JPH0760978B2 - Power amplifier - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power amplifier, and more particularly to a class AB power amplifier using a power MOSFET.

[0002]

2. Description of the Related Art FIG. 1 shows a conventional class AB amplifier using a power MOSFET, particularly a gate bias voltage control circuit. In FIG. 1, TR1 is a power MOSFET that constitutes a power amplifier whose source is grounded.
The circuit configured by the adder / subtractor circuit 11 and the resistors R3 to R6 is a differential amplifier having a gain of 1, and supplies the power supply voltage V _CC to the resistor R.
2. E _c difference E _a − of voltage E _a and V _CC divided by the variable resistor RV1 divided by resistor R1 and the temperature sensor 12.
E _c is applied to TR1 as the gate bias voltage V _GS . Here, the variable resistor RV1 is for adjusting the gate bias voltage V _GS of TR1 to allow an appropriate drain idle current to flow through TR1, and the temperature sensor 12 is a thermistor or diode attached to the radiator of TR1. MOSF having a negative temperature coefficient by detecting the temperature of the radiator and controlling the bias voltage of TR1.
The gain fluctuation due to the temperature change of ET is compensated.

Generally, in a class AB power amplifier circuit using MOSFETs, a signal current is superimposed on a large idle current. Therefore, the peak value of the signal current can be changed by changing the idle current, so that the bias voltage is controlled. By doing so, it is possible to control the gain apparently.

The transistor TR2 is for turning on and off the bias voltage. Power MOSFET in general
However, when trying to obtain good linearity, the operating point current (idle current) of the drain is often large, so leaving the idle current flowing is large even when the input signal is applied to the amplifier. Power is lost, and it is useless. Therefore, the idle current is turned on / off by turning on / off the TR2 and turning on / off the bias voltage by the signal detecting the presence or absence of the input signal or the electronic key signal.

However, the above-mentioned circuit has the following problems. That is, in this circuit, since the internal loss of TR1 is zero when there is no signal, the junction temperature of TR1 is low.
When an input signal is applied in this state, the bias voltage turns on, the signal current superimposed on the idle current flows to the drain, and a large internal loss occurs, resulting in an increase in junction temperature. In the case of general power MOSFET,
Since the idle current in the class AB linear amplifier circuit is large and the internal loss due to this is large, the junction temperature rises and the gain decreases even when the signal current is small.

Considering the above state from the viewpoint of the temporal change of the junction temperature, the following is obtained. That is, since the radiator to which TR1 is attached has a large heat capacity, the thermal time constant is considerably large and the temperature does not change suddenly.
Since the thermal time constant formed by the internal thermal resistance of 1 (between the case and the junction), the contact thermal resistance (between the case and the radiator) and the heat capacity of TR1 is small, the temperature at the junction will not decrease after the loss occurs. Rise in a relatively short time. This thermal time constant is TR
Depending on the size of 1 and the heat dissipation system,
Even large ones take 2-3 seconds. When the loss continues, the temperature of the radiator rises due to the small thermal constant in minutes.

In the circuit of FIG. 1, the temperature sensor 12 attached to the radiator controls the bias voltage. However, since this sensor can detect only the slow temperature change of the radiator, the signal rises. It is not possible to compensate for changes in gain due to changes in temperature that sometimes rise sharply.

FIG. 2 is a diagram showing these time relationships. (A) is the input signal, (b) is the bias on / off signal synchronized with the input signal, (c) is the temperature change at the junction of TR1, (d) shows a change in gain of TR1 and (e) shows a change in output of TR1.

Now, when the bias (b) is turned on, the junction temperature (c) rises by ΔT _{A in a} short time t, and then ΔT _B rises as the temperature of the radiator rises. As described above, the circuit of FIG. 1 can compensate ΔT _B , but cannot compensate the temperature change due to ΔT _A.
In general, since the MOSFET has a large negative temperature coefficient, the gain of TR1 rapidly changes from large to small during the rising t seconds as shown in (d). That is, if the rated output of the power amplifier is a thermally stable output level after t seconds, an abnormally large output is generated for t seconds at the time of rising. Therefore, during this t seconds, the output voltage is saturated, distortion is generated, and an overcurrent flows in TR1, which causes damage to TR1 and the operation of the overcurrent protection circuit of the power supply unit, which causes many problems. .

[0010]

Therefore, the present invention is
It is intended to obtain a power amplifier in which the output for t seconds at the time of start-up is as similar as possible to the thermally stable rated output after t seconds. More specifically, MOSFE
Among the thermal characteristics of T, it is intended to obtain a power amplifier capable of compensating for a temperature rise having a small thermal time constant due to an internal thermal resistance or the like, which has been overlooked in the past.

[0011]

According to the present invention, the temperature change of the junction portion of TR1 is divided into the heat sink ΔT _B having a long thermal time constant and the internal temperature change ΔT _A having a short thermal time constant. Separately, it has a configuration in which the gain change caused by the internal temperature change of TR1 is compensated by each separate compensation circuit. That is, according to the present invention, a power MOSFET and this power MOS
A temperature sensor attached to the FET, an on / off signal generating means for changing a bias control voltage into a bias on / off signal of a desired voltage, a first input terminal for receiving an output signal of the temperature sensor, and a second input terminal for the bias. A class AB power amplifier configured to receive an on / off signal and add / subtract a circuit for supplying a difference voltage between these two input signals to the power MOSFET as a gate bias voltage, and to compensate a gain variation due to a temperature change of the power MOSFET. In the circuit, a means for changing the bias control voltage into an on / off signal of a predetermined voltage in parallel with the on / off signal generating means and a thermal time constant of an internal temperature change of the power MOSFET for the on / off signal of the predetermined voltage. Another on / off circuit that includes an integrator circuit (time constant circuit) that gives a given time constant. A signal generating means is provided, the power amplifier is obtained, characterized in that the on-off signal applied to the time constant was set to be input to the second input terminal with said bias on-off signal.

Further, according to the present invention, in the class AB power amplifier circuit configured to compensate the gain variation due to the temperature change of the power MOSFET as described above, the gate bias from the adder / subtractor circuit to the power MOSFET. A power amplifier characterized in that an integrator circuit (time constant circuit) that gives a time constant matching a thermal time constant of an internal temperature change of the power MOSFET to the gate bias voltage is provided in a path for supplying a voltage. To be

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 3 is a circuit diagram of an embodiment of the present invention. R7 and R8 divide V _CC to form a compensation voltage E _b for compensating the internal temperature change ΔT _B of TR1, and R9 and capacitor C1 are an integrating circuit. Reference numeral 13 is a voltage follower having a gain of 1, and provides the adder / subtractor circuit 11 with a compensation voltage obtained by integrating the _Eb voltage with R9 and C1. Integrator circuit R9, C
If the time constant of 1 is matched with the thermal time constant of the internal temperature change of TR1, and the _Eb voltage is turned on / off by TR3 by the bias control signal, the voltage follower 13 is provided with a compensation voltage commensurate with the internal temperature change of TR1. Output.

Further, the bias voltage component E _b for compensating the gain change amount caused by the change in the internal temperature of TR1 is considered as follows. That is, in the case of a class AB linear amplifier using a MOSFET, since the drain idle current is large, as shown in FIG. 4, TR1 loses power even when the amplifier output is rated or when there is no signal (only idle current). Does not change significantly. Therefore, the internal temperature of TR1 largely depends on whether the bias voltage is on or off (whether or not there is an idle current). Therefore, if the bias voltage _Eb commensurate with compensating for this difference is determined, then TR1 will be approximately It is possible to compensate for the change in gain caused by the change in internal temperature.

FIG. 5 shows the time relationship of each part according to the circuit of the present invention. When the bias control signal (f) is turned on, the gate bias voltage V _GS is for flowing an idle current as shown in (h). The fixed bias voltage E _a rises first, the voltage E _b by the integrating circuit of R9 and C1 rises for t seconds, and then the compensation voltage E _c for the temperature change of the radiator.
Is added. In this way, V compensated by this circuit
By applying _GS to TR1, it is possible to compensate for the gain change caused by the junction temperature change (g) of TR1, so that the output of the amplifier becomes constant as shown in (i), and the output changes at the rising edge of the input signal. It is possible to eliminate the phenomenon that becomes excessive.

FIG. 6 is a diagram showing a circuit configuration of the second embodiment of the present invention. CD is a Zener diode, the Zener voltage is E _Z , R11 is a resistance for flowing the leakage current of CD, and the resistance value is large. If R10 << R11, the time constant of this integration circuit is R10 × C. is there. In FIG. 6, when TR2 is turned on and off in response to a bias control signal, an output E _B of the adding and subtracting circuit 11 rises as shown in FIG. 7 of the (j). If E _Z <E _B , the adder / subtractor circuit 11
In the rise of the voltage _{E B,} the output voltage _{E Z}
The charge current flows to C2 because the CD is non-conducting until the temperature exceeds the threshold, but when _EZ is exceeded, charging starts and R10 ×
The voltage rises with the time constant of C2. When the voltage at which the output of the adder / subtractor 11 is turned off falls, the charge of C2 becomes C
Since the discharge is performed through D and R10, the cathode terminal voltage of CD, that is, the gate bias voltage V _G of TR1 is as shown in (h) of FIG. 7, and the same voltage as (i) of FIG. 5 can be obtained.

[0017]

As described above, according to the present invention, the temperature change of the junction portion of the power MOSFET is calculated by the change ΔT _{B of the} radiator having a long thermal time constant and the internal temperature change Δ of the short thermal time constant Δ. The output of the amplifier can be made constant by separating into T _A and compensating the gain change caused by the internal temperature change of the power MOSFET by each separate compensating circuit. It is possible to avoid an excessive phenomenon and prevent damage to the power transistor.

[Brief description of drawings]

FIG. 1 is a diagram showing a class AB amplifier using a conventional power MOSFET, particularly focusing on a gate bias voltage control circuit.

2 is a diagram showing an input signal, a bias on / off signal, a junction temperature, a gain, and an output signal in the device of FIG. 1, respectively.

FIG. 3 is a diagram showing a configuration of an exemplary embodiment of the present invention.

FIG. 4 is a diagram showing a relationship between inputs and outputs in the circuit of the present invention.

FIG. 5 is a diagram showing a time relationship of each part in the circuit of the present invention.

FIG. 6 is a diagram showing a circuit configuration of a second exemplary embodiment of the present invention.

FIG. 7 is a diagram showing waveforms of an output of an addition / subtraction circuit and a gate bias voltage in the second embodiment.

[Explanation of symbols]

 11 Addition / subtraction circuit 12 Temperature sensor 13 Voltage follower TR1 Power MOSFET TR2 Bias voltage on / off transistor RV Addition / change resistance CD Zener diode

Claims (2)

[Claims] 1. A power MOSFET and this power MO
A temperature sensor attached to the SFET, an on / off signal generating means for changing a bias control voltage into a bias on / off signal of a desired voltage, a first input terminal for receiving an output signal of the temperature sensor, and a second input terminal for the bias. And an adder / subtractor circuit that receives an on / off signal and supplies a difference voltage between these two input signals to the power MOSFET as a gate bias voltage, and is configured to compensate the gain variation due to the temperature change of the power MOSFET.
In the class power amplifier circuit, a means for changing the bias control voltage into an on / off signal of a predetermined voltage in parallel with the on / off signal generating means, and a thermal time constant of an internal temperature change of the power MOSFET for the on / off signal of the predetermined voltage. Another on / off signal generating means including an integrator circuit (time constant circuit) for giving a time constant that matches the above is provided, and the on / off signal given the time constant is input to the second input terminal together with the bias on / off signal. A power amplifier characterized in that 2. A power MOSFET and this power MO
A temperature sensor attached to the SFET, an on / off signal generating means for changing a bias control voltage into a bias on / off signal of a desired voltage, a first input terminal for receiving an output signal of the temperature sensor, and a second input terminal for the bias. And an adder / subtractor circuit that receives an on / off signal and supplies a difference voltage between these two input signals to the power MOSFET as a gate bias voltage, and is configured to compensate the gain variation due to the temperature change of the power MOSFET.
In the class power amplifier circuit, an integration circuit that gives a time constant that matches the thermal time constant of the internal temperature change of the power MOSFET to the gate bias voltage is provided in the path for supplying the gate bias voltage from the addition / subtraction circuit to the power MOSFET. A power amplifier characterized by being provided.