JPH1038928A - Attenuation circuit for oscilloscope - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress a change in the frequency characteristic and the square- wave characteristic of an attenuation circuit by a method wherein a voltage to be applied to the resistance network of the attenuation circuit is controlled by a voltage detected by a resistance in which range information is weighted by every input from a control input terminal used to select an attenuation range and a base potential is not changed. SOLUTION: An analog input waveform signal is input an input terminal 1. Transistors Q1 to Q8 are composed of transconductance stages Q11/Q12 which are called cascode amplifiers and of an amplifier in which base connection stages are combined and whose high-frequency characteristic is excellent. Transistors Q9, Q10 are emitter followers which are separated from a next stage, transistors Q11 to Q16 are cascode amplifiers and emitter followers whose configuration is identical, and a potential which is raised by an NPN transistor at a previous stage is lowered to 0V. An operational amplifier U1 stabilizes the common-mode potential of an output in a DC feedback route.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a signal attenuating circuit used for a waveform observation device such as an oscilloscope.

[0002]

2. Description of the Related Art Generally, an input circuit of an oscilloscope is divided into a high-impedance first attenuator provided before the input section impedance converter and a second attenuator provided after the impedance converter. A wide range of signals such as 40 mV to 40 V full scale can be handled by combining the ratios. First
Since the voltage of the signal to be handled is high and the breakdown voltage is high, the damping circuit generally uses a mechanical switch such as a relay.
The attenuation ratio is, for example, 1: 1/10: 1/100.
On the other hand, since the second attenuating circuit has a low impedance and a low voltage to be handled, as shown in the example of FIG. FIG. 2 does not show the first attenuation circuit.

The operation of this conventional example will be described.

The input signal VIN is converted into a current by the transconductance stage transistors Q1 / Q2, and turned on by raising the base potential of one of the common base pairs Q7 / Q8, Q5 / Q6, Q3 / Q4. The voltage generated between the transistors Q9 / Q10 is controlled by the sum of the resistors R11 to R16 connected between the changed collectors.

In this example (re means the dynamic resistance of the emitter of the transistor): (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 + R1
4 + R15 + R16) / (R5 + R6 + 2re) = 40
0 / (39 + re).

(2) When the control voltage source VLA is set to 0 V, VLB is set to 5 V, and VLC is set to 0 V, the gain Av is intermediate, and Av = Vc / VIN = (R12 + R13 + R14 + R1
5) / (R5 + R6 + 2re) = 200 / (39 + r)
e).

(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 + R
6 + 2re) = 100 / (39 + re).

As described above, when the maximum gain is set to 1, an attenuating circuit having a ratio of attenuating in order of 1: 1/2: 1/4 can be realized. The output voltage VC passes through the emitter follower Q9 / Q10 and the next-stage amplifier circuits Q11 to Q16, and is output to the output terminal 2,
3 is transmitted.

[0009]

However, actually, in the conventional example, the operating point of the subsequent amplifier circuit fluctuates when the attenuation ratio is switched. For this reason, there is a disadvantage that the shape of the square wave characteristic changes depending on the range. The cause of the change in the shape of the square wave characteristic is the transconductance stage Q11 / Q12 of the next stage.
The amount of self-heating changes depending on the operating point due to the thermal distortion of the element, and the shape of the rising portion of the square wave response caused by the change in the amount of thermal positive feedback changes.

The operating point of the amplifier circuit varies because the path through which the bias current Ic of the circuit flows varies depending on which of the common base pair Q7 / Q8, Q5 / Q6, and Q3 / Q4 is turned on. When the damping ratio is 1, the base voltage of Q9 is changed from + 8V through R17, R13, R12 and R11 to Q
7, but when the damping ratio is 1/4, R1
7 and R13 are determined by the current flowing into Q7 only through R13, so that the potential is Ic · (R11 + R1
This is because only 2) goes up.

[0011]

According to the present invention, range information is detected from a control input terminal for selecting an attenuation range by a resistor weighted by each input, and is taken out as a voltage change at a base of an added emitter follower circuit.
This controls the voltage applied to the resistor network of the damping circuit,
A transistor and a resistor network are added to prevent the base potential of the emitter followers Q9 / Q10 from fluctuating.

As a result, since the correction voltage is applied when the attenuation ratio is switched, the fluctuation of the operating point of the subsequent amplification circuit can be reduced, and the fluctuation of the frequency characteristic and the square wave characteristic of the circuit can be reduced.

[0013]

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

Reference numeral 1 denotes an input terminal for an analog input waveform signal, and an amplifier composed of transistors Q1 to Q8 is a differential amplifier having excellent high frequency characteristics in which a transconductance stage Q11 / Q12 generally called a cascode amplifier and a grounded base stage are combined. is there. Transistors Q9, Q1
Numeral 0 is an emitter follower for separating from the next stage. Transistors Q11 to Q16 are cascode amplifiers of the same configuration, and the emitter follower is an emitter follower. In this example, the potential increased by the NPN transistor is reduced to 0V by using a PNP transistor in this example. ing.

The operational amplifier U1 is a direct current feedback path provided for stabilizing the output common mode potential.

As described with reference to the conventional example of FIG. 2, the attenuation ratio (gain) is determined based on the pair of common base transistors Q7 / Q8 and Q5.
Switching is performed by a resistance value connected between the collectors of the transistors turned on in one of / Q6 and Q3 / Q4. At this time, the bias current Ic of the circuit is supplied from the added transistor QA. This transistor QA
The emitter potential is the resistance connected to the base RD, RA, R
Determined by B and RC. The values of RA, RB, and RC are selected so that the base potential of QA fluctuates optimally according to the logic levels of the control input terminals A, B, and C, so that the base potential of Q9 / Q10 does not fluctuate finally.

This value can be easily determined by determining the RD and the base potential of Q9 / Q10 and establishing simultaneous equations for each state.

That is, these relations can be minimized if the following changes are made.

Hereinafter, description will be made with reference to FIG.

It is assumed that Vc is given and does not change.

VLA, VLB and VLC are generally 0 V,
As described above, the attenuation ratio is determined by setting any one to 5v.

Now, when VLA is 5V, the attenuation ratio is 1/1, when VLB is 5V, the attenuation ratio is 1/2, VLC
Is 5v, the attenuation ratio is 1/4.

Vc, R11, R12, and R13 are given from the attenuation ratio and the bias condition. In this case, for example, R11 = 100Ω, R12 = 50Ω, R13 =
When Vx at which Vc is constant is determined for each state of 50Ω, the following is obtained.

When the attenuation ratio is 1/1, VxA = Vc + (R
11 + R12 + R13) Ie + VBE = 6.5v When the attenuation ratio is 1/2, VxB = Vc + (R12 + R1)
3) Ie + VBE = 6.0V When the attenuation ratio is 1/4, VxC = Vc + R13Ie + VB
E = 5.75v When VLA, VLB, and VLC are in each state, an equation is established to satisfy this condition. When the attenuation ratio is 1/1, (Vcc-VxA) / RD = (V
xA-5v) / RA + VxA / RB + VxA / RC When the attenuation ratio is 1/2, (Vcc-VxB) / RD = Vx
B / RA + (VxB-5v) / RB + VxB / RC When the attenuation ratio is 1/4, (Vcc-VxC) / RD = (V
xC-5v) / RA + VxC / RB + VxC / RC For example, if RA = 1 and simultaneous equations are obtained for RB and RC, the respective ratios can be obtained.

The following is one example.

1.5 / 1 = 1.5 / RD + 6.5 / RB
+ 6.5 / RC 2.0 / 1 = 6.0 / RD + 1.0 / RB + 6.0 / R
C 2.25 / 1 = 5.75 / RD + 5.75 / RB + 0.
75 / RC In this way, level fluctuations are minimized.

[0027]

As described above, according to the present invention,
The problem that the shape of the square wave characteristic changes when the attenuation ratio is changed can be reduced, and a high-quality oscilloscope can be provided.

[Brief description of the drawings]

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

FIG. 2 is a circuit diagram showing a conventional example.

[Explanation of symbols]

1 input terminal, Q1-Q16 transistor, R1-R
D resistance, C1 to C4 capacitors, A, B, C attenuation range switching control input terminals, 2, 3 output terminals, VIN, V
LA, VLB, VLC voltage source, I1, I2 current source

Claims (1)

[Claims] 1. An emitter-grounded differential transistor pair for converting an input signal into a voltage and a current, a plurality of grounded base transistor pairs having an emitter commonly connected to each collector of the pair of grounded emitter differential transistors, and a plurality of grounded base transistor pairs. A control voltage source connected to the bases of the common-base transistor pair, a plurality of control voltage sources connected between the collectors of the plurality of common-base transistor pairs, connected in series with each other at an arbitrary resistance ratio, and connected at an intermediate point to a fixed potential; And a resistance network between collector outputs by selectively increasing a base potential of a pair of common bases among a plurality of common base transistor pairs connected to the resistance network. A circuit for selecting a resistance value and controlling a transmission gain, wherein an emitter follower circuit in which an emitter is connected to a connection terminal of the resistor network at the fixed potential. A resistor between the base of the emitter follower circuit and each base of the pair of grounded base transistors and a fixed potential, and controls an emitter potential of the emitter follower circuit according to a gain setting. Oscilloscope attenuation circuit.