JPS63252005A - High-band negative feedback amplifier - Google Patents

Abstract

PURPOSE:To improve characteristics by negative feedback by connecting a delay line whose delay time corresponds to the propagation delay time of an amplifier between the inverted input terminal and output terminal of the amplifier through resistances. CONSTITUTION:This negative feedback amplifier consists of an amplifier Amp which has a linear phase in a target band and also has flat closed loop gain characteristics, the delay line Td, and resistance elements Ri, Rf and Rf'. The resistance element Ri is connected to the inverted input terminal of the amplifier Amp and the delay line Td and resistance elements Rf and Rf' are connected in series. This series circuit is connected between the inverted input terminal (1) and output terminal (4) of the feedback circuit of the amplifier Amp. The circuit gain of a negative feedback amplifier of this constitution is expressed by an equation I. Here, when the input impedance from a signal source Vi to the amplifier Amp is 200 OMEGA, the equation I becomes an equation II. When the feedback resistance Rf is set to, for example, 800 OMEGA, the circuit gain is expressed by an equation III. Consequently, the circuit characteristics are improved.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a wideband negative feedback amplifier that can cover a frequency range from direct current to approximately 500 MHz.

[Conventional technology]

Traditionally, it has generally been
Deep negative feedback tank covering up to about 00 MHz +11
1D was not possible. As a conventional example, from DC to 100
FIG. 4 shows a negative feedback amplifier that covers up to approximately MHz.

In FIG. 4, Amp is an amplifier. This amplifier has a flat gain in the band from DC to 100MHz, and one phase is close to a linear phase.The input resistor R1 is connected to the inverting input terminal, and the connection between the inverting input terminal and the output terminal is A series circuit of feedback resistors Rf' and Rf is connected between them. Non-inverting input terminal is reference potential point C
Connected to OM. With this configuration, the amplifier Amp is configured as a negative feedback type inverting amplifier. The circuit gain of this amplifier is (Vo/Vi) = -(Rf/ (Ri +Rf'), where Vi is the input voltage and Vo is the output voltage.
). Note that if the gain of A m p is A, then A=-(Rf/Rf') Here, if the input voltage Vi is a pulse voltage shown in FIG. 5 (a) having a frequency higher than 100 MHz.

Even though the amplifier A m p is in linear phase, the propagation delay in A m p distorts the pulse response at the non-inverting input point as shown in FIG. Furthermore, the input impedance is not constant.

[Problem that the invention seeks to solve]

The present invention was made in order to solve these problems, and the amplifier Am
To provide a wideband negative feedback amplifier which is free from distortion due to propagation delay of signal p, has a constant input impedance as seen from a signal source Vi, has a predetermined loop gain at low frequencies, and has improved characteristics by negative feedback. It is intended for this purpose.

[Means for solving problems]

In order to achieve the above object, the present invention includes an amplifier having a linear phase and a flat gain in a target frequency band, and a delay time between an inverting input terminal and an output terminal of the amplifier. There is a structure in which a delay line corresponding to a propagation delay time of 1 is connected via a resistor. below,
Examples will be described in detail.

〔Example〕

FIG. 1 is a circuit diagram of an embodiment of a broadband negative feedback amplifier according to the present invention. In the figure, Amp is an amplifier that has a linear phase in the target band and has flat open-loop gain characteristics. Ri is an input resistor connected to the inverting input gate of the amplifier A m p. R'f and Rf' are resistance elements, respectively, and Td is a delay line. The delay line Td and the resistance elements Rf' and Rf are connected in series, and this series circuit is an amplification@
It is connected between the inverting input terminal ■, which is the feedback circuit of Amp, and the output terminal ■. A coaxial cable with an impedance Zo of 50Ω is used as the delay line Td. Here, the connection point between the feedback resistors Rf' and Rf is denoted by ``2'', and the connection point between the delay line Td and the feedback resistor Rf' is denoted by ``2''. When the delay line Td with Zo=50Ω is used in this way, the resistance value of the feedback resistor Rf' is selected to be the same <50Ω. By making the values of Zo and Rf' equal in this way,
The input wave is not reflected at point 0, and the input wave and output V at point ■
``Addition'' with the return wave from o can be performed correctly. As a result, the voltage at point ■ becomes zero.

What is the circuit gain in a negative feedback amplifier with this configuration?

(Vo/Vi)"-(Rf/ (Ri+Rf'))・
...(2) Represented by: Here, the amplifier A seen from the signal source Vi
When the input impedance of m p is 200Ω, (
2) The formula is (V o / Vi) = −(Rf / 200
) -(3) However, the gain A of the amplifier Amp is A=-(Rf/Rf')=-(Rf150)Here, for example, if the feedback resistor Rf is selected to be 800Ω, the first
The circuit gain in the figure is (Vo/Vi)=-4, and the gain of the amplifier Amp is A=-16.In a negative feedback amplifier with such a configuration, the operation when the input Vi is a pulse voltage is as follows. The explanation using the waveform diagram in the figure is as follows.

In FIG. 2, [A] shows the pulse waveform of the input Vi, and the horizontal axis represents time t. For such a pulse input Vi, the inverting input terminal (2) of the amplifier Amp has a voltage of 1 due to the input resistance Rin and the impedance Zo of the delay line Td.
As shown in Figure 2 (b), (1/4) attenusimine is received. The voltage detected at point The voltage becomes Vo.Delay time T of delay line Td
delay is designed to be equal to the propagation delay time Tpd of amplifier AmP. As a result, the delay line Td and the feedback resistance R
At the connection point (3) with f', at the same time as the output voltage Vo is generated, a pulse waveform generated at the inverting input terminal appears as shown in FIG. Therefore, the connection point (2) between the feedback resistors Rf and Rf'' is maintained as a voltage zero point as shown in FIG. 2 (E). Note that the feedback resistor Rf.

and Rf may be one resistance element.

On the other hand, in the negative feedback tank shown in Fig. 1, the feedback rate βda at low frequency input is βd c= (Ri/ (Ri+Rf' +Rf)) -
It is expressed as (3).

Here, as in the example (Ri=150Ω, Rf'=50Ω
, Rf=800Ω, the feedback rate βdc expressed by equation (3) is (150/1000)=15%. Therefore, the loop gain A·βdc is IAI·βdc=16×0.15=2.4.

Here, in the circuit shown in FIG. 1, a phase shift corresponding to Tdelay=Tpd is performed on the high-frequency loop gain. Therefore, it oscillates at an extremely high frequency such as f=(1/2Tpd)=1/2×0.25m5=2GHz. In order to prevent this, the first
By inserting and connecting a series circuit of a resistive element Rc and a capacitor Cc as shown in FIG. 2 to the connection point (2) between the feedback resistors Rf and Rf', which is the point of zero voltage in the feedback circuit of the negative feedback amplifier shown in the figure.

The feedback rate βact+ at ultra-high frequencies can be reduced as described below.

βac=Rc/ (Rc+Rf' +Rf)= (50
/900)=5.6% As a result, IAl·βac=16Xo, 056=0.89<1, and no oscillation occurs even at ultra-high frequencies.

〔Effect of the invention〕

As explained above, in the present invention, a delay line whose delay time corresponds to the propagation delay time of this amplifier is connected between the inverting input terminal and the output terminal of the amplifier via a resistor. (a) The input impedance is constant up to frequencies exceeding 200 MHz.

(b) It has a constant loop gain at frequencies up to about 100 MHz, and linearity, flatness of frequency characteristics, etc. can be obtained by the effect of negative feedback. Furthermore.

(c) At frequencies exceeding 200 MHz, the loop gain decreases, making it possible to avoid problems such as unstable oscillation due to deep negative feedback.

It is possible to obtain a wideband negative feedback amplifier having the following characteristics.

[Brief explanation of the drawing]

FIG. 1 is a connection diagram of one embodiment of the negative feedback amplifier according to the present invention, FIG. 2 is a diagram for explaining the operation of the circuit shown in FIG. 4 is a connection diagram of an example of a conventional amplifier of this type, and FIG. 5 is a diagram for explaining the operation of the circuit shown in FIG. 4. Amp...Amplifier, Ri...Input resistance, Rf, Rf
'...Feedback resistance, Td...Delay line. Figure 1 Figure 2

Claims (2)

[Claims] (1) An input resistor is connected to the inverting input terminal, and the amplifier is provided with a linear phase and a flat gain in the target frequency band, and the delay time between the inverting input terminal and the output terminal of the amplifier is A wideband negative feedback amplifier configured to connect a delay line corresponding to the propagation delay time of , via a feedback resistor. (2) The broadband negative feedback amplifier according to claim (1), characterized in that the feedback resistor is divided into two parts, and a series circuit consisting of a resistance element and a capacitor is connected between the connection point of the feedback resistor and a common potential point.