JP4767051B2 - Variable gain amplifier - Google Patents

Description

  The present invention greatly reduces the number of switches compared to the type of gain, and further cancels an error component due to the on-resistance of the switch so as to realize a highly accurate gain amount. It is about.

  As a conventional variable gain amplifier, there is a circuit having a configuration shown in FIG. 5 that adjusts the attenuation of a feedback circuit of an operational amplifier. In FIG. 5, 1 is a signal input terminal, 9 is an operational amplifier, and 10 is an output terminal. The switch groups S50 to S70 and the attenuating units 11 to 13 constitute a feedback circuit. The switch group S50 includes switches S51 to S58, and the attenuation unit 11 includes resistors R111 to R119. The switch group S60 includes switches S61 to S68, and the attenuation unit 12 includes resistors R121 to R129. Further, the switch group S70 includes switches S71 to S78, and the attenuation unit 13 includes resistors R131 to R139.

  In this feedback circuit of the variable gain amplifier, the values of the resistors R111 to R119, R121 to R129, R131 to R139 are set as appropriate, and only one of the switches S51 to S58 is selected and turned on, thereby making a 1 dB step. By selecting only one of S61 to S68 and turning on the attenuation of 0 dB to 7 dB, the attenuation of 8 dB to 15 dB is selected in 1 dB step, and only one of S71 to S78 is selected. By turning it on, attenuation of 16 dB to 23 dB can be realized in 1 dB steps, and therefore the gain of the variable gain amplifier can be adjusted in a plurality of stages.

  However, the gain variable amplifier of FIG. 5 has a problem that the K scale gain adjustment requires K switches, which increases the circuit scale.

  An object of the present invention is to provide a variable gain so that multi-stage gain adjustment can be realized with a small number of switches, and the set gain is not affected by the on-resistance of the switch, and a highly accurate gain amount can be obtained. An amplifier is provided.

In order to solve the above-described problem, the invention according to claim 1 is configured such that a non-inverting input terminal of an operational amplifier is connected to a signal input terminal, an output terminal of the operational amplifier is connected to a signal output terminal, and the operational amplifier includes: In a variable gain amplifier in which a feedback circuit is connected between an output terminal and an inverting input terminal, the feedback circuit includes first to Nth (N ≧ 2) attenuations connected in series to the output terminal of the operational amplifier. N series circuit consisting of a switch, one switch having one end connected to the output side of each of the first to Nth attenuating sections, and one resistor, and the other of the N series circuits A switching unit configured by one series circuit including one switch and one resistor connected between the end and the ground, and the N series circuits and the one series circuit of the switching unit. An (N + 1) th attenuation unit connected to the common connection point, and the (N + 1) th decrease unit M switches for selecting the amount of attenuation of the unit, and a terminal opposite to the (N + 1) th attenuation unit in the M switches is connected to the inverting input terminal of the operational amplifier, and the N series Each switch of the circuit and the switch of the one series circuit have the same on-resistance, and each resistance of the N series circuit and the resistance of the one series circuit have the same resistance value.
According to a second aspect of the present invention, in the variable gain amplifier according to the first aspect, the N + 2th attenuating unit connected to the output terminal of the operational amplifier and the L (2) selecting the attenuation amount of the N + 2th attenuating unit. = M) switches, and a terminal on the opposite side of the N + 2 attenuation section in the L switches is connected to the inverting input terminal of the operational amplifier.

  According to the first aspect of the present invention, the attenuation in the feedback circuit can be obtained in multiple stages with a small number of switches. In addition, since the on-resistance of the switch is canceled, not only can a highly accurate gain amount be realized, but also the size of the variable gain amplifier can be reduced in size because it is not necessary to increase the size of the MOS transistor constituting the switch. be able to. According to the second aspect of the present invention, when the M switches of the (N + 1) th attenuating unit and the L (= M) switches of the (N + 2) th attenuating unit are respectively configured by MOS transistors, the N + 1th attenuating unit is provided. Since one common switch and one N + 2 attenuation unit switch can be driven by a common drive circuit, the N + 1th and N + 2 attenuation units can be driven by M (= L) drive circuits. The drive circuit can be halved.

  FIG. 1 is a block diagram for explaining the principle of the variable gain amplifier of the present invention. 1 is a signal input terminal, 2 to 7 are attenuation units of attenuation amounts A1 to A6, 8 is a switching unit of attenuation amount A7, 9 is an operational amplifier, and 10 is a signal output terminal. S1 to S7 are switches. In particular, the switches S3 to S6 of the switching unit 8 have the same on-resistance. The resistors R1 to R5 of the switching unit 8 are resistors having the same value. In relation to the claims, the attenuation units 4 to 7 correspond to the first to Nth attenuation units, the attenuation unit 3 corresponds to the (N + 1) th attenuation unit, and the attenuation unit 2 corresponds to the (N + 2) th attenuation unit.

  In this variable gain amplifier, when only the switch S1 is turned on, the attenuation amount A1 by the attenuation unit 2 can be realized in the feedback circuit. At this time, since the operational amplifier 9 has a large input impedance, no current flows through the switch S1, and the ON resistance of the switch S1 does not affect the circuit.

  Next, when the switches S1, S4 to S6 are turned off and the switches S2, S3, S7 are turned on, in addition to the attenuation amount A2 of the attenuation unit 3 and the attenuation amount A3 of the attenuation unit 4, the resistance of the switching unit 8 An attenuation amount obtained by adding the attenuation amount A7 due to R1 and R5 can be realized in the feedback circuit. At this time, since the resistors R1 and R5 have the same resistance value and the switches S3 and S7 have the same on-resistance, the attenuation A7 is ½ (= −6 dB). Accordingly, the attenuation in this case is “A2 + A3 + A7”. At this time, the attenuation A7 does not change from -6 dB regardless of the values of the on-resistances of the switches S3 and S7 (however, the same value). Since the operational amplifier 9 has a large input impedance, no current flows through the switch S2, and the on-resistance of the switch S2 does not affect the circuit.

  Next, when the switches S1, S3, S5, and S6 are turned off and the switches S2, S4, and S7 are turned on, the attenuation amount A2 of the attenuation unit 3, the attenuation amount A3 of the attenuation unit 4, and the attenuation amount of the attenuation unit 5 In addition to A4, an attenuation amount obtained by adding the attenuation amount A7 by the resistors R2 and R5 of the switching unit 8 can be realized in the feedback circuit. At this time, since the resistors R2 and R5 have the same resistance value and the switches S4 and S7 have the same on-resistance, the attenuation A7 is ½ (= −6 dB). Accordingly, the attenuation in this case is “A2 + A3 + A4 + A7”. At this time, the attenuation A7 does not change from −6 dB regardless of the values of the on-resistances of the switches S4 and S7 (however, the same value).

  Similarly, when the switches S1, S3, S4, and S6 are turned off and the switches S2, S5, and S7 are turned on, the amount of attenuation is “A2 + A3 + A4 + A5 + A7”, the switches S1, S3 to S5 are turned off, and the switches S2, S2 are turned off. The attenuation when S6 and S7 are turned on is “A2 + A3 + A4 + A5 + A6 + A7”.

  As described above, the on-resistances of the switches S3 to S7 are canceled, and the on-resistance does not affect the circuit. In addition, since it is not necessary to make the on-resistance specially small, it is not necessary to make the size of the MOS transistor constituting it specially small.

  FIG. 2 is a circuit specifically showing the attenuating units 4 to 7 and the switching unit 8 of FIG. 1, and the attenuating units 4 to 7 are configured by resistors R11 to R18. FIG. 3 is another example circuit specifically showing the attenuation units 4 to 7 and the switching unit 8, and the attenuation units 4 to 7 are configured by resistors R 21 to R 33. In the attenuating units 4 to 7 shown in FIG. 3, the input impedance can be set to a substantially constant value regardless of which of the switches S3 to S6 is turned on by appropriately setting the resistance values of the resistors R21 to R33. it can.

  FIG. 4 is a circuit diagram showing a specific configuration of the variable gain amplifier. The attenuating section 2 is composed of resistors R201 to R208, and the switch S1 is composed of switches S11 to S18. If only the switch S11 is turned on, 0 dB is obtained. If only the switch S12 is turned on, only -1 dB and S13 are produced. If turned on, −2 dB,..., If only S18 is turned on, an attenuation of −7 dB can be realized.

  The attenuating section 3 is composed of resistors R301 to R308, and the switch S2 is composed of switches S21 to S28. If only the switch S21 is turned on, 0 dB is obtained. If only the switch S22 is turned on, only -1 dB and S23 are produced. If turned on, −2 dB,..., If only S28 is turned on, an attenuation of −7 dB can be realized. That is, the attenuation unit 3 and the switch S2 have the same configuration as the attenuation unit 2 and the switch S1.

  The attenuation units 4 to 7 employ the configuration shown in FIG. 3. For example, the attenuation unit 4 has −10 dB, the attenuation unit 5 has −8 dB, the attenuation unit 6 has −6 dB, and the attenuation unit 7 has an attenuation of −4 dB. realizable. The switching unit 8 can realize an attenuation of −6 dB as described above in any combination of the switch S7 and the switches S3 to S6.

  From the above, in the variable gain amplifier of FIG. 4, the attenuation unit 2 can realize eight types of attenuation by the switches S11 to S18. Since 8 × 4 = 32 types of attenuation can be realized by the combination of ˜S6, a total of 40 types of attenuation can be realized. For this purpose, 21 switches are necessary. That is, it is possible to realize an attenuation amount twice as many as the number of switches (amplification degree in terms of a variable gain amplifier). On the other hand, in FIG. 5 of the related art, 24 switches are required to realize 24 types of attenuation, and the number of switches and types of attenuation remain the same.

  In the variable gain amplifier of FIG. 4, the switches S3 to S7 used in the portion not on the input side of the operational amplifier 9 are set to the on-resistance having the same resistance value and the resistors R1 to R5 as described above. By making the same resistance value, the on-resistance can be canceled. If the on-resistance exists in the switch, the amount of attenuation will deviate from the design value and the accuracy will decrease. Therefore, in order to reduce the on-resistance, it is necessary to increase the size of the MOS transistor that constitutes the switch. Although the circuit scale increases, this embodiment can solve this problem.

  Further, since only one of the switches S11 to S18 and the switches S21 to S28 can be controlled to be turned on, these switches are constituted by MOS transistors, and the switches S11 and S21, S12 and S22,. And S28 can be driven by the same drive circuit, and the number of drive circuits can be reduced.

It is a basic block diagram of the variable gain amplifier of the present invention. FIG. 7 is a circuit diagram of specific circuits of the attenuation units 4 to 7 and the switching unit 8 of FIG. FIG. 8 is a circuit diagram of a specific circuit of another example of the attenuation units 4 to 7 and the switching unit 8 in FIG. 1. It is a circuit diagram of a specific variable gain amplifier of the present invention. It is a circuit diagram of a conventional variable gain amplifier.

Explanation of symbols

1: Signal input terminal 2-7: Attenuator 8: Switching unit 9: Operational amplifier 10: Signal output terminal 11-13: Attenuator

Claims (2)

The non-inverting input terminal of the operational amplifier is connected to the signal input terminal, the output terminal of the operational amplifier is connected to the signal output terminal, and a feedback circuit is connected between the output terminal and the inverting input terminal of the operational amplifier. In the variable gain amplifier, the feedback circuit includes:
First to Nth (N ≧ 2) attenuation units connected in series to the output terminal of the operational amplifier;
N series circuits including one switch having one end connected to the output side of each of the first to Nth attenuation units and one resistor, and the other end of the N series circuits and the ground A switching unit constituted by one series circuit composed of one switch and one resistor connected therebetween,
An N + 1th attenuation unit connected to a common connection point of the N series circuits and the one series circuit of the switching unit;
M switches for selecting the attenuation amount of the (N + 1) th attenuation unit,
A terminal opposite to the (N + 1) -th attenuation unit in the M switches is connected to the inverting input terminal of the operational amplifier;
Each switch of the N series circuits and the switch of the one series circuit have the same on-resistance, and each resistance of the N series circuits and the resistance of the one series circuit have the same resistance value. A variable gain amplifier. The variable gain amplifier according to claim 1,
An N + 2th attenuation unit connected to the output terminal of the operational amplifier;
L (= M) switches for selecting the attenuation amount of the N + 2 attenuation unit,
A variable gain amplifier, wherein a terminal opposite to the N + 2 attenuation section in the L switches is connected to the inverting input terminal of the operational amplifier.

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