JPS6141298Y2 - - Google Patents

Description

[Detailed description of the invention] This invention relates to an improvement of a high-frequency amplifier circuit using a grounded base transistor, particularly an amplifier circuit in which a PIN diode is inserted between the emitter and the ground to attenuate excessive input.

Conventionally, reverse AGC type transistor amplifiers have been widely used because their operating current is small and power supply circuits can be advantageously configured. However, since the operating current of this type of amplifier is small, high-frequency distortion occurs in response to excessive input signals, and cross-modulation characteristics deteriorate. These deficiencies can be overcome by the use of PIN diodes, which provide low-distortion attenuation of excessive input signals; for example, as shown in Figure 1,
It was proposed to use a PIN diode 2 inserted between the emitter and ground of the grounded base transistor 1. A high frequency input signal is supplied to the connection point between the PIN diode 2 and the emitter resistor 3 in the figure, an AGC signal is supplied to the base of the transistor 1, and a high frequency output signal is obtained at the collector. In this circuit, the emitter current I _E of transistor 1 flows through the series circuit of PIN diode 2 and base resistor 3 due to the AGC signal applied to the base.
The high frequency resistance of the PIN diode 2 to the input signal is utilized within its effective operating current to attenuate excessive input.

However, the effective operating current range of PIN diode 2 is about 2 mA or less, whereas the input/output characteristics of general transistors have good linearity at emitter _currents exceeding 2 mA, which is within the effective operating current range of PIN diode 2. At 2 mA or less, effective attenuation characteristics by the PIN diode could not be obtained in the range where the distortion of the transistor's input/output characteristics becomes large and the transistor's operating current is large and the linearity is good.

Therefore, the present invention has been proposed in view of the above, and is based on a reverse AGC method using a general RF amplification transistor, preferably a transistor with flat input/output characteristics regarding the transistor operating current, and especially a transistor with good linearity even at low current. Provides RF amplification circuits. That is, the object of the present invention is to provide a grounded base transistor amplifier circuit in which a second emitter resistor is connected in parallel to a series circuit of a PIN diode and a resistor so that the operating current of the PIN diode can be used in a different state from the operating current of the transistor. Therefore, the amplifier circuit of the present invention helps to attenuate excessive input in the linear region of the input/output characteristics of the transistor, resulting in improved cross-modulation characteristics and harmonic interference characteristics.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 2 shows the main parts of a grounded base transistor amplifier circuit according to an embodiment of the present invention. This circuit is substantially the same as the conventional circuit shown in FIG. 1 except that a second emitter resistor 10 is inserted. That is, a series circuit of a PIN diode 12 and a first emitter resistor 13 is inserted between the emitter of the transistor 11 and the ground, and the input terminal 1 is connected to the connection point 14 between the PIN diode 12 and the first emitter resistor 13.
A high frequency signal is input from 5 through a coupling capacitor 16, and a high frequency output signal is obtained at the collector. Note that 17 is an AGC signal input terminal. Further, a high frequency input/output circuit is not shown.

This amplification circuit has the problem that when the input circuit is detuned for miniaturization, especially in TV tuners, if the low channel signal is strong, harmonics are generated in the RF amplification stage, causing interference when receiving the high channel channel. It is possible to make significant improvements. That is, in the present invention, the attenuation effect of the PIN diode 12 can be effectively obtained even in the operating range of 2 mA or more with good linearity with respect to the input/output characteristics of the transistor shown in FIG. Figure 4 shows the characteristic curves of the conventional circuit in Figure 1 and the circuit of the present invention in Figure 2 with dotted lines a for the 1% harmonic (doubled) disturbance characteristics.
and is shown by a solid line b. This characteristic curve is the desired wave signal D
This was measured using a 200 MHz 60 dBμ standard, a 100 MHz interference signal U, and a D/U ratio of 40 dB (1%). Furthermore, FIG. 5 shows the 1% cross-modulation characteristics for the circuits of FIGS. 1 and 2 by dotted lines a and solid lines b, as in FIG. 4. The measurement conditions in this case were that the desired wave signal was 185 MHz, the interfering wave signal was 195 MHz with a 60 dBμ reference, and the modulation degree was 30%. These signal frequencies correspond to the 6th and 8th channels of the Japan channel, and are signals that are likely to cause problems in cross-modulation characteristics. As is clear from FIGS. 4 and 5, the circuit of the present invention has the following characteristics as shown by the characteristic curve b.
Significant improvements are made in both properties.

Although the circuit of FIG. 2 is particularly useful for use in input untuned circuits, it can also be used in input tuned circuits. In the case of a non-tuned circuit, since there is matching with the input impedance, the resistance value of each emitter resistor is necessarily limited. For example, when the input impedance of a television channel is 75Ω, the standing wave ratio is
If the VSWR is 3.0 or less, the resistance value when the PIN diode is conductive at the full gain point of the input signal is 10Ω.
It is necessary to select a resistance value of the emitter resistor 13 that is about 10 times larger than the input impedance (R component) of the grounded base transistor, which is about 10Ω, which is negligible. In this case, VSWR is approximately 3.7
(≒75/10+10). On the other hand, the cutoff point
Since the PIN diode is several kΩ, it is desirable to set the resistance value of the first emitter resistor 13 to around 225 (=75×3) considering the input impedance of 75Ω and VSWR of 3.0. Furthermore, a second emitter resistor 10
must be selected to determine the emitter current I _E around 0.7V, which is the voltage V _F at which the PIN diode starts operating. That is, Set so that the relational expression is satisfied. here,
R ₁₀ is the resistance value of the second emitter resistor 10, and R ₁₃ is the resistance value of the first emitter resistor 10.
The resistance value of the emitter resistor 13 and R _D are the resistance values of the PIN diode 12.

According to the circuit of the present invention, a large degree of gain attenuation near the cut-off point can be obtained.For example, whereas a general forward AGC transistor amplifier circuit obtains an attenuation of about 40 dB, it is possible to achieve a large gain attenuation near the cut-off point. The circuit of this invention can obtain attenuation of about 55 dB. This is because, as mentioned above, if the first emitter resistor 13 is set to about 200 Ω and the second emitter resistor 10 is set to several hundred Ω and is additionally inserted, mismatching near the cut-off point increases, and the voltage applied to the transistor increases. This is because the gain attenuation increases as the value decreases.

In addition, in the implementation circuit of this invention, the PIN diode has a horizontal structure developed as a low-cost PIN diode.
ISV77 type is used, and 2SC is used as the amplification element.
-2353 type general amplification transistors with little change in gain due to current were used. In particular, the use of the latter transistor is more effective than the case of using an AGC transistor because there is no beat disturbance in the non-linear characteristic portion.

The amplifier circuit according to the present invention uses a reverse AGC method.
As shown in FIGS. 4 and 5, significant improvements in harmonic interference characteristics and cross-modulation characteristics can be achieved by using inexpensive active elements and a simple circuit configuration in the RF amplifier circuit.

[Brief explanation of the drawing]

Figure 1 is an amplifier circuit diagram that is the premise of this invention, Figure 2
The figure shows an amplifier circuit diagram as an embodiment of the present invention, FIG. 3 shows a transistor characteristic diagram, and FIGS. 4 and 5 show harmonic distortion and cross-modulation characteristics for interference waves caused by the circuits of FIGS. 1 and 2, respectively. It is a diagram. 10... Second emitter resistor, 11... Transistor, 12... PIN diode, 13... First emitter resistor, 15... Input side signal terminal, 17...
AGC signal terminal.

Claims (1)

[Scope of utility model registration request] A series circuit of a PIN diode and a first emitter resistor is provided between the emitter of the base-grounded transistor and the ground, a high-frequency input signal is supplied to the connection point of the PIN diode and the first emitter resistor, and an AGC is connected to the base.
In a reverse AGC-type high-frequency amplifier circuit in which a signal is applied to the collector to obtain a high-frequency output signal, a second emitter resistor is connected in parallel to the series circuit to direct the effective operating range of the PIN diode in the direction of larger emitter current. A high-frequency amplification circuit using a reverse AGC method.