JPS5839405B2 - high frequency amplifier - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a high frequency amplifier that is less likely to cause cross modulation.

As shown in Figure 1, in a conventional emitter-grounded high-frequency amplifier, a DC current feedback (emitter bias) resistor RB is connected between the emitter of a transistor Q and the ground, and the high-frequency signal is bypassed in parallel with the resistor. It is formed by connecting capacitors Ct in parallel.

Therefore, in this circuit, the impedance for high frequencies is zero, and the impedance for low frequencies and direct current is RB.

However, such a circuit has the disadvantage that cross-modulation is likely to occur when amplifying an amplitude-modulated high frequency wave.

Note that the cross modulation discussed in Section 5 refers to a phenomenon in which a desired signal is modulated by a modulation component contained in an interfering signal.

The present invention aims to improve these points and prevent the occurrence of cross-modulation using extremely simple means.

The present invention provides a high-frequency amplifier having a transistor for amplifying an amplitude-modulated high-frequency signal, in which a capacitor for passing a modulation component is connected in parallel to a resistor connected to the emitter of the transistor to provide an emitter bias, in order to minimize cross-modulation. The present invention is characterized in that a series circuit with a resistor selected to have a value of

The reason why cross-modulation occurs in the circuit shown in FIG. 1 is as follows.

An amplitude-modulated high-frequency signal S1 is applied to the base of the transistor Q, but a part of the low-frequency modulation component contained in the carrier wave of the signal is extracted due to the nonlinear characteristics of the diode composed of the base and emitter. appears at resistor RE, which modulates the desired signal, resulting in cross-modulation.

Therefore, it has been found that changing the emitter resistance REO value changes the degree of cross-modulation, and that this becomes minimum when the value of the emitter resistance with respect to the modulation component is a predetermined value.

Figure 2 shows the first actual case of the present invention based on this knowledge, in which the bias resistor RB and the high frequency bypass capacitor Ct
Furthermore, a series circuit of a resistor r and an electrolytic capacitor Cp is connected in parallel to the resistor r.

In this way, the impedance for high frequencies of the circuit of the present invention is zero due to the capacitor Ct, the impedance for direct current is reduced to zero by the resistor RE, and the impedance for low frequencies is reduced to zero by the capacitor Ct.
RE/(r+RE), whose approximate value is r, and when this resistance r is changed, the cross modulation changes as shown in FIG. 3, for example.

In this figure, the horizontal axis is r (Ω), the vertical axis is the modulation degree Mu (%) of cross modulation measured on the collector side, and the transistor used is 2SC710, RB = Ω, Ct = 0.022μF1:
7 Rector current Ic = 1mA.

Desired input is 80dBp, 1000 KHz, interference wave 1K
Hz, 30% modulation.

The modulation degree Mu for the resistance r is 100 to 8 when the disturbance force S2 is
It changes as shown in the figure as it changes to 0 dBμ, but since the disturbing input that is a problem is about 80 dBμ, the value of the resistor r that provides the lowest modulation degree, 22Ω in this example, may be used.

When a resistor r with such a value is used in series with the capacitor Cp, a part of the modulation component voltage of the interference wave is given to the emitter I, and by appropriately selecting the value of the resistor r, the DC bias between the pace emitters can be adjusted. Voltage fluctuations can be minimized, and in this state cross-modulation is minimized.

The resistance r that minimizes cross modulation is 22 Ω in the above example, and is generally several Ω to several hundred Ω, but if this resistance value provides an appropriate emitter bias voltage at the desired collector current, the emitter bias The resistor RE can be omitted.

FIG. 4 is an explanatory diagram of this.

However, as in this circuit, the relationship RB''-r cannot always be maintained, so if the resistance r is set with priority, it will be disadvantageous in terms of DC bias.

Therefore, in the present invention, as shown in FIG. 2 or 5, the resistor RE
Separate and r.

FIG. 5 shows another embodiment, in which a resistor r having the above value is connected in series with a parallel circuit of a small capacitance capacitor C1 for high frequency bias and a large capacitance electrolytic capacitor Cp for low frequency bias, and these are emitter bias resistors. Connected in parallel with RE.

In this case, the impedance to high frequencies is also r,
Although the gain for high frequencies is slightly lower, this circuit can be used if the gain is sufficient.

The circuit of the present invention can be applied to amplifiers, frequency converters, mixers, etc. that handle amplitude modulated waves, examples of which are shown in FIGS. 6 to 9.

FIG. 6 and FIG. 1 show an example of application to a balanced mixer.

G2 is a transistor, C and L are tank circuits tuned to the intermediate frequency, S3 is a local oscillator output signal, and S4 is a carrier wave signal.

There is no need to take any special measures on the side where the local oscillator signal S3 enters, but
There is a risk of cross modulation occurring on the side where the carrier wave signal S4 enters.
In the example of FIG. 6, a series circuit of the capacitor Cp and a resistor r is connected in parallel with the emitter resistor RE, and in FIG. 7, the value of the emitter resistor REO is set to r.

FIG. 8 shows an example of application to an intermediate frequency amplifier, in which a series circuit of a capacitor CP and a resistor r is connected in parallel to a resistor RE.

Further, FIG. 9 shows an example in which the present invention is applied to a differential amplifier that amplifies a carrier wave signal S4, and in this case, the emitter resistance RB is set to r.

■ is a constant current source. There are methods to prevent cross-modulation, such as providing a trap circuit or an automatic gain control circuit, but these methods reduce the interference level and avoid cross-modulation.

On the other hand, the method of the present invention does not lower the interference level, but rather attempts to reduce <SL by causing cross-modulation even if a certain level of interference input is received.

Next, an experimental example will be given.

The influence of various factors on cross modulation was investigated using the measurement circuit shown in FIG.

In this figure, 01 is the desired signal source, G2 is the interference signal source, and R1-
R7 is a resistor, C1 and C2 are capacitors, Ll is an inductance, A is an ammeter, and other symbols are the same as above.

When the collector current Ic of the transistor Q in this circuit was measured using a spectrum analyzer, the results shown in FIG. 11 were obtained.

In this figure, fs is the frequency of the desired signal, fmd is the modulation frequency of the interference signal, and M is the amplitude difference between the desired signal and the number of sidebands due to cross modulation.

Next, the results of completely bypassing the emitter side of the transistor Q by setting r=o are shown in FIGS. 12 and 13.

In this example, R6=0. Cp=47μF1R1=2.2Ω
, R2=470Ω, ■c=0.7 mA.

fs=1000KHz, frequency of interference signal fo=110
0 KHz, fmd = I KHz, and the modulation degree of the interfering signal md = 30%.

FIG. 12 shows the above M with respect to the interference signal level Vd,
The desired signal level vs=80 dB.

FIG. 13 shows the desired signal level vs. M (dB), where the interfering signal level vd=30 dB.

It can be seen from FIG. 13 that M is proportional to the square of the interfering signal, and it can also be seen from FIG. 13 that cross modulation does not depend on the magnitude of the desired signal.

Further, FIG. 14 shows M with respect to the collector current Ic, and v
s, fs, vd, fd, fmd, and md are the same as above.

It can be seen from this figure that M is influenced by the collector current and increases as the collector current increases.

Next, we tried to make the impedance on the emitter side finite at the modulation frequency of the interference wave.

Figure 15 shows the relationship between vd and M, r=o, r=22Ω
, r=47Ω, r-100Ω, r=IKΩ as parameters.

In this example, R6-0, Cp, R1°R2s VS, f
s, fd, fmd, md are the same as above, Ic-1
It is mA.

FIG. 16 shows this diagram with the resistance r plotted on the horizontal axis, and is similar to FIG. 3.

Further, FIG. 11 shows the optimum value of M for the interference signal level vd using r and Ic as parameters, and FIG. 18 shows the optimum value of r for the interference signal level vd using Ic*I as a parameter.

The RswmdO value is the same as above.

From FIG. 18, it can be seen that when the disturbance input is small, the optimum value of r is a constant value, and that value is determined by Ic.

Next, the optimum value of the resistance r determined through experiments will be shown.

Next, we will show the results of analyzing the effect of emitter-side impedance on cross-modulation.

In the circuit shown in FIG. 1, if the impedance between the emitter and the ground is Z, then the pace emitter voltage Ve, the collector current Ic, and the base voltage E are expressed by the following equations.

E=Ve+IZ Ve=E IZ=Vo+v=E□+e IOZ i
Z here v.

, ■o, Eo are the direct current components of Ve, I, and E, vtis
e is the same sentence.

Therefore, the nonlinearity of a transistor can generally be expressed as follows.

Since it is up to the third-order term that affects cross-modulation, Isε
αvO=a□, IsεαV□α=a1α2
♂ IsεavO−=a2, IsεαV O=2 s
Then, 21 31 Ic=ao+alv+a2v2+a3v3I□+i
=ao+al(e-i Z)+a2(e-i Z
)”+a3(e-i Z)3 I for the DC component.

= ao, i(1+a1Z+2 a2eZ+-3
a3 e2Z) -12(a3Z"+3ageZ2)113Z3a3=ale+a2e2+a3e3i is a minute value and Z is several tens of ohms, the terms i2.i3 can be omitted, so the above equation becomes as follows.

Here, the modulation degree mu of cross modulation is expressed as E2, the voltage of the interfering wave, and m2, the modulation degree of the interfering wave.

From this equation, it can be seen that cross-modulation depends on the magnitude of impedance Z, and that there is a point depending on Z where cross-modulation becomes minimum.

As explained in detail above, according to the present invention, cross-modulation in an amplitude-modulated high-frequency transistor amplifier circuit is minimized by a simple means of setting the value of the emitter-side impedance to an appropriate value for the modulation component. This provides practical advantages.

[Brief explanation of drawings]

Figure 1 is a circuit diagram showing the main parts of a conventional high-frequency amplifier;
The figure is a circuit diagram showing an embodiment of the present invention, FIG. 3 is a graph showing the relationship between resistance r and modulation depth Mu, FIG. 4 is a circuit diagram when emitter resistance and intermodulation optimization resistance are shared, and FIG. The figure is a circuit diagram showing another embodiment of the present invention, Figures 6 to 9 are main circuit diagrams showing an example of application of the present invention, Figure 10 is a circuit diagram of a measurement circuit used in the experiment, and Figure 11 is a circuit diagram showing another embodiment of the present invention. Figures 1 to 18 are graphs showing various characteristics obtained through experiments. In the drawing, Q is a transistor, RE is an emitter bias resistor, Cp is a capacitor that passes modulation components, and r is a resistor selected to have a value that minimizes cross modulation.

Claims (1)

[Claims] 1. In a high-frequency amplifier that has a transistor that amplifies an amplitude-modulated high-frequency signal, a capacitor that passes the modulation component is connected in parallel to a resistor that is connected to the emitter of the transistor and provides an emitter bias, and has a value that minimizes cross-modulation. A high frequency amplifier characterized by connecting a series circuit with a resistor selected as follows.