JPH0716139B2 - Wideband variable gain amplifier - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wide band amplifier suitable for amplifying a signal over a wide band from direct current to high frequency with a plurality of selectable gains.

[Prior Art and Problems to be Solved by the Invention] In the vertical amplifier and the trigger amplifier of the oscilloscope,
Wideband amplifiers are used that uniformly amplify wideband input signals ranging from direct current (DC) to high frequencies with selectable gain.

A conventional amplifier having such a wide band characteristic generally has a source follower type field effect transistor as an input stage and a combination of an amplification stage and an output stage of a bipolar transistor, especially when a high input impedance is required. Target. Further, when the gain of the amplifier is switched and selected to a plurality of values, a configuration in which a variable attenuator is connected to the front stage or the rear stage of the amplifier is used.

However, in such a conventional configuration, an expensive wideband variable attenuator is required to stably amplify an input signal from DC to a high frequency exceeding 100 MHz with a desired gain. It was necessary to adjust the value of the variable capacitor of the variable attenuator for each attenuation ratio and to calibrate the frequency characteristic in order to make it uniform over the entire range.

Therefore, the object of the present invention is to perform amplification of a desired gain while maintaining uniform characteristics over a wide band from direct current to high frequencies, and to eliminate the need for a conventional wide band variable attenuator, and to calibrate frequency characteristics. It is an object of the present invention to provide a wide band variable gain amplifier which does not require work and is suitable for integration into an integrated circuit.

[Means for Solving the Problems] A wide-band variable gain amplifier according to the present invention includes a plurality of series resistors 76 to 78, one end of which is connected to an input terminal for receiving an input signal,
A low-frequency amplifier 82 that receives the input signal via the plurality of series resistors, variable attenuators 86 and 88 that attenuate the output of the low-frequency amplifier, and connection points that individually correspond to the plurality of series resistors. A plurality of wide band amplifiers 70 having different gains, each of which has its input end connected to each other via a coupling capacitor.
And 72 and one of these multiple broadband amplifiers,
The output of the variable attenuator is selected and supplied to the input terminal of the selected wide band amplifier in conjunction with the selection,
Bias selecting means 100 for deactivating a wide band amplifier other than the selected wide band amplifier is provided, and an output signal having a substantially uniform frequency characteristic over a wide band is obtained from the selected wide band amplifier.

[Operation] According to the wide band variable gain amplifier of the present invention, the high frequency path and the low frequency path are separated, the output of the low frequency amplifier is attenuated by the variable attenuator at a desired attenuation ratio, and the low frequency component is selected in the wide band. Since it is supplied to the input terminal of the amplifier to obtain an excellent wide band characteristic, it is not necessary to use a variable attenuator having a wide band characteristic, and there is no need to calibrate the frequency characteristic.

[Embodiment] FIG. 1 is a circuit diagram showing a configuration of a preferred embodiment of the present invention. First, the input signal to be amplified is applied to the input terminal 10. One end of a plurality of series resistors 76, 77 and 78 which form a voltage divider is connected to the input terminal 10, and the other ends of these series resistors are connected to the inverting input end of the operational amplifier 82.
This operational amplifier 82 has a non-inverting input terminal connected to a DC balanced voltage source (DC BAL) and a negative feedback resistor 80 between the output terminal and the inverting input terminal, and functions as an inverting low-frequency amplifier as a whole. . The output of this low frequency amplifier is switch 84
Is supplied to the variable attenuators 86 and 88 via. The switch 88 of this variable attenuator is preferably an electronic switch. The outputs of the variable attenuators 86 and 88 are supplied to the inverting input terminal of the operational amplifier 92 via the resistor 90.

On the other hand, the connection points of the series resistors 76 to 78 are, for example, 0.00
The input terminals of the wide band amplifiers 70 and 72 are connected via the high frequency coupling capacitors 50a and 50b of 2 μF. The wide band amplifiers 70 and 72 have substantially the same structure, and are a conventional type circuit mainly composed of source follower FET input stages Q12 and Q13 and emitter follower output stages Q14 and Q15, respectively.
Diodes D3 and D4 are for protection. It goes without saying that amplification stages having different gains can be added in order to set the gains of the wide band amplifiers 70 and 72 to different values as desired. The output terminals of the broadband amplifiers 70 and 72 are commonly connected to the output terminal 104. Output terminal 104 and operational amplifier
A negative feedback resistor 99 is connected between the inverting input terminals of 92, and the operational amplifier 92 functions as an inverting amplifier. Eventually, the low frequency component of the input signal is inverted by the operational amplifier 82, and the variable attenuator 86
After being attenuated at a predetermined attenuation ratio at 88 and 88, it is inverted again at the operational amplifier 92. The output of the operational amplifier 92 is supplied to the input terminal of either one of the wide band amplifiers 70 and 72 selected by the selection means 100. The selection means 100 comprises transistors Q10 and Q11, resistors 94a and 94a, as shown.
b, consisting of diodes D1 and D2 and associated resistors and power supplies, which supplies a low frequency component signal to one of the wide band amplifiers and the other wide band amplifier in response to a control signal supplied to control input 102. Set to inactive state. The control signal supplied to the control input terminal 102 and the switching of the switch 88 of the variable attenuator are interlocked to surely achieve the matching of the attenuation ratio of the low frequency component and the high frequency component, and the flat frequency characteristic over the wide band. Is configured to obtain.

The operation of the circuit of FIG. 1 will be further described. When switches 84 and 88 are set to the positions shown, control input 102
The control signal of becomes high level, and the selection means 100 supplies the low frequency component to the input terminal of the wide band amplifier 70. That is, when the control signal is at a high level, the transistor Q10 is turned off, so that the transistor Q11 is turned on and the diode D1 is turned on.
Turns on and the diode D2 turns off. Therefore, the output of the operational amplifier 92 is supplied to the input terminal of the wide band amplifier 70 via the diode D1 and the resistor 94a, and is combined with the high frequency component supplied via the coupling capacitor 50a and amplified. It should be noted that the input terminal of the wide band amplifier 72 is maintained at a low potential and thus set in the inactive state, and the output signal does not contribute. At this time, the voltage division ratios of the series resistors 76 to 78 and the attenuation ratios of the variable attenuators 86 and 88 are matched, and a flat frequency characteristic can be realized over a wide band.

Next, when the control signal supplied to the control input terminal 102 becomes low level, the switch 88 of the variable attenuator is also switched to the side opposite to the illustrated position. At this time, the transistor Q10 turns on and the transistor Q11 turns off.
1 turns off and diode D2 turns on. Therefore, the output of the operational amplifier 92 is supplied to the input terminal of the wideband amplifier 72 through the diode D2 and the resistor 94b, and the input terminal of the other wideband amplifier 70 is maintained at a low potential and set to the inactive state. In this case as well, the voltage division ratio of resistors 76 to 78 and variable attenuator 86
And 88, and the substantially uniform frequency characteristic is maintained over a wide band.

As described above, interlocking the selection of the wide band amplifier and the variable attenuator for adjusting the output of the low frequency amplifier, which are respectively connected to the connection points of the series resistors connected to the input terminals through the coupling capacitors. This makes it possible to obtain a desired gain while maintaining the frequency characteristics substantially uniform over a wide band. Since the output of the low-frequency amplifier is variably attenuated, there is no need for a variable attenuator having a wide band characteristic as in the prior art, and a frequency characteristic calibration operation for each attenuation ratio is also unnecessary. Further, since a wide band amplifier is individually connected to each high path, there is no switch in series in the high frequency path, there is no change in the frequency characteristic due to the stray capacitance, and the integrated circuit is easy.

In the above embodiment, the circuit that selects two different attenuation ratios has been described, but it is also easy to configure the circuit to select three or more different attenuation ratios and realize a wide dynamic range.

[Advantages of the Invention] A wide-band variable gain amplifier according to the present invention separates a plurality of high-frequency paths having different gains from a low-frequency path, attenuates the output of the low-frequency amplifier with a variable attenuator, and selects the low-frequency component. It is supplied to the input of the wide band amplifier and combined with the high frequency component to realize a wide band characteristic with substantially uniform frequency characteristics. Therefore, unlike the prior art, an expensive variable attenuator having a wide band characteristic is not required, and the work of calibrating the frequency characteristic for each attenuation ratio is also unnecessary. Further, since the broadband amplifier is provided for each of the plurality of high frequency paths, no switch is required in the high frequency paths, which contributes to stabilization of frequency characteristics and integration into an integrated circuit.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing the configuration of a preferred embodiment of the present invention. 70, 72: Wideband amplifier 76, 77, 78: Series resistor 82: Low frequency amplifier 86, 88: Variable attenuator 100: Bias selection means

Claims (1)

[Claims] 1. A plurality of series resistors, one end of which is connected to an input terminal for receiving an input signal, a low-frequency amplifier for receiving the input signal via the plurality of series resistors, and an output of the low-frequency amplifier. A variable attenuator for attenuating, a plurality of wide band amplifiers having different gains, each input terminal of which is connected to a corresponding connection point of the plurality of series resistors via coupling capacitors, One is selected, the output of the variable attenuator is adjusted and supplied to the input end of the selected wide band amplifier in conjunction with the selection, and wide band amplifiers other than the selected wide band amplifier are deactivated. A wide band variable gain amplifier, characterized in that an output signal having substantially uniform frequency characteristics over a wide band is obtained from the selected wide band amplifier.