit 2-2. 3
4 is a time constant circuit, 4 is a heat sink temperature sensor diode, and 5 is a bias constant voltage power supply. First, the output voltage of the transistor type AB class linear amplifier 1 is input to the compensation voltage detection circuit 2 due to transistor internal loss. In step 2, V ₀ is linearly detected to obtain an output envelope. Now, when the transistor collector instantaneous voltage changes, the internal loss of the transistor is expressed by the following equation.

P _D = E _B・ε _C /πR _C −ε _C ² /4R _C (W) ……(1) P _D …… Internal power loss of the transistor E _B …… Collector power supply voltage of the transistor ε _C …… 〃 Instantaneous Voltage R _C …… 〃 Load resistance where ε _C <E Load resistance R _L of _B transistor class AB linear amplifier 1
If is a stepped-up value of the transistor collector load resistance R _C , then R _L = ηR _C ……(2) V _O =√ε _C ……(3) holds true.

From equations (1), (2), and (3), the following equation holds true.

If the internal thermal resistance of the transistor is θ (° C./W), then the temperature rise T _JI due to internal loss at the transistor junction is expressed by the following equation.

Since the dependence of the junction temperature change on the operating point is 2 mV/°C when converted to the base bias voltage, the operating point fluctuation occurs due to the junction temperature rise T _JI , and the base bias voltage to compensate for this is 2 mV/℃.
V _OC is obtained by the following equation.

Since equation (6) is a quadratic equation with respect to the output voltage V _O of the linear amplifier, the coefficient and quadratic approximation correction are performed by a quadratic approximation circuit using diode broken line approximation to obtain V _OC . Next, by passing through an electrical time constant circuit 3 that approximates the thermal time constant of the transistor, a correction voltage V _OC ' corresponding to internal loss fluctuations of the transistor is obtained.

On the other hand, the temperature sensor diode 4 attached to the radiator is driven with a constant current to obtain a forward voltage V _D corresponding to the radiator temperature. Further, V _D and V _OC ' are added to obtain the base bias compensation voltage V _BC . V _BC is a compensation voltage for changes in transistor junction temperature due to changes in ambient temperature, changes in heatsink temperature, and changes in transistor internal loss.

By subtracting this V _BC from the bias constant voltage V _BS , the base bias applied voltage V _BE is obtained.

By applying a compensated base bias V _BE to the transistor base, the operating point of the transistor is compensated, and the operating point of the amplifier remains constant even if the output level changes. becomes good, and as shown in FIG. 5, the input/output linearity also becomes good.

As explained above, when the operating point compensation circuit according to the present invention is attached to a class AB transistor linear power amplifier, it is possible to solve the problems of IMD and linearity at medium and low levels as shown in Figures 1 and 2. This is effective because these phenomena are more pronounced as the transistor handles a larger amount of power.

[Brief explanation of the drawing]

Figure 1 is a characteristic diagram of intermodulation distortion (IMD) with respect to output changes without operating point compensation, and Figure 2 is
A characteristic diagram showing the input/output linearity when temperature compensation is not performed. FIG. 3 is a block diagram of an example of an implementation circuit of the present invention, and FIGS. 4 and 5 are obtained by using the compensation circuit according to the present invention. A characteristic diagram of intermodulation distortion and input/output linearity is shown. 1...Transistor class AB linear amplifier, 2...
...Compensation voltage detection circuit, 2-1...Detection circuit, 2-
2...Detection circuit, 2-3...Approximate circuit, 3...Time constant circuit, 4...Radiator temperature sensor diode,
5... Constant voltage power supply for bias.

Claims (1)

[Claims] 1. A circuit for compensating for fluctuations in the operating point of a transistor linear amplifier due to temperature changes, comprising a detection circuit for detecting an output voltage change that causes a change in transistor internal loss of the transistor linear amplifier, and a voltage detected by the detection circuit. a detection circuit that detects the detection circuit, a diode polygonal line approximation circuit that performs quadratic approximation correction on the detection output of the detection circuit, and a thermal time constant of the transistor that extracts a voltage according to the internal loss fluctuation of the transistor from the output of the polygonal line approximation circuit. A time constant circuit having an approximate time constant, a temperature sensor diode that generates a voltage according to the heat sink temperature of the transistor, and a constant voltage power source for biasing the transistor linear amplifier, and a constant voltage power source for biasing the transistor linear amplifier. A voltage according to the internal loss fluctuation of the transistor and a voltage generated by the temperature sensor diode are subtracted from the bias voltage, and the obtained voltage is added as the bias voltage of the transistor to improve intermodulation distortion and input/output nonlinearity. An operating point compensation circuit for a transistor linear amplifier, characterized in that: