JPH0720064B2 - Transistor power amplifier - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a transistor power amplifier for amplifying an AM modulated wave, and more particularly to a transistor power amplifier used in a television broadcast machine.

[Conventional technology]

Conventionally, a transistor power amplifier used in this type of television broadcaster is required to have good linearity, high output, and low power consumption, so that it is often used in transistor class AB operation, and particularly high output is required. The final stage transistor power amplifier has a class AB operation closer to class B.

[Problems to be Solved by the Invention]

The above-described conventional transistor power amplifier has a drawback that the linearity changes depending on the average picture level (hereinafter referred to as APL) because it is a class AB operation, so-called APL fluctuation is large.

APL is the average image level, and since the television video signal has a level according to light (white) and dark (black), the APL changes depending on the light / dark state. In addition, in the television video signal, the level that the signal can take (0 to 100
%), The characteristics of the transmitted signal are measured at a white level APL 90% point, a black level APL 10% point, and an intermediate APL 50% point. On the other hand, in the transmission of the television signal, the carrier wave is amplitude-modulated by the negative video signal. Therefore, the carrier wave amplitude is large at the black level and becomes small at the white level. This is shown in FIG.

It is generally known that in the final stage class AB power amplifier using bipolar transistors, the output characteristics differ depending on the value of APL. That is, when the APL is 50% or less, that is, when the average value of the amplitude of the television broadcast wave is above a certain level and a current above the certain level flows through the transistor, the linearity near the white level is good and the differential gain is flat , APL 90%, that is, when the amplitude of the television broadcast wave is small and the current flowing through the transistor is small, the linearity near the white level becomes poor, and the gain decreases,
As a result, the differential gain tends to be blocked.

[Means for Solving the Problems]

The transistor power amplifier of the present invention has a detector for detecting a part of an input signal and an emitter follower circuit of a transistor for impedance-converting the output of the detector, and the class AB operation transistor power according to the output of the above-mentioned detector. It controls the base bias voltage of the amplifier. For this reason APL90
When the current flowing through the transistor power amplifier is reduced by% and the differential gain on the white level side is blocked, it operates in the direction of increasing the base bias voltage and extending the differential gain, and it is possible to reduce the change in differential gain due to APL change. I am trying.

〔Example〕

Next, the present invention will be described with reference to the drawings. FIG. 1 is a system diagram of an embodiment of the present invention.

According to FIG. 1, the signal input from the input 1 of the transistor power amplifier is partially extracted by the directional coupler 2 and detected by the detector 7. Since the detector 7 employs negative detection, a large negative voltage is detected when APL is low, such as 10%, that is, when the amplitude of the carrier is large, and a small negative voltage is detected when APL is high, such as 90%. To be detected. That is, when the APL is high, the output of the detector 7 is higher than when the APL is low. Next, the output of the wave detector 7 is impedance-converted through the emitter follower circuit 8 of the transistor, the base bias resistor 9, the RF blocking coil.
It is supplied as a base bias to the base of the class AB operation transistor 4 through 11. Therefore, in the case of APL 90%, when the current flowing in the class AB transistor power amplifier is small, the gain on the white level side decreases, and the differential gain on the white side is blocked, the output voltage of the detector 7 rises and the base voltage increases. It is possible to raise the bias voltage and add the current flowing in the class AB operation transistor to extend the differential gain on the white side and make it flat.

FIG. 2 and FIG. 3 are circuit diagrams in which FIG. 1 is made more concrete. In FIG. 2, a part of the input signal is taken out by the voltage dividing resistor 21 and the terminating resistor 22 instead of the directional feeder 2 of FIG.

Next, the signal detected by the detection diode 23 is the high frequency bypass capacitor 24, the resistor 25, and the DC blocking capacitor 26.
The voltage waveform is shaped according to the average level of the image components, that is, APL. The voltage waveform corresponding to the above APL has a DC voltage clamped by a clamp level setting variable resistor 27 and a clamping diode 28, and then impedance-converted by an emitter follower transistor 29, a resistor 31, a bias circuit diode 32, and a bias voltage. It is added to the conventional base bias circuit including the setting conversion resistor 33 and the base bias circuit series resistor 34.

Further, in FIG. 3, a part of the input signal 1 is taken out by the directional coupler 2, passes through the DC blocking capacitor 26, and is detected by the detection diode 23. The detected signal is charged and discharged in the capacitor 38. Moreover, the voltage generated across the capacitor 38 changes depending on the average level (APL) of the image component, but the changing speed can be arbitrarily selected by the values of the resistors 39, 40, 41 and the capacitor 38, and the APL changes. However, it is possible to follow steady changes in the differential gain characteristics and the like. Further, the DC voltage is clamped by the clamp level setting variable resistor 27 and the resistor 37, the impedance is converted by the transistor emitter follower circuit 29, and it is added to the base bias circuit of the AB class operating transistor 4.

Therefore, the circuit of FIGS. 2 and 3 changes the base bias voltage according to the average level (ALP) of the image component, unlike the conventional case, and can reduce the ALP fluctuation.

〔The invention's effect〕

As described above, the present invention detects a part of the input signal in the class AB transistor power amplifier for amplifying the television broadcast wave by using the wave detector, and after the waveform shaping, the impedance is converted by the emitter follower circuit of the transistor. By adding it to the rear base bias circuit, the base bias voltage is controlled so as to suppress the characteristic fluctuation due to the APL fluctuation, and it has the effect of reducing the APL fluctuation such as the differential gain characteristic.

In addition, a circuit consisting of a resistor and a capacitor should be installed between the detector that detects a part of the input signal and the emitter follower circuit of the transistor that impedance-converts the detector output, and the values of these resistors and capacitors should be set arbitrarily. This has the effect of being able to follow steady changes such as the differential gain characteristics even if the APL changes.

[Brief description of drawings]

FIG. 1 is a system diagram of one embodiment of the present invention, FIG. 2 and FIG.
FIG. 4 is a circuit diagram embodying FIG. 1, and FIG. 4 is a conventional circuit diagram. FIG. 5 is a diagram showing a modulated wave amplified by the transistor power amplifier of the present invention. 1 ... Input of transistor power amplifier, 2 ... Directional coupler, 3 ... Input matching circuit of transistor power amplifier,
4 ... AB class operation transistor, 5 ... Transistor power amplifier output matching circuit, 6 ... Transistor power amplifier output, 7 ... Detector, 8 ... Transistor emitter follower circuit, 9 ... Base bias circuit series Resistance, 10
…… Bypass capacitor, 11 …… RF blocking coil of bias circuit, 21 …… Dividing resistor, 22 …… Terminating resistor, 23 …… Detection diode, 24 …… High frequency bypass capacitor, 25 ……
Resistance, 26 ... DC blocking capacitor, 27 ... Variable resistance for setting clamp level, 28 ... Clamping diode, 29 ...
… Emitter follower transistor, 30,31 …… Resistance, 32…
… Bias circuit diode, 33 …… Bias voltage setting variable resistor, 34 …… Base bias circuit series resistor, 35,3
6 …… DC bias input terminal for base bias, 37 …… Clamp level setting resistor, 38 …… Capacitor, 39,40,41…
… Resistors, 42… Variable resistors, 43… High-frequency bypass capacitors.

Claims (1)

[Claims] 1. A class AB operation transistor power amplifier for amplifying an AM modulated wave, comprising: a detector for detecting a part of an input signal; and an emitter follower circuit of a transistor for impedance-converting an output of the detector. In a transistor power amplifier that controls the base bias voltage of the class AB transistor power amplifier according to the output of the detector, a detector that detects a part of the input signal and an emitter follower circuit of the transistor that impedance-converts the output of the detector And a capacitor for charging and discharging the detected signal, and a circuit including a resistor and a capacitor connected to the capacitor are incorporated, and the speed at which the voltage generated across the capacitor for charging and discharging the detected signal changes is incorporated. Follows steady changes in differential gain characteristics, etc. by arbitrarily setting the value of each element in the circuit Transistor power amplifier, characterized in that that can.