JPH043607A - Wide band amplifier - Google Patents

Abstract

PURPOSE:To smooth high band and low band characteristic and to obtain an excellent overall characteristic by providing a high band amplifier, a low band amplifier, and an equalizer amplifier by coupling an amplified low band signal and an amplified high band signal so as to obtain a smooth frequency characteristic. CONSTITUTION:When the frequency characteristics of a high band amplifier 21 and a low band amplifier 22 are allowed to overlap, there are an area (a) where the gain of the high band amplifier does not exist but only that of the low band amplifier exists, an area (b) where the gain of the high band amplifier starts to appear and the low band amplifier has the same frequency characteristic as the gain for a DC signal, an area (c) where frequency characteristics of gains of both amplifiers are flat, an area (d) where the gain of the high band amplifier is flat and that of the low band amplifier is reduced in accordance with the rise of the frequency, and an area (e) where the gain of the low band amplifier does not exist but the flat frequency characteristic of the high band amplifier exists. The open loop gain of an equalizer amplifier 20 is so set that it is sufficiently high in areas (a) and (b) and is sufficiently low in areas (d) and (e). Thus, the smooth frequency characteristic from a DC to a high frequency is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a wideband amplifier. More specifically, the present invention aims to provide a broadband DC amplifier with high input impedance and low output impedance, which is a combination of a high-band AC amplifier and a low-band DC amplifier.

[Prior Art] Amplifiers with high input impedance and low output impedance, which have flat frequency characteristics from DC to nearly 1GH7, are used in the input section of oscilloscopes, etc.
An example of the circuit is shown in FIGS. 3(a) and 3(b).

In FIG. 3, an input signal is applied to a high impedance input terminal 13, and is outputted at a low impedance from an output terminal 19 via a field effect transistor 36 and a resistor 46, which form a source follower.

The field effect transistor 37 is the field effect transistor 36
The resistance value of the resistor 47 is the same as that of the resistor 4.
equal to the resistance value of 6. Therefore, DC offset and temperature drift do not appear at the output terminal 19.

In the conventional example shown in FIG. 3(b), the high frequency component of the signal applied to the input terminal 13 is applied to the field effect transistor 36, which serves as a source follower, via the capacitor 67, and the output thereof is applied via the resistor 46. is obtained at the output terminal 19. Among the signals applied to the input terminal 13, low frequency components including direct current are passed through a resistor 53 to an operational amplifier (operational amplifier).
24, its output is amplified by the transistor 32, and the output is obtained at the output terminal 19 on its collector side. From the output terminal 19, an operational amplifier 2 is connected via a resistor 52.
For example, since negative feedback is applied to resistor 53
If the resistance values of and 52 are equal, the input terminal 13
The voltage amplification degree for low frequency components between the output terminal and one output terminal is 1. Resistor 48 and resistor 46 constitute a self-bias circuit for field effect transistor 36, and determine the source current and emitter current of field effect transistor 36 and transistor 32.

[Problems to be Solved by the Invention] Although the conventional example shown in FIG. 3(a) is characterized by an extremely simple circuit configuration, the field effect transistor 36 is
．． It must have a good set of 37 characteristics, making it very expensive. Furthermore, if a gallium arsenide field effect transistor is used to generate a broadband source follower, it is difficult to obtain flat frequency characteristics from direct current to high frequencies because the output impedance changes with frequency. It was difficult.

The conventional example shown in FIG. 3(b) is intended to solve the problems of the conventional example shown in FIG. 3(a), and uses a gallium arsenide field effect transistor 36 with excellent frequency characteristics and It uses an excellent operational amplifier 24 to divide and amplify the high frequency region and the low frequency region, respectively.
It is characterized by the ability to realize high-performance circuits at low cost. However, for example, there are cases where the capacitance value of the capacitor 63 cannot be increased due to physical conditions such as ultra-miniaturization, and the low cutoff frequency cannot be lowered sufficiently;
A very large resistor is used as the resistor 53, and the operational amplifier 24
If the high-frequency cutoff frequency of the input signal cannot be raised sufficiently, it becomes extremely difficult to flatten the synthesized overall frequency characteristic, which remains a problem to be solved.

[Means for Solving the Problem] The present invention has been made to solve this problem, and includes a high-frequency amplifier that amplifies a high-frequency signal, a low-frequency amplifier that amplifies a low-frequency signal, and an amplifier. An equalizer amplifier was provided to combine the amplified low-frequency signal with the amplified high-frequency signal to obtain a flat frequency characteristic.

[Effects] If the frequency characteristics of the high-frequency amplifier and the low-frequency amplifier are made to overlap, the gain of the high-frequency amplifier is virtually nonexistent;
There is a region a where only the gain of the low-frequency amplifier exists, and a region where the gain of the high-frequency amplifier also begins to appear, and the frequency characteristics of the low-frequency amplifier are substantially the same as the gain for DC signals.
There is a region C in which the frequency characteristics of the gains of the high-frequency amplifier and the low-frequency amplifier are both flat; a region d in which the gain of the high-frequency amplifier is flat and the gain of the low-frequency amplifier decreases as the frequency increases; There is a region e in which the high frequency amplifier has substantially no gain and the high frequency amplifier has flat frequency characteristics. Here, the open loop gain of the equalizer amplifier was set to be sufficiently large in regions a and b, and sufficiently small in regions d and e. by this,
It has become possible to obtain extremely flat frequency characteristics from direct current to high frequencies.

[Embodiment] An embodiment in which a wideband amplifier according to the present invention is applied to a differential probe having high input impedance and low output impedance is shown in FIG. 1 and will be described. Here, components corresponding to those of the conventional example shown in FIG. 3 are given the same symbols.

A differential input signal is applied to the input terminals 11 and 12 in FIG. The output thereof is applied to the base of the transistor 31 which forms an emitter follower via a transmission line 71 which is a coaxial cable. The resistor 43 has a resistance value equal to the characteristic impedance of the transmission line 71, which is a coaxial cable, and acts as a terminating resistor. A high frequency component output from the emitter of the transistor 31 is obtained at the output terminal 19 via a resistor 44 (resistance value RS).

Of the differential input signals applied to input terminals 11 and 12,
The low frequency component is amplified by a differential low frequency amplifier 22 via a high resistance 41, 42 and a transmission line (not necessarily a coaxial cable) 72, 73, and its output is output via a resistor 51 via a broken line. It is applied to the equalizer amplifier 20 shown by . The equalizer amplifier 20 includes an operational amplifier (operational amplifier) 23 and a low-pass filter 59, and the low frequency components that have passed through these are applied to the base of the transistor 32, amplified, and obtained at the output terminal 19. . The signal obtained at the output terminal 19 is negatively fed back to the operational amplifier 23 via the resistor 52.

Based on the polarity of the signal on the input terminal 11 side, the high-frequency amplifier 21 is a non-inverting amplifier, and the low-frequency amplifier 22 is an inverting amplifier. The amplifier consisting of the operational amplifier 23 and the transistor 32 has negative feedback applied from its output terminal 19 through the resistor 52, and the input signal is applied through the resistor 51, so it is an inverting amplifier, and its gain is the value of the resistor 52. Ratio (R2/R1) of the value (R2) and the value (R1) of the resistor 51
Determined by

The high frequency component path of the input signal, that is, the capacitor 61
,62. High frequency amplifier 21. Transmission line 71, transistor 31. The transfer function of the path of the resistor 41 is GH(s), and among the paths of the low frequency component, the resistors 41, 42 . Transmission line 72
．． 73. The transfer function of the low-frequency amplifier 22 is GL (s)
, operational amplifier 23. Low-pass filter 59. Assuming that the open loop gain transfer function of the equalizer amplifier is A (S), the overall transfer function G (S) from the input heirs 11 and 12 to the output terminal 19 is determined by the following equation.

±R3-1GH(S) cox (R1+R2>"A(s)2-' To simplify this equation, set R1=R2=R, and 2R>>R8
, that is, equalizer amplifier 20 and transistor 3
If the gain of the negative feedback amplifier consisting of 2 and resistors 51 and 52 is set to -1, and the value is set such that the resistor 52 (R2> does not become a load at the output terminal 19), then G(s) "" [(2RE) - 'R3A(s) G
1 (S)+GH(S)kox [(2RE) -1R8A(s) +1
] −1 is obtained. From this equation, it can be seen that the gain is, for example, 1 in all frequency bands, that is, the overall transfer function G(S
) -1 to obtain a flat characteristic for the open loop transfer function A(S).

Figure 2 shows the low-frequency transfer functions GL (s) and GH (s).
is the solid line, and the open loop transfer function A(S) is reduced by the dashed line.

In the frequency region a on the horizontal axis, the high-frequency transfer function GH (
s) is substantially O, and the low-frequency transfer function GE(S)
The value of is 1.

In the frequency domain, the value of the high-frequency transfer function GH(S) starts to increase but is smaller than 1, and the value of the low-frequency transfer function G1 (s) is 1.

In the frequency region C, the values of the high-frequency transfer function GH(S) and the low-frequency transfer functions G,(S) are both 1.

In the frequency region d, the value of the high-frequency transfer function GH(S) is 1, but the low-frequency transfer function GL(S) has begun to decrease and its value is smaller than 1.

In the frequency region e, the value of the high-frequency transfer function GH (S) remains 1 and remains flat until reaching high frequencies, but the value of the low-frequency transfer function G, (S) is sufficiently attenuated. It is substantially O. It should be noted that here, when we say that the transfer function is 1, we mean not only that its absolute value (gain) is close to 1, but also that the phase rotation is so small that it can be ignored.

When calculating the overall transfer function G (s) for these five frequency regions a, b, c, d, and e, in region a, G1 (s) = 1. Since GH(s) = 0, G(s
) = (R3/2RE > (R3/2RE+ 1 /
A(s) ) -1 Here, if △(S) - flash, G(s) = 1 In the area, GL (S) = 1, GH(S) = g(0<Q
< 1) Therefore, if A(s) = ■, then G(s) = 1 In area C, GL(s) -1, GH(S) = 1, so in area d, G1 (s) = g(0 <Ω<1>, GH(S)=1, so if A(S)=O then G(S)=1 In region e, if A(s)=O then G(s)=1 is obtained.

Open equalizer amplifier in each region a-e
To illustrate the condition of the loop transfer function A(S), the second
As shown by the broken line in the figure, the loop transfer function A(S) is in the areas a, b! , : is sufficiently large that it can be considered to be essentially infinite in the area d.

It can be seen that if the value is small enough to be regarded as O in e, the overall transfer function G (s) exhibits 1 in all frequency regions a to e, regardless of the value in region C. However, this equalizer amplifier is
Since it operates as a feedback amplifier using an external feedback resistor, it is necessary to consider the phase margin in the feedback loop.

FIG. 1 shows an example in which the present invention is implemented in a probe suitable for use in an oscilloscope or the like.

For this purpose, transmission lines 71 to 73 are used between the tip of the probe and the output section. Therefore, when the transmission lines 71 to 73 are not required, the transmission lines 71 to 73 are not required.
73 and the resistor 43 may be omitted. The low-frequency amplifier 22 can be configured with a circuit including an operational amplifier with negative feedback. The high-frequency amplifier 21 can be easily demonstrated by, for example, the same amplifier as that of a wideband oscilloscope.

Further, the low-pass filter 59 can be easily formed into a ring by combining a resistor and a capacitor or an inductance and a capacitor. Although differential input terminals 11 and 12 are illustrated in FIG. 1, it will be clear from the above description that the present invention can be applied even to a single input terminal.

[Effect of the invention] As is clear from the above explanation, according to the present invention,
This invention makes it possible to obtain excellent overall characteristics by using a high-frequency amplifier with excellent high-frequency characteristics and a low-frequency amplifier with excellent low-frequency characteristics and flattening both characteristics with an equalizer amplifier. The effect is extremely large.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram of an embodiment of the wideband amplifier of the present invention, Fig. 2 is a frequency characteristic diagram for explaining the frequency characteristics of the wideband amplifier shown in Fig. 1, and Fig. 3 is a diagram of the conventional wideband amplifier. It is a circuit diagram. 1-48.51-53...-Resistor 9...Low-pass filter 1-63...Capacitor 1-73...Transmission line.

Claims (1)

[Claims] A high-frequency amplification means (21, 31) for amplifying a high-frequency signal and a low-frequency amplification means for amplifying a low-frequency signal having a flat part gain equal to the flat part gain of the high-frequency amplification means. an amplifying means (22); a frequency region a in which there is substantially no gain of the high-frequency amplifying means and only a gain of the low-frequency amplifying means; and a frequency region a in which the high-frequency amplifying means has substantially no gain; in a frequency region b in which the low-frequency amplification means has a gain in the flat part, and the high-frequency amplification means and the low-frequency amplification means both have a gain in the flat part. a frequency region c in which the high frequency amplifying means has a gain in a flat portion, and a frequency region d in which the low frequency amplifying means has a gain smaller than the gain in the flat portion; There is a frequency region e in which the amplifying means has a flat gain and the low frequency amplifying means has substantially no gain, and the open loop gain is equal to the flat gain in the regions a and b. In the regions d and e, the gains are sufficiently large relative to the gains in the flat portions, and the respective outputs of the high-frequency amplification means and the low-frequency amplification means are and equalizer means (20, 32, 51, 52) for obtaining a flat frequency characteristic by combining.