JPH1070418A - Multistage cascode amplifier - Google Patents

Abstract

(57) [Problem] To provide a multi-stage cascode amplifier having a level shift circuit suitable for monolithic ICs with good high-frequency characteristics over a wide band, little variation, low noise, and low noise. SOLUTION: A PNP transistor formed on the same chip is operated as a grounded base having a gain of about 1 for direct current level shift, and a capacitor having a relatively small capacitance is provided from an input of a grounded base to an output collector. The grounded base is responsible for amplification and level shift from to the middle band, and high-frequency components above the middle band are bypassed by the capacitor, thereby realizing a stable and low-noise level shift circuit over a wide band.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a voltage level shift circuit suitable for a wideband amplifier, particularly a monolithic IC, and more particularly, to a multi-stage having a level shift circuit suitable for a wideband DC amplifier, such as an oscilloscope amplifier and a high definition video signal amplifier. The present invention relates to a cascode amplifier.

[0002]

2. Description of the Related Art A cascode differential amplifier is known as a circuit such as an oscilloscope for amplifying a wide range from a direct current to a wide band. FIG. 8 shows an example of a wide-band DC differential multi-stage cascode amplifier in which the cascode amplifiers are connected in multiple stages.

In FIG. 1, this amplifier comprises a first-stage cascode amplifier 1a, a level shift circuit 2, and a second-stage cascode amplifier 1b. Focusing on the cascode amplifier 1a in such a well-known conventional wideband DC differential amplifier, the amplifier is composed of a transistor pair Q
4, a transconductance stage of Q5 and a common-base pair transistor Q6, Q7 connected to its collector.

If all the cascode amplifiers are constituted by NPN transistors, the DC potential rises by Vbc of the transistors Q4 and Q5 in the transconductance stage and Vce of the transistors Q6 and Q7 in the common base stage.

When it is desired to gain a gain by connecting a plurality of cascode amplifiers, the potential rises by the number of stages, so it is necessary to perform a level shift somewhere to lower the potential.

For example, the potential can be lowered by changing the transistors Q10, Q11, Q12, Q13 of the cascode amplifier 1b of FIG. 8 to PNP transistors. However, in a monolithic IC, generally, a high ft (gain bandwidth product) P
It is difficult to make NP transistors. For this reason, FIG.
As shown in (1), all are composed of NPN transistors.

In the example shown in FIG. 8, cascode amplifiers composed of transistors having the same configuration are connected in two stages 1a and 1b, and a level shift made up of emitter followers Q1 and Q2 and zener diodes Dz1 and Dz2 is connected between the stages. Circuit 2
Is used to temporarily lower the DC level.

FIG. 9 shows only one polarity of the level shift circuit 2. Emitter follower transistors Q1, Q2 (not shown in FIG. 9)
Although the output potential is lowered by Vbe and the Zener diode Dz1, there is a problem that the Zener diode easily generates noise and the Zener voltage varies.

On the other hand, there is a well-known method shown in FIG. 10 in which a level is shifted by a resistor without using a Zener diode. In the figure, only one polarity of the level shift circuit 2 is extracted,
It is shown. The level shift voltage is determined by the current flowing through the resistor R1, but the resistor R1 forms a low-pass filter with the next-stage circuit and the capacitive component of the impedance of the current source I1, and it is difficult to obtain a wide band characteristic. It is necessary to provide a large capacitor C1 in parallel.

[0010]

SUMMARY OF THE INVENTION In a level shift circuit of a broadband DC differential cascode amplifier in which cascode amplifiers are connected in multiple stages: PNP is used for the transistor constituting the cascode amplifier.
Although a method using a transistor does not require a level shift circuit, it is difficult to produce a transistor having good high-frequency characteristics, and the performance of a PNP transistor becomes a bottleneck.

2. In the method using the Zener diode, the Zener diode is apt to generate noise, and the Zener voltage is apt to vary. If the Zener diode is used in a balanced circuit as in this embodiment, a voltage offset occurs due to the variation.

3. In the conventional example using a resistor instead of a Zener diode, a relatively large capacitance element such as a high-capacity ceramic capacitor having a capacitance of several μF or a tantalum capacitor is indispensable in order to obtain broadband characteristics, and cannot be realized with a chip capacitor. Yes, it is not suitable for monolithic IC.

Each has its own drawbacks.

An object of the present invention is to make a monolithic IC, which has a wide band, good high frequency characteristics, little variation,
An object of the present invention is to provide a wideband DC differential level shift circuit having low noise and suitable for, for example, an oscilloscope or a high-definition video circuit.

[0015]

According to the present invention, in order to achieve the above object, in a multistage cascode amplifier, a PNP transistor formed on the same chip is operated with a base ground for direct current level shift (for example, a gain of 1). And a relatively small-capacitance capacitor is provided from the input of the common-base transistor to the output collector.

As a result, the base-grounded transistor is responsible for amplification and level shift from DC to the middle band, and high-frequency components above the middle band are bypassed by the capacitor. As a result, a stable level shift circuit can be realized over a wide band. .

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows only one polarity of the level shift circuit of the multi-stage cascode amplifier.

Q20 is an emitter follower transistor,
Q21 is a PNP transistor. Transistor Q2
Reference numeral 1 denotes a bell-shifted common base transistor, and a transistor Q23 forms an emitter follower for a buffer with the next stage.

The transistor Q21 has its base connected to the first predetermined potential Vb2, and the resistor R2 connected to the collector is connected to the second predetermined potential V-.

Further, C is a capacitor for passing a high-frequency signal, and is a capacitor having a relatively small capacitance, for example, a capacitance value of 5 pF and a chip size of 2 μm × 2 μm. The gain of the common base transistor Q21 is the resistance of the resistor R1.
And R2. In this case, for example, R1 + re = R2 is selected and set to a gain of 1.

Here, re is the emitter dynamic resistance of the transistor 21.

The voltage Vb3 after the level shift is equal to the base potential Vb1 of the transistor Q20 and the transistor Q2.
Vb3 = (Vb1−Vb2− (Vbe1 + Vbe2)) determined by the base potential Vb2 of 1 and the resistors R1 and R2.
R2 / R1 + V.

Considering the case where there is no capacitance C of the capacitor, the high-frequency pass band is
Since it is determined by the gain bandwidth product ft of the transistor Q21,
As described above, it is difficult to make a high ft PNP transistor due to process restrictions, and it is not possible to expect a wide band. However,
This disadvantage can be covered by adding a capacitance element (capacitor) C. By providing this capacitive element, the frequency characteristic becomes a solid line c as a composite characteristic of the characteristic a shown by the dotted line and the characteristic b shown by the one-dot chain line as shown in FIG.

The characteristic a in the low frequency band becomes an equivalent circuit shown in FIG. 2, and the impedance of the collector of the common base transistor becomes the capacitor C and the resistor R2, and the cutoff frequency becomes 1 / 2πC · R2.

The characteristics in the high frequency band are as shown in the equivalent circuit of FIG. 3, which is a high-pass filter characteristic having the same cutoff frequency of 1 / 2πC · R2 composed of the capacitor C and the resistor R2. Has almost no effect.

FIG. 5 is a circuit diagram of a wideband multistage cascode amplifier for amplifying an input signal from DC to wideband, embodying the present invention. Such a wideband multistage cascode amplifier is used for, for example, an oscilloscope.

In the figure, the same components as those in FIG. 8 are denoted by the same reference numerals.

This amplifier comprises a first-stage cascode amplifier 1a, a level shift circuit 3, and a second-stage cascode amplifier 1b.

Next, this operation will be described.

The transistors Q 6 and Q 7 of the first-stage cascode amplifier 1 a are input to the level shift circuit 3. The differential signal input to the transistors Q20 and Q24 is amplified (however, the gain is 1), and its emitter output is connected to a resistor R1,
The signal is input to the emitters of the transistors Q21 and Q25 via R4, and the low-frequency component of the input signal is output to the collector. At this time, the high frequency component passes through the capacitors C2 and C3 and is input to the bases of the transistors Q23 and Q26. The emitter outputs of the transistors Q23 and Q26 are output to the bases of the transistors Q10 and Q11 of the cascode amplifier 1b at the next stage.

Next, an example in which the wideband DC differential amplifier of the present invention is applied to a digital oscilloscope will be described with reference to FIGS.

FIG. 6 shows an example in which the cascode amplifying circuit of the present invention is used for the amplifier of the A / D converter of the digital oscilloscope.

In the figure, reference numeral 4 denotes an input terminal of a signal to be observed, 5 denotes a first attenuator including a buffer amplifier, 6 denotes a second attenuator, and the second attenuator corresponds to the cascode amplifier 1a described above. FIG. 7 shows a detailed circuit diagram thereof. 12 is an A / D conversion circuit, 13 is a memory, 14 is a display circuit including a display such as LCD, CRT or EL, 15 is a trigger circuit, 16 is a time base circuit, and 17 is a CPU. Other symbols indicate the same components as those in FIG. The configurations of the A / D converter 12 and the trigger circuit 15 and below are the same as those of a well-known digital oscilloscope, and a description thereof will be omitted.

Next, this operation will be described.

The signal to be observed input to the input terminal 4 is attenuated by a first attenuator circuit 5 including a buffer amplifier (not shown) at an attenuation ratio of coarse intervals. Next, in order to set the attenuation ratio of the observed signal with the attenuation ratio at a smaller interval, the second
An attenuator circuit 6 is used. As described above, the input circuit of the oscilloscope is divided into the high-impedance first attenuator circuit 5 provided before the input section impedance conversion circuit and the second attenuator circuit 6 provided after the impedance conversion circuit. A wide range of signals such as 40 mV to 40 V full scale can be handled by a combination of attenuation ratios. The first attenuator circuit 5 generally uses a mechanical switch such as a relay because the voltage of a signal to be handled is high and the withstand voltage is high. The attenuation ratio is, for example, 1: 1/10: 1/100. On the other hand, since the second attenuator circuit 6 has low impedance and low handling voltage, it is electronically switched by a transistor or the like as shown in the example of FIG. Next, this operation will be described.

The input signal of the second attenuator 6 is converted into a current by the transconductance stage transistors Q68 / Q69. This current is applied to the common base versus transistor Q6.
By raising the base potential of any of 3 / Q67, Q62 / Q65, and Q61 / Q64, the current flows through the resistors R11 to R16 connected between the collectors that are turned ON. The level shift circuit 3 is determined by the sum of the currents flowing through the resistors.
Is controlled between the transistors Q20 and Q24.

For example, in this example, (1) the control voltage source VLA is 5 V, VLB is 0 V, VLC
Is set to 0 V, the gain Av becomes maximum, and Av = Vc / VIN = (R11 + R12 + R13 + R14 + R15 + R1
6) / (R5 + R6 + 2re) = 400 / (39 + r)
e).

(2) When the control voltage source VLA is set to 0V, VLB is set to 5V, and VLC is set to 0V, the gain Av is intermediate, and Av = Vc / VIN = (R12 + R13 + R14 + R15) / (R5 + R6 + 2re) = 200 / (39 + re) Becomes

(3) When the control voltage source VLA is set to 0 V, VLB is set to 0 V, and VLC is set to 5 V, the gain Av becomes minimum, and Av = Vc / VIN = (R13 + R14) / (R5 + R6 + 2re) = 100 / (39 + re) Becomes

As described above, an attenuator having a ratio of 1: (1/2) :( 1/4) that attenuates sequentially when the maximum gain is 1 can be realized. The output voltage of the second attenuator circuit 6 is applied to the emitter follower Q20 / Q24 of the level shift circuit 3 of the next stage and the amplifier circuits Q21 and Q2 of the next stage.
23, Q25, Q26, cascode amplifier 1b
Is transmitted to

Hereinafter, this output is passed through a well-known digital oscilloscope circuit, and the waveform of the input signal is displayed on a display (not shown) of the display circuit 14. The display waveform according to the present embodiment has high quality because there is little noise in the circuit and there is little offset shift. In addition, since the shape can be reduced, a larger effect can be obtained by implementing the invention on a portable digital oscilloscope.

[0043]

As described above, according to the present invention, since the high-frequency characteristics do not depend on the common base, there is no restriction on the gain bandwidth product ft of the PNP transistor, and the capacitance C and the resistance R
By appropriately selecting 2, a level shift circuit having a wideband frequency characteristic can be easily realized, and a high-performance multistage cascode amplifier can be provided.

Further, the capacitor is suitable for a monolithic IC, since a capacitor having a capacity of several pF that can be formed on a chip can be obtained by optimizing a method of selecting a cutoff frequency. Therefore, the circuit can be downsized.

Further, since this level shift circuit does not use a Zener diode, the generation of noise is small, and the shift voltage is determined by the base-emitter voltage Vbe having a small variation on the same chip as the base voltage of the common base, so that the level shift circuit can be stabilized. Even when used in a balanced circuit, the amount of offset deviation can be reduced.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of an embodiment of a level shift circuit of a multi-stage cascode amplifier according to the present invention.

FIG. 2 is an equivalent circuit diagram showing a low-frequency operation of the embodiment of the present invention.

FIG. 3 is an equivalent circuit diagram showing a high-frequency operation according to the embodiment of the present invention.

FIG. 4 is a frequency characteristic diagram of the embodiment of the present invention.

FIG. 5 is a circuit diagram of an embodiment of the multistage cascode amplifier of the present invention.

FIG. 6 is a block diagram of an oscilloscope according to an embodiment of the present invention.

FIG. 7 is a circuit block diagram of an oscilloscope according to an embodiment of the present invention.

FIG. 8 is a circuit diagram showing a conventional example of a multistage cascode amplifier.

FIG. 9 is a circuit diagram of a level shift circuit of a conventional multistage cascode amplifier.

FIG. 10 is a circuit diagram of a level shift circuit of a conventional multistage cascode amplifier.

[Explanation of symbols]

1a, 1b: cascode amplifier, 2: level shift circuit, Q1 to Q26: transistor, R1 to R16, R
s: resistance, C: capacitor, 4: input terminal, 5: first attenuator circuit, 6: second attenuator circuit, 12: A
/ D converter, 13: memory, 14: display circuit, 15: trigger circuit, 16: time base circuit, 17: CPU

Claims (5)

[Claims] 1. A PNP transistor is operated as a grounded base for DC level shift, and a capacitance element is provided from an input to an output collector of the grounded base transistor.
A multi-stage cascode amplifier having a level shift circuit, wherein the PNP base-grounded transistor is responsible for amplification and level shift from a direct current to a middle range, and high-frequency components above the middle range are bypassed by the capacitive element. 2. A base-grounded transistor having a base connected to a first predetermined potential and a collector serving as an output terminal, and a resistor having one terminal serving as an input terminal and the other terminal connected to the emitter of the base-grounded transistor. A level shift circuit including a resistor having one terminal connected to the second predetermined potential, the other terminal connected to the collector of the common base transistor, and a capacitor connected from the input terminal to the output terminal. A multi-stage cascode amplifier. 3. The level shift circuit according to claim 2, further comprising an emitter follower buffer transistor having an emitter connected to an input terminal of said common base transistor and having a base as an input terminal. Features a multi-stage cascode amplifier. 4. An emitter follower buffer transistor having an output signal of a preceding cascode amplifier as a base input, a collector connected to a predetermined potential, and an emitter as an output, and a base having a base connected to a predetermined potential and a collector as an output terminal. A ground transistor, a resistor connected between the emitter of the common base transistor and the emitter follower transistor, a resistor having one terminal connected to a predetermined potential and the other terminal connected to the collector of the common base transistor, A multi-stage cascode amplifier comprising: a level shift circuit including a capacitor connected between an emitter output terminal of the emitter follower buffer transistor and a collector output terminal of the common base transistor; 5. The multistage cascode amplifier for an oscilloscope according to claim 4, wherein the multistage cascode amplifier of the preceding stage is formed by cascading attenuator circuits.