JPH0817292B2 - Wideband compound amplifier - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wideband composite amplifier, and more particularly to improvement of maximum output amplitude.

<Prior Art> FIG. 4 is a circuit diagram showing an example of a conventional composite amplifier. In the figure, IN is an input terminal to which the input signal e _in is input, and OUT is an output terminal to which the output signal e _out is output. Reference numeral 1 is a high-speed amplifier having an in-band gain A over a relatively high frequency region but having a relatively large DC offset and drift, and its output signal e ₂ is connected to one input terminal of an adder 2. Although 3 and 4 are inferior to the high speed amplifier 1 in frequency characteristics, they are operational amplifiers which are superior in direct current offset and drift to the high speed amplifier 1. The non-inverting input terminal of the operational amplifier 3 is connected to the common potential point, and the inverting input terminal is connected to the input terminal of the high speed amplifier 1 via the resistor R _{1 and} to the output terminal of the operational amplifier 3 via the resistor R _2. It is connected. The output terminal of the operational amplifier 3 is connected to the inverting input terminal of the operational amplifier 4 via the resistor R _3, and the inverting input terminal of the operational amplifier 4 is connected to the output terminal of the adder 2 via the resistor R ₄ and a capacitor. It is connected to the output terminal of the operational amplifier 4 and the other input terminal of the adder 2 via C ₁ . Here, the resistance ratio R ₁ : R ₂ of resistors R ₁ and R ₂ is set to 1: 1 and the resistance ratio R ₃ : R ₄ of resistors R ₃ and R ₄ is set to 1: A. ing.

In such a configuration, the input signal e _in is input to the high-speed amplifier as the input signal e _1, amplified as e ₂ = A · e _in and output, and is inverted by the operational amplifier 3 to be −
Output as e _in . To the inverting input terminal of the operational amplifier 4, -e _in which is inversely output from the operational amplifier 3 and the adder 2
The output signal e _{out of} is current added at a ratio of A: 1. This current addition value corresponds to an error component between the product A · e _in of the input signal e _in and the gain A of the high speed amplifier 1 and the output signal e _out . This error component is amplified by the operational amplifier 4, added to the adder 2, and added to the output signal of the high speed amplifier 1. That is, the operational amplifiers 3 and 4 form an error amplifier.

As a result, the high frequency component of the input signal e _in is amplified by the high speed amplifier 1 and the low frequency component and the direct current component are amplified by the operational amplifier.
It will be amplified by the error amplification unit composed of 3,4,
The frequency characteristic of the entire circuit is as shown in FIG.

FIG. 6 is an operation waveform diagram when a square wave of ± V _in as shown _in (a) is input as the input signal e _in of FIG. The high-speed amplifier 1 amplifies the input signal by A times as shown in (b), but at the same time, the offset and the drift which the self-confidence possesses are also amplified by A times and added to the output. That is, the H level of the output signal e ₂ becomes + A (V _in + V _off ), and the L level becomes A (−V _in + V _off ). Note that V
_off is the sum of the offset and drift converted into the input signal of the high speed amplifier 1. Further, if the low-frequency characteristic of the high-speed amplifier 1 is bad, the level 2 of the output signal e ₂ will fluctuate like V _d with the passage of time. The error amplifying section complements these DC and low-frequency errors generated in the high speed amplifier 1, that is, the output signal e _{3 of the} operational amplifier 4.
Is A times the input signal (e ₃ = −A ·
V _off ). Finally the output signal e ₂ and the output signal e ₃ of the error amplifying section of high-speed amplifier 1 are added by adder 2, the output signal e _out of the adder 2, as shown in (d), a square wave The input signal ± V _in is faithfully multiplied by A, the H level becomes + A · V _in , and the L level becomes −A · V _in .

By the way, in such a circuit, it is necessary to use one in which a bias voltage does not appear between the input and output terminals of the high speed amplifier 1.
There is a case where a desired voltage level cannot be obtained due to generation of a bias voltage of several V or restriction of a power source. This kind of high-speed amplifier cannot be used in the circuit connection shown in FIG.

Therefore, for example, as shown in FIG. 7, it has been proposed to provide a coupling capacitor C ₂ at the input terminal of the high speed amplifier 1 to AC-couple the input signal and to provide a level shift circuit 6 at the output terminal side. That is, in FIG. 7, the input terminal IN is connected to one input terminal of the adder 5 via the coupling capacitor C _2, and the output terminal of the operational amplifier 4 is connected to the other input terminal of the adder 5, The output terminal of the container 5 is connected to the input terminal of the high speed amplifier 1. A level shift circuit 6 is connected between the output terminal OUT and the output terminal of the high speed amplifier 1.

In such a configuration, since the input terminal IN is connected to the input terminal of the high speed amplifier 1 via the coupling capacitor C ₂ , no bias voltage appears at the input terminal of the high speed amplifier 1. On the other hand, the voltage generated in the output signal of the high speed amplifier 1 when there is no input signal is almost canceled by the level shift circuit 6. Further, the reduction of the passing amount at the low frequency by the coupling capacitor C _2, the error of the level shift circuit 6 and the offset and drift of the high speed amplifier 1 are corrected by the feedback circuit centered on the operational amplifier 4.

FIG. 8 is a circuit diagram showing a specific example of the level shift circuit 6 used in FIG. 7, and shows an example using the Zener diode ZD. Here, as the Zener diode ZD, the Zener voltage is a signalless output voltage e of the high speed amplifier 1.
Use a value almost equal to.

However, when such a Zener diode ZD is used, the influence of the non-linear characteristics of the Zener diode ZD appears on the AC response, and when the output voltage changes, the current flowing through the Zener diode ZD changes and the drop voltage decreases. Change. As a result, the output voltage also changes, but over time, the feedback circuit centered on the operational amplifier 4 catches up with the change and is corrected to an accurate output voltage. This means that when a square wave is input, for example, nonlinear distortion that is difficult to correct occurs immediately after the level changes. In order to avoid the occurrence of distortion due to such non-linear characteristics of the Zener diode, a level shift circuit that does not generate distortion may be used, but such a circuit has a complicated configuration.

As another circuit configuration, as shown in FIG. 9, a coupling capacitor is connected to each of the input terminal and the output terminal of the high speed amplifier 1.
It has also been proposed to connect C ₂ and C ₃ .

According to such a configuration, the input terminal of the high speed amplifier 1,
The DC component generated at each of the output terminals is cut by the coupling capacitors C ₂ and C ₃ , and does not appear at the input terminal IN and the output terminal OUT of the entire circuit. The same applies to drift and offset. The feedback circuit centered on the operational amplifier 4 corrects the deterioration of the low-frequency characteristic due to the coupling capacitors C ₂ and C ₃ . As a result, the problem of nonlinear distortion as described above can be solved without increasing the number of parts.

However, such a circuit has a problem that the output amplitude cannot be obtained. FIG. 10 shows the input signal e _in as in FIG.
FIG. 4 is an operation waveform diagram when a ± V _in square wave as shown in FIG. Here, the period of the square wave is the sum of the signal source impedance and the operational amplifier 3 and the coupling capacitor C ₂
It is assumed to be sufficiently larger than the time constant τ due to the product of and. Since the coupling capacitor C ₂ is connected to the input terminal of the high speed amplifier 1, the input voltage e ₁ of the high speed amplifier 1 immediately before the level of the square wave changes is substantially equal to the input bias voltage thereof. Then, the input voltage e ₁ of the high-speed amplifier 1 changes by the amplitude as shown in the moment (b) when the level of the square wave changes, but when time passes, it becomes the input bias voltage due to the time constant τ of CR described above. . After that, when the level of the square wave returns to its original value, this time, the amplitude of the square wave changes in the opposite direction to the previous direction. Eventually, the maximum input voltage e ₁ of the high speed amplifier 1 changes twice the amplitude of the square wave (2V _in ). Therefore, the maximum output amplitude of the entire circuit is (c),
As is apparent from (d), the maximum output amplitude of the high speed amplifier 1 becomes 1/2.

<Problems to be Solved by the Invention> The present invention focuses on such a point, and an object thereof is to provide a wideband composite amplifier in which the magnitude of the maximum output amplitude is improved.

<Means for Solving the Problem> A wideband composite amplifier of the present invention includes a high-speed amplifier, a first capacitor for coupling connected to the input terminal side of the high-speed amplifier, and a coupling first capacitor connected to the output terminal side. A second capacitor; an adder for receiving the output signal of the second capacitor at one input terminal; an amplifier for receiving the input signal applied to the first capacitor and inverting its polarity and outputting the input signal; Equipped with an operational amplifier to which a capacitor is connected,
The output of the amplifier and the output of the adder are current-added and input to the input terminal of the operational amplifier at a ratio of A to 1 (A is the gain of the high speed amplifier), and the output signal of the operational amplifier is added to the adder. R _in / (Rf + R _in ) = 1 / A, while R _in is input to the input terminal of the high speed amplifier via a resistor Rf that satisfies the relationship of the combined input impedance of the high speed amplifier. It is characterized by comprising an error correction circuit configured as described above.

<Operation> Since a part of the output signal of the error correction circuit according to the present invention is also applied to the input terminal of the high speed amplifier, the level of the input signal is kept constant even when the level of the input signal changes.

<Examples> Examples of the present invention will be described in detail below with reference to the drawings.

FIG. 1 is a block diagram of the principle of the present invention, and the same portions as those in FIG. 9 are designated by the same reference numerals. In FIG. 1, 7
Is an error correction circuit. The input signal of the coupling capacitor C ₂ is input to one input terminal of the error correction circuit 7, the output signal of the adder 2 is input to the other input terminal, and the output signal is the other input of the adder 2. A part of it is input to the input terminal of the high speed amplifier 1 via the resistor Rf.

FIG. 2 is a circuit diagram showing an embodiment of the present invention.
The same parts as those in the figure are denoted by the same reference numerals. 2 and 9
The difference between the figures is that a part of the output signal of the operational amplifier 4 is a resistor.
That is, the signal is input to the input terminal of the high speed amplifier 1 via Rf. In FIG. 2, an emitter follower is used as the adder 2, for example.

If the change amount at the input end is Δe _in , the output change amount Δe _out = A × Δe _in ... (1) multiplied by A occurs at the output end. Since the output side of the capacitor C ₃ is a DC-cut output, the final value of the output of the amplifier 4 (which is Δe ₄ ) for correcting this is equal to Δe _out .

_{Δe 4 = Δe out ...... (2} ) Δe 1 = Δe 4 from the output DC variation .DELTA.e ₄ and the high-speed relationship between the variation .DELTA.e ₁ input terminal of the amplifier 1 resistance division ratio of the amplifier _{4 × R in / (Rf +} R _in ) (3) However, R _in is associated with the combined input impedance of the high-speed amplifier 1.

From the above equations (2) and (3), Δe ₁ = Δe _out × R _in / (Rf + R _in ) ... (4) Substituting the equation (1) into the equation (4), Δe ₁ = A × Δe _in × R _in / (Rf + R _in ) (5) Finally, Δe ₁ = Δe _in , so (5)
The formula is R _in / (Rf + R _in ) = 1 / A (6).

Therefore, as the resistor Rf connected to the output end of the amplifier 4 and the input end of the high speed amplifier 1, a resistor satisfying the relationship of the above expression (6) is used.

FIG. 3 is a waveform diagram illustrating the operation of FIG. 2 when a square wave is input. In the case where a square wave as shown in (a) is input, after the level of the input signal e _in changes, the output signal e ₂ of the high speed amplifier 1 is delayed by the first order as shown by the broken line in (b). Although trying to change, a part of the output signal of the error correction circuit 7 which is mainly composed of the operational amplifier 4 is fed back to the input terminal of the high speed amplifier 1 through the resistor Rf whose resistance value is set as described above. Therefore, the output signal e ₂ of the high speed amplifier 1 is kept constant as shown by the solid line in (b). Also, when the level returns to the original level, the same correction operation is executed. In addition, in (b), _Vb is a bias voltage that appears in the input terminal and the input signal of the high speed amplifier 1 when no signal is input. Therefore, the waveform of the output signal e ₂ becomes substantially the same as the waveform of the input signal e _in . This allows
As shown in (c), the maximum output amplitude of the high speed amplifier 1 becomes the maximum output amplitude of the entire circuit as it is.

<Effects of the Invention> As described above, according to the present invention, the maximum output amplitude of a high-speed amplifier having a relatively simple structure, in which a bias voltage appears at the input terminal and the input signal when no signal is input, is used. A wideband composite amplifier that does not decrease can be realized, and the practical effect is great.

[Brief description of drawings]

1 is a block diagram of the principle of the present invention, FIG. 2 is a circuit diagram showing an embodiment of the present invention, FIG. 3 is a waveform diagram for explaining the operation of FIG. 2, and FIG. 4 is a conventional composite amplifier. FIG. 5 is a circuit diagram showing an example, FIG. 5 is a frequency characteristic example diagram of FIG. 4, FIG. 6 is a waveform diagram explaining the operation of FIG. 4, and FIG. 7 is a circuit diagram showing another example of a conventional composite amplifier. FIG. 8 is a circuit diagram showing a concrete example of the level shift circuit used in FIG. 7, FIG. 9 is also a circuit diagram showing another example of the conventional composite amplifier, and FIG. 10 explains the operation of FIG. It is a waveform diagram. 1 ... High-speed amplifier, 2 ... Adder, 7 ... Error correction circuit, C ₂ , C ₃ ... Coupling capacitor, Rf ... Feedback resistor.

Claims (1)

[Claims] 1. A high-speed amplifier and a first coupling device connected to the input terminal side of the high-speed amplifier.
And a second coupling capacitor connected to the output terminal side, an adder that receives the output signal of the second capacitor at one input terminal, and an input signal that is applied to the first capacitor An amplifier for inverting the polarity and outputting, and an operational amplifier in which a capacitor is connected to the feedback path are provided, and the output of the amplifier and the output of the adder are A-to-1 (A is the high-speed amplifier) at the input terminal of the operational amplifier. Current) at the ratio of (1), and the input signal is input to the other input terminal of the adder, and R _in / (Rf + R _in ) = 1 / A, where R _in is A wideband composite amplifier comprising: an error correction circuit configured to input to an input terminal of the high speed amplifier via a resistor Rf satisfying a relation of combined input impedance of the high speed amplifier.