JP2855998B2 - Linearity compensation circuit for insulated gate field effect transistor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a linearity compensation circuit for an insulated gate field effect transistor, and more particularly to a linearity compensation circuit for an insulated gate field effect transistor for high frequency power amplification for amplifying power of video signal information.

[0002]

2. Description of the Related Art FIG. 3 shows a schematic configuration of a high frequency power amplifier circuit using an insulated gate type field effect transistor (hereinafter simply referred to as a transistor) as an amplifier. An input matching circuit 1 is provided at a gate input of the transistor 2, and an input signal IN becomes a gate input of the transistor 2 via the input matching circuit 1, and power is amplified. This power-amplified drain output is derived as an output OUT via the output matching circuit 3.

In this case, the circuit constants of the input / output matching circuits 1 and 3 are determined so that optimum matching can be obtained at the maximum output power. On the other hand, with respect to the linearity of the power amplifier circuit, only a certain degree of characteristics is secured by optimizing the bias condition of the transistor 2, and no compensation for the linearity is taken into account. .

Here, a transistor (insulated gate field effect transistor) has a device structure on the output side (drain side) formed by a PN junction. Therefore, the capacitance of the PN junction changes according to the change of the output voltage, and as a result, there is a fact that the dynamic impedance seen from the output side to the amplifier circuit changes according to the output voltage.

The PN junction capacitance Cja and the output voltage Vo
Is expressed by the following equation.

[0006]

(Equation 1)

In the above equation (1), Cja is the average junction capacitance for one cycle when the output voltage change is assumed to be a sine wave, and Cjo is the junction capacitance when Vo = 0. Then, the dynamic impedance on the output side changes in accordance with the change in the output voltage, and as a result, a mismatch is given to the output matching circuit 3 by the output voltage of the amplifier circuit.
This mismatch affects the linearity at low output power.Therefore, in a high-frequency high output power amplifier circuit, when a small signal with small output power is input, the amplifier circuit is used with poor linearity. Will be.

[0008] In particular, in a television broadcaster that handles video signals, it causes adverse effects on characteristics such as differential gain (DG), differential phase (DP), and third-order cross modulation that largely depend on linearity. I have.

Therefore, a power amplifier circuit shown in FIG. 4 to which a linearity compensation circuit is added has been proposed. In FIG. 4, a voltage proportional to the gate input voltage of the transistor 2 is applied to the varactor diode 10 via the input isolator 9, and a change in the capacitance of the varactor diode 10 is supplied to the drain output side of the transistor 2 via the output isolator 11. I am trying to do it.

The inductance 13 supplies the bias voltage 12 to the varactor diode 10.
The isolator 9 transmits the input voltage on the amplification input side to the varactor diode 10 of the compensation circuit and prevents the impedance of the compensation circuit from being fed back to the amplification input side. The isolator 11 transmits the capacitance value of the varactor diode 10 that changes according to the amplification input voltage to the amplification output side, and does not feed back the voltage change on the amplification output side to the compensation circuit.

In the circuit of FIG. 4, the capacitance of the varactor diode 10 is changed according to the input voltage, and this capacitance is supplied to the output side (drain side) of the transistor 2, so that the transistor 2 is viewed from the amplified output. Since the impedance is compensated and the compensation is performed on the time axis, the linearity compensation characteristics are extremely excellent.
Is used, the circuit scale is increased, and the cost is increased.

Further, since the change in the output capacitance of the transistor 2 is canceled by the change in the capacitance of the compensation circuit, it is necessary to set the relationship between the voltage change on the compensation circuit side and that on the amplification circuit side such that the phases are inverted. Therefore, the electrical length on the compensation circuit side must always be λ / 2 (λ indicates a signal wavelength), and there is a disadvantage that circuit design is difficult.

[0013]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a low-cost insulated gate field effect transistor linearity compensation circuit with a simple structure.

Another object of the present invention is to provide a linearity compensation circuit for an insulated gate field effect transistor which eliminates restrictions on circuit design.

[0015]

According to the present invention, there is provided a linearity compensating circuit for an insulated gate field effect transistor for amplifying high frequency power, a detecting circuit for detecting a gate input power of the transistor, and inverting the detected output. an inverting circuit, a variable capacitance element the capacitance value is controlled according to the inverted output, seen including a deriving means for deriving the capacitance value of the variable capacitance element to the drain output of the transistor versus the gate input voltage
To compensate for the drain output capacitance that changes in the positive phase
Thus , a linearity compensating circuit is obtained.

[0016]

Embodiments of the present invention will be described below in detail with reference to the drawings.

FIG. 1 is a circuit diagram of an embodiment of the present invention, and portions equivalent to those in FIGS. The detection circuit 4 rectifies and smoothes the gate input voltage of the transistor 2 to obtain a voltage proportional to the input voltage, and includes a detection diode 41, a smoothing choke coil 42, and a capacitor 4.
3,44.

The inverting amplifying circuit 5 inverts the phase of the output from the detecting circuit , and includes an operational amplifier 51 and resistors 52 to 54.
Consists of This inverted output is the voltage applied to the varactor diode 6 which is a variable capacitance element. The capacitance value of the varactor diode 6 is used to control the DC blocking capacitor 8 to cancel and compensate for the change in the output capacitance of the transistor 2. Through the transistor 2 to the drain side.

The resistor 7 provided in parallel with the varactor diode 6 is a damping resistor for compensating a sink current characteristic of the inverting amplifier circuit 5.

In FIG. 1, the input voltage from the amplifier circuit input IN is applied to the gate input of the transistor 2 via the input matching circuit 1, but since the gate input side impedance of the transistor 2 is extremely high, this input power is It is almost consumed as the gate input voltage. Therefore, this gate input voltage is applied to the detection circuit 4 and converted into a DC voltage.

Referring now to FIG. 2, there is shown a simplified equivalent circuit of the transistor 2, wherein an output power id proportional to the gate input voltage applied to the gate terminal 21 flows from the drain terminal 23 to the drain side. . This current flows to the grounded source 22, but when the voltage applied to the gate 21 is AC, the voltage of the drain 23 is applied to the gate 21 due to the effect of the inductance 25 (for supplying the drain bias voltage 26). It decreases as the input voltage increases.

In this case, the average value Cja of the output junction capacitance 24
Is expressed by the above equation (1). Since the relationship between Cja and the output Vo is inversely proportional, the relationship between Cja and the gate input voltage is directly proportional (in-phase). Accordingly, a voltage proportional to the gate input voltage is obtained by the detection circuit 4, and this voltage is inverted by the inverting amplifier circuit 5 and applied as the bias voltage of the varactor diode 6. It works so as to cancel the change of the output junction capacitance 24.

The reason why the transistor is limited to the insulated gate field effect transistor is as follows.
As shown in the equivalent circuit of FIG. 2, the insulated gate type field effect transistor has only the gate capacitance on the input side, and therefore has a high input impedance because only a small amount of high-frequency current flows on the input side. Therefore, since the input voltage is almost applied to the gate as a voltage, a sufficient voltage is supplied to the varactor diode. However, if the amplifying element is a bipolar transistor, most of the input voltage is consumed as a current, so that a voltage change to the varactor diode hardly occurs. Therefore, in the present invention, the insulated gate type amplifying element is used. It is limited to a field effect transistor.

[0024]

As described above, according to the present invention, the input voltage is detected by detecting the gate input voltage of the power amplifying transistor and supplying the inverted output of the detected voltage as the bias voltage of the variable capacitance diode. On the other hand, since the compensation capacitance that changes in the opposite phase is added to the drain output capacitance of the transistor, the drain output capacitance that changes in the positive phase with the input voltage can be compensated.

Therefore, in a television transmitter that handles video signals, the linearity of the transistors is improved, and DG,
The DP and the cross modulation characteristics can be greatly improved.

Further, since most of the compensation circuit can be constituted by a DC circuit, it is inexpensive and easy to design, and the adjustment of the circuit is only required to adjust the gain and offset voltage of the inverting amplifier circuit. It becomes.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of an embodiment of the present invention.

FIG. 2 is a simplified equivalent circuit diagram of an insulated gate field effect transistor.

FIG. 3 is a block diagram of a high-frequency power amplifier circuit using a conventional insulated gate field effect transistor.

FIG. 4 is a block diagram of a high-frequency power amplifier circuit using a conventional insulated gate field effect transistor having a linearity compensation circuit.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 input matching circuit 2 insulated gate field effect transistor 3 output matching circuit 4 detection circuit 5 inverting amplifier circuit 6 varactor diode 8 DC blocking capacitor 21 gate 22 source 23 drain 24 drain output capacitance

Claims (3)

(57) [Claims] 1. A linearity compensation circuit for an insulated gate field effect transistor for high frequency power amplification, comprising: a detection circuit for detecting a gate input power of the transistor; an inversion circuit for inverting the detection output; a variable capacitance element the capacitance value is controlled according to an output, see it contains a derivation means for deriving the capacitance value of the variable capacitance element to the drain output of the transistor, to vary the positive phase with respect to the gate input voltage
A linearity compensation circuit for compensating drain output capacitance . 2. The linearity compensation circuit according to claim 1, wherein said variable capacitance element is a varactor diode. 3. The linearity compensation circuit according to claim 2, wherein said deriving means is a DC blocking capacitor provided between a cathode of said varactor diode and said drain.