JPH0619312U - Wideband amplifier - Google Patents

Abstract

(57) [Summary] [Purpose] A differential input voltage feedback type amplifier is a multi-stage feedback system in which negative feedback is performed via two stages of transistors. Negative feedback improves linearity and is resistant to noise, but has the drawback that it becomes unstable due to excessive phase rotation of the feedback loop. To solve this problem, we added resistors and capacitors to open loop operation without using a multistage feedback loop at high frequencies. [Structure] Explaining with reference to FIG.
1 and C2 become conductive, the transistor Q1 operates as a base ground, and the multistage return loop is killed and becomes stable. The high frequency gain is determined by the gain of the grounded base transistor Q1, and the combined resistance R of the resistors R3, R4 and the collector of Q3
It suffices to match the ratio of c to the gain in the low frequency band determined by the resistors R1 and R2.

Description

[Detailed description of the device]

[0001]

[Industrial applications]

 The present invention mainly relates to a wide band amplifier used in an oscilloscope.

[0002]

[Prior art]

 A conventional wide band amplifier has a differential input voltage feedback type circuit as shown in FIG. 4, for example. The advantage of this circuit is that the DC potential between the input IN and the output OUT can be made the same, and a relatively wide band amplifier can be constructed with a small number of parts. The voltage gain Av of this circuit becomes Av≈1 + R2 / R1 (1), and the gain is determined by controlling the feedback ratio with the resistors R1 and R2, which is also easy to use. However, in the state where the gain is small and a large amount of negative feedback is applied, it is likely to become unstable, and excessive phase rotation of the feedback loop causes oscillation or ringing of a large step response. As a method of stabilizing this, there is a method in which C3 is provided and a high-frequency feedback current is directly returned to the base of the transistor Q2, but this causes the closed-loop band to be greatly reduced, which is not suitable for widening the band. . If a wide band characteristic up to about 1 GHz is required, the wiring length of the feedback loop around the transistors Q2, Q3 and the resistors R2, R1 will start to work, especially when the circuit is composed of discrete components. Is very unstable, and it is difficult to take a band.

[0003]

[Problems to be solved by the device]

 The above-mentioned conventional technique has a drawback that it is likely to become unstable at high frequencies. The present invention alleviates this drawback and provides a circuit capable of stable amplification from DC to a wide band.

[0004]

[Means for Solving the Problems]

 In order to achieve the above-mentioned object, the present invention does not use a multistage feedback loop through the transistors Q2 and Q3 at high frequency, but operates the transistor Q2 with the base grounded. FIG. 1 is a diagram showing the configuration of the present invention. In the figure, resistors R3 and R4 and capacitors C1 and C2 are added. At high frequencies, the capacitors C1 and C2 are in a conductive state, and the transistor Q2 operates as a base ground.

[0005]

[Action]

 As a result, there is no multi-stage feedback path in terms of high frequency, and the excessive phase rotation due to the wiring length of the above-described feedback path becomes irrelevant, so that a stable amplifier can be constructed over a wide band.

[0006]

【Example】

An embodiment of this invention will be described below with reference to FIG. The signal is input to the base of Q1 of the differential transistor pair composed of the transistors Q1 and Q2 and the resistor R3, and is transmitted to the collector of the transistor Q2 as a change in current. This change in current is converted into a voltage by the combined resistance Rc of the resistors connected to the collector of the transistor Q2 such as the input impedance of the transistor Q3 and the internal resistance of the current source I1 and output to the emitter of the emitter follower Q3 and the output terminal OUT. The output voltage is attenuated by the resistors R1 and R2 and fed back to the base of Q2 of the differential transistor pair to determine the voltage gain Av of the circuit. The low frequency operation of this circuit is considered to be an open circuit, as shown in FIG. 3, because the impedances of the capacitors C1 and C2 are large and open. The open loop gain Ao is determined by the emitter internal resistance re and the resistance R3 of the differential transistor pair Q1, Q2 Ao≈Rc / R3 + 2re ... (2), and the above-mentioned resistance Rc. AVL≈1 + R2 / R1 (3), and the open loop gain becomes smaller and the gain error increases a little because the resistor R3 is added as compared with the conventional circuit diagram 4, but the collector combined resistance Rc of the transistor Q3 becomes smaller. It doesn't matter if you take care to make it bigger. At such a low frequency, it operates almost the same as a conventional circuit. On the other hand, at a high frequency where the capacitors C1 and C2 appear to be conductive, an equivalent circuit as shown in FIG. 2 can be considered. The base of the transistor Q2 is grounded by the capacitance C1, and the resistor R4 is added in parallel to the collector of the transistor Q2 by the capacitance C2. The voltage gain at this time is Aυ _H ≈ R4 // Rc / R3 + 2re ... (4). The circuit at this time has no feedback loop that causes excessive phase rotation, and a stable and wide-band circuit can be obtained. From (3) and (4), 1 + R2 / R1 ≒ R4 // Rc / R3 + 2re ... (5) In order to match the gain relationship between high and low frequencies, (5) In addition, the transition frequency of high and low frequencies Let fc be fc = 1 / 2πC1 (R1 // R2) ≈1 / 2πC2R4, and the frequency characteristics will be flat and smoothly connected.

[0007]

[Effect of device]

 According to the present invention, while taking advantage of the negative feedback such as good linearity at low frequencies and noise suppression of multistage feedback, excessive phase rotation due to the negative feedback loop can be ignored at high frequencies. Restrictions such as shortening the wiring length can be alleviated, and a stable and wide-band amplifier can be realized.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention.

FIG. 2 is an AC equivalent circuit diagram showing the high frequency operation of the present invention.

FIG. 3 is an AC equivalent circuit diagram showing the low frequency operation of the present invention.

FIG. 4 is a circuit diagram showing a conventional example.

[Explanation of symbols]

 IN input terminal Q1 to Q3 transistors I1 to I4 current source OUT output terminal

Claims (2)

[Scope of utility model registration request] 1. A wide band amplifier having a structure in which a multi-stage feedback loop is used in a low frequency region and a multi-stage feedback loop is not used in a high frequency region. 2. A differential transistor pair in which the emitters of which one base is an input terminal and the other base is a feedback terminal are coupled to each other, an emitter follower in which the collector of the feedback terminal side transistor is connected to the base, and the emitter. In a differential input voltage feedback type amplifier comprising a follower emitter and a resistance attenuator provided between a feedback terminal, a resistor coupling the emitters of the differential transistor pair and a collector of the feedback input side transistor are capacitively coupled. And a time constant for grounding with a series impedance of a resistor and a capacitor for grounding the feedback input terminal at high frequencies.