JPS6056322B2 - Wideband balanced amplifier with gain switching function - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is applicable to wideband oscilloscopes (e.g. 200MHz band).
2) relates to an amplifier circuit in which the voltage gain used on the vertical axis can be switched.

To switch the vertical axis deflection sensitivity of the oscilloscope,
A method as shown in FIG. 1 has been conventionally considered as a method for switching the voltage gain of an amplifier.

First, the gain switching circuit shown in FIG. 1 will be explained. The state with the smallest voltage gain is when the switch 42 is in position a, and in this state the transistors 27, 28, 33, 3
Only resistor 4 is active, and the voltage gain is determined by the ratio of resistor 16 to resistors 25 and 26. A medium voltage gain condition (2.5 times the lowest condition) is obtained when switch 42 is in the position. In this state, outside of transistors 27, 28, 33, 34, 29, 30, 35
becomes active. Voltage gain is resistor 16 and resistor 25,2
It is determined by adding the ratio of the resistor 17 and the resistors 25 and 26 to the ratio of the resistor 17 and the resistor 25 and 26. The highest voltage gain condition (approximately five times the lowest voltage gain) is obtained when switch 42 is in position c. In this state, the transistor 31
, 32 become active. The voltage gain is determined by adding the ratio of the resistor 18 to the resistors 25 and 26 to the gain when the voltage gain is medium. Here, variable resistor 38
is for adjusting the bias current distribution of the transistors 29 and 30 so that the potential difference between the output terminals 2 and 3 does not change when the transistors 29 and 30 are activated (when the gain is medium). be. Also, variable resistor 4
Similarly, when transistors 29, 30, 31, and 32 are activated (when the gain is maximum), 1 adjusts the bias current distribution of transistors 31 and 32 so that the potential difference between 2 and 3 does not change. belongs to. The transistor 35 is for supplying the collector current through the resistors 21 and 22 when the transistors 29 and 30 become active, and the current value is equal to that of the resistors 51, 50, 49, and 4.
8. Furthermore, when transistors 31 and 32 become active, their collector currents are supplied from transistor 35 and the current values are determined by resistors 51, 50, and 49. The diodes 43 to 47 are provided so that the selector switch 42 can be a simple switch of one circuit. Resistors 52, 53, 54, 55 and capacitor 56,
57 is for lowering the frequency bandwidth when the gain is minimum to make it approximately equal to the bandwidth when the gain is increased. In order to use this amplifier circuit in a vertical amplifier circuit of an oscilloscope, it is necessary to reduce the drift so that the potential difference between the output terminals 2 and 3 does not change even if the ambient temperature of the amplifier changes. It is also necessary to prevent the potential difference between 2 and 3 from changing even when the gain is switched. Therefore, in the past, transistors 27 and 28, 29 and 30, 3
Bare transistors with well-matched static characteristics must be selected and used for 1 and 32, 33 and 34, respectively, and the labor and cost of selection and the cost of managing the selected components will increase the cost of the amplifier circuit. It was rising.

Further, even after the variable resistors 38 and 41 have been adjusted once, if there is a change in the transistor or resistor over time, the variable resistors 38 and 41 must be adjusted again. Furthermore, when transistors 29 to 32 become active, the current value supplied from transistor 35 is determined by the values of resistors 48, 49, 50, and 51, so if high-precision resistors are not used for these, There is a drawback that variations occur in the operating points of the transistors 29 to 32, resulting in variations in characteristics when the amplifier circuit is mass-produced.

The present invention eliminates these drawbacks and provides a wideband amplifier circuit that uses unselected components by using a bias circuit using an inexpensive general-purpose operational amplifier, has fewer adjustment points, and has a gain switching function with less drift. The purpose is to obtain.

The present invention will be explained in detail below.

FIG. 2 shows an embodiment of the invention.

1 is a signal input terminal, and 2 and 3 are output terminals.

61, 68, and 80 are operational amplifiers.

The operational amplifier 68 receives the outputs of the output terminals 2 and 3, amplifies the potential difference between the output terminals 2 and 3, and supplies the amplified potential difference to the operational amplifier 80. The operational amplifier 80 compares the potential difference (differential signal) between the input signal at the terminal 1 and the output terminals 2 and 3, amplifies the difference, and provides negative feedback to the base of the transistor 28. This negative feedback compensates for drift caused by the amplifier circuit composed of transistors 27 to 34 and fluctuations in potential difference between terminals 2 and 3 due to gain switching. This feedback action is performed only in a low frequency region including direct current. Capacitor 78 determines the high cutoff frequency of operational amplifier 80. The resistors 81 and 82 are used to divide the input signal and apply it to the terminal 80, since if the input terminal of the operational amplifier 80 is directly connected to the terminal 1, the signal at the terminal 1 will be attenuated by the input capacitance. In order for the outputs of terminals 2 and 3 to have flat gains from direct current to high frequencies, it is necessary to change the amount of negative feedback in accordance with the gain of the amplifier. When the voltage gain is minimum, that is, when only the transistors 27, 28, 33, and 34 of the amplifier circuit are active (when the switch 42 is in position a), the output of the operational amplifier 68 is connected to the resistor 72. but when the voltage gain is medium (switch 4
2 is at position b), the resistor 73 is grounded, so the output of the resistor 68 is divided by the resistors 72 and 73. The operational amplifier 61 and transistor 69 circuit has two purposes.

One is for supplying the collector current of the transistors 29, 30, 31, 32, and the other is for supplying the collector current of the transistors 29, 30, 31, 32.
The goal is to always keep the common mode voltage between the two at a constant value. terminal 2,
The common mode voltage between the two is detected as the potential at the connection point between the resistors 25 and 26, and the potential is shifted by the resistors 58, 59 and 60 and led to the input terminal of the operational amplifier 61, where it is compared with the potential at the ground point. The difference signal is amplified and supplied to the base of transistor 69. As a result, transistors 29, 30
, 31 and 32 are activated, the common mode voltage between terminals 2 and 3 is maintained constant, and as a result, the necessary current flows from transistor 35 to transistors 29 to 32 through resistors 21 and 22. will be supplied. operational amplifier 68
is provided for the purpose of amplifying only the differential voltage between terminals 2 and 3, but if there is a fluctuation in the common mode voltage of terminals 2 and 3,
The output of the operational amplifier 68 also fluctuates due to the influence of errors in the resistors 64, 65, 66, and 67. Therefore, the provision of operational amplifiers 61 and 35 to suppress fluctuations in the common mode voltage at terminals 2 and 3 greatly contributes to simplifying the configuration of the differential feedback circuit. FIG. 3 is a circuit diagram showing still another embodiment of the present invention.

In this embodiment, the respective output currents of the transistors 27 and 28 are connected to resistors 156, 152, 154 inserted into the collectors.
It is attenuated by 157, 153, 155 and the transistor 33
, 34 emitters. In addition, each output current of the transistors 29 and 30 is similarly controlled by the resistors 156 and 152,
154, 157, 153, and 155 and is applied to each emitter of transistors 33 and 34, but the amount of attenuation is smaller than the amount of attenuation for each output current of transistors 27 and 28. With the above configuration, the values of the feedback resistors 16, 17, and 18 can be set to almost similar values, so it is possible to reduce the change in bandwidth when switching the gain, and further reduce the attenuation caused by the resistors. Since it is a circuit network, there is no need to narrow the band. The general-purpose operational amplifier used in the above embodiments is inexpensive and can easily be obtained with little drift, so the amplifier transistors 27 to 3
The cost is much lower than selecting 4.

In addition, the amplifier circuit of a wideband synchroscope with a band of 100 to 300 MHz or more is often composed of a hybrid IC or monolithic IC, but if the circuit can tolerate differences in the characteristics of the elements, the yield of the IC can be improved. There are some effects that can be significantly improved. As explained above, according to the present invention, it is possible to easily obtain a gain-switchable wideband amplifier circuit with less drift and fewer adjustment parts even when using components whose characteristics are not selected. If used, a vertical deflection system with little drift can be easily obtained, and furthermore, drift etc. due to variations in elements after manufacturing can be significantly reduced.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing a conventional example, and FIGS. 2 and 3 are circuit diagrams showing an embodiment of the present invention.

Claims (1)

[Claims] 1. A common emitter amplifier having a first input terminal and a first feedback resistor connected between each emitter, and one of each base being connected to the first input terminal. The first to configure
, a second transistor, and third and fourth transistors constituting a common emitter amplifier, each having a base connected to each emitter of the first and second transistors, and having a second feedback resistor between each emitter. a transistor, and means for turning on and off the bias of the third and fourth transistors;
first and second balanced output terminals, and a first combining means that combines collector currents of the first and third transistors and supplies them to the first balanced output terminal; a second synthesizing means for synthesizing the respective collector currents of the transistors and supplying it to the second balanced output terminal;
a first amplifier that amplifies the common mode voltage of a balanced output terminal of and applies it to the neutral point of the first combining means and the second combining means, and the difference between the first and second balanced output terminals; a second amplifier that amplifies the dynamic component; and an output of the second amplifier
, amplification switching means that switches in conjunction with the switching operation of the means for turning on and off the bias of the fourth transistor, and amplifying the difference between the output of the amplification switching means and the signal of the first input terminal, and a third amplifier for applying voltage to the base of the transistor whose base is not connected to the first input terminal among the first and second transistors, means for turning on and off the bias, and the amplification factor. A broadband balanced amplifier having a gain switching function configured to switch the amplification gain by a switching means. 2. The first and second synthesizing means are configured to directly couple the respective collectors of the first and third transistors and to directly couple the respective collectors of the second and fourth transistors. A broadband balanced amplifier having a gain switching function according to claim 1. 3. The first and second combining means include a circuit that shunts the collector currents of the first and third transistors, and a circuit that shunts the collector currents of the second and fourth transistors. A broadband balanced amplifier having a gain switching function according to claim 1, characterized in that the amplifier is configured as follows.