JPH04357706A - Variable gain amplifier - Google Patents

Abstract

PURPOSE:To adopt simple structure for a part varying a gain with respect to the variable gain amplifier used for an input stage of an A/D converter so as to prevent overflow. CONSTITUTION:The variable gain amplifier provided with an amplifier 1 and decreasing the gain through feedback when an output voltage exceeds a prescribed value so as to keep the output voltage constant is provided with a silicon diode 2 whose anode connects to an output of the amplifier 1, a voltage latch section 3 connecting to a cathode of the silicon diode 2 and a gain variable section 4 to decrease the gain of the amplifier 1 in response to a voltage in excess of a threshold voltage of the silicon diode 2.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a variable gain amplifier used in the input stage of an analog-to-digital converter to prevent overflow. The present invention particularly aims to simplify the structure of the portion that makes the gain variable.

0002

2. Description of the Related Art FIG. 5 is a diagram showing a conventional variable gain amplifier. The variable gain amplifier in this figure includes an amplifier 1 and an amplifier 1.
a rectifying unit 20 that rectifies the feedback output of and obtains an output average value;
The voltage value obtained from the rectifier 20 is the target value (for example, 1V)
Threshold part 2 that monitors whether the value exceeds
1, and a voltage control attenuation section 22, which is provided at the input stage of the amplifier 1 and is configured of, for example, a volume limiter, which throttles the input signal using a threshold section 21. Note that the same applies even if the order of arrangement of the rectifying section 20 and the threshold section 21 is reversed.

The rectifying section 20 used in the variable gain amplifier uses a silicon diode, which lowers the rectification sensitivity due to the forward voltage drop. Conventionally, germanium diodes, which have a small forward voltage drop, have been used. FIG. 6 is a diagram showing an example of the rectifying section in FIG. 5. As shown in this figure (a), an ideal diode using an operational amplifier may be used, and as shown in this figure (b), the diode has an input/output characteristic of an output signal e0 with respect to an input signal ei. As shown in Figure (c), the input signal ei is rectified to obtain the output signal ei.

FIG. 7 is a diagram showing the input/output characteristics of a variable gain amplifier. As shown in this figure, if the input signal is less than approximately 1V, the output signal is proportional to the input signal (proportionality constant = 1
) When the input signal attempts to exceed approximately 1V, the rectifier 20,
The input signal is attenuated by the threshold section 21 and the voltage controlled attenuation section 22, and the output signal is suppressed to 1V or less. In this way, for example, in a digital signal processing circuit for audio signals, a variable gain amplifier is provided before an analog-to-digital converter to prevent overflow of the analog-to-digital converter.

[0005]

[Problems to be Solved by the Invention] However, in conventional variable gain amplifiers, a rectifier section, a threshold section, and a voltage control attenuation section are required to vary the gain of the amplifier, resulting in a complicated circuit configuration. was there. Therefore, in view of the above problems, it is an object of the present invention to provide a variable gain amplifier with a simplified circuit configuration.

[0006]

[Means for Solving the Problems] FIG. 1 is an equivalent circuit diagram showing the principle configuration of the present invention. In order to solve the above-mentioned problems, the present invention provides a variable gain amplifier which includes an amplifier 1 and which lowers the gain by feedback to keep the output voltage constant when the output voltage exceeds a certain value. 3 and a gain variable section 4 are provided.

The anode of the silicon diode 2 is connected to the output of the amplifier 1. Voltage holding section 3 is connected to the cathode of silicon diode 2 . The variable gain section 4 reduces the gain of the amplifier 1 according to the amount exceeding the threshold voltage of the silicon diode 2.

[0008]

[Operation] According to the variable gain amplifier shown in FIG. 1, when the output of the amplifier 1 exceeds the forward drop voltage (approximately 0.7V) of the silicon diode 2, the portion beyond which the output voltage exceeds the forward voltage drop of the silicon diode 2 is a capacitor. Since the gain of the amplifier 1 is reduced by the variable gain section 4, which is charged by the voltage holding section 3 and is formed of a transistor, the configuration of the variable gain amplifier is simplified.

[0009]

[Examples] Examples of the present invention will be described below. FIG. 2 is a diagram showing a variable gain amplifier according to an embodiment of the present invention. The configuration of this figure will be explained. The variable gain amplifier in this figure is a non-inverting operational amplifier 11 to which a power supply voltage Vcc is supplied;
A resistor 12 is fed back from the output of the operational amplifier 11 and connected to the inverting input terminal of the operational amplifier, one of which is connected to the operational amplifier 1.
an amplifier 1 consisting of a resistor 13 connected to an inverting input terminal of the amplifier 1; a capacitor 14 having one end connected to the other end of the resistor 13 and the other end grounded; and a silicon diode 2;
A voltage holding section 3 consisting of a capacitor 32, one of which is connected to the cathode of the silicon diode 2 and the other grounded, and a resistor 31 connected in parallel to the capacitor 32.
and a resistor 41, one of which is connected to the cathode of the silicon diode 2, an NPN transistor 42 whose base is connected to the other of the resistor 41, whose base is connected to the emitter of the transistor 41, and whose emitter is grounded. a gain variable section 4 consisting of an npn type transistor 43 and a resistor 44, one of which is connected to the collector of the transistor 42 and the other connected to the voltage power supply Vcc;
A resistor 51, one of which receives an input signal, and a resistor 52, one of which is connected to the other of the resistor 51 and the other is grounded.
and a resistor 53, one of which is connected to the other of the resistor 51, and the other of which is connected to the collector of the transistor 43,
A capacitor 5 whose one end is connected to the other end of the resistor 53 and the other end connected to the non-inverting input terminal of the operational amplifier 11
4, a capacitor 55, one of which is connected near the voltage power supply Vcc and the other grounded, and a capacitor 56, one of which is connected near the voltage power supply Vcc that supplies the operational amplifier, and the other grounded. A resistor 57 whose one end is connected to the voltage power supply Vcc, a resistor 58 whose one end is connected to the resistor 57 and whose other end is grounded, and a capacitor whose one end is connected to the other end of the resistor 57 and whose other end is grounded. 59, a resistor 60, one of which is connected to the other of the resistor 57 and the other to the non-inverting input terminal of the operational amplifier 11, a capacitor 61, one of which is connected to the output of the operational amplifier 11, and one of which is connected to the non-inverting input terminal of the operational amplifier 11; a resistor 62, one of which is connected to the other of the resistor 62, and the other of which is connected to the anode of the silicon diode 2; a capacitor 63, whose cathode is connected to the anode of the silicon diode 2; A silicon diode 64 whose cathode is grounded, a resistor 65 whose one end is connected to the other end of the capacitor 61 and whose other end serves as an output, and a resistor 66 whose one end is connected to the other end of the resistor 65 and whose other end is grounded. including.

FIG. 3 is a diagram showing forward voltage drop characteristics of a silicon diode. As shown in this figure (a), the forward drop voltage Vf of the silicon diodes 2 and 64 is approximately 0.7.
When one of the resistors is connected to the cathode of the silicon diode and the other resistor is grounded as shown in this figure (b), and a voltage V is applied to the anode of the silicon diode, the current flowing through the resistor I is I if the resistance value is RL
=(V-Vf)/RL.

Next, the operation of this embodiment will be explained. FIG. 4 shows the input/output characteristics of the variable gain amplifier according to the embodiment of the present invention. From the equivalent circuit of the operational amplifier 11, the gain G is the resistance 12
, 13, 51, 52, 53, 60 as R1
2, R13, R51, R52, R53, R6
0, input signal υi and output signal υ0, then G=υ0 /υi = (R12+R13) /R13×R52/(R51+
R52) ×R60/(R53+R60), R12=47KΩ, R13=1KΩ, R51=
10KΩ, R52=220Ω, R53=2.2K
Ω, R60=100 Substituting KΩ into the above formula, G=(47+1)/1×0.22/(10+0.22)
×100/(100+2.2)≒1.0.

Next, if the internal resistance of the transistor 43 is r, the equivalent circuit G(r) of the operational amplifier 11 is G
(r) = (R12+R13) /R13×R52/(
R51+R52) ×R(R+2.2)
≒R/(R+2.2)=1/(1+2.2/R) where R=r×R60/(r+R60) =r×100
/(r+100).

As shown in FIG. 4, when the input signal υi is smaller than the forward drop voltage Vf of the silicon diode 2, the output of the operational amplifier 11 is divided by resistors 65 and 66 to form an output signal υ0. input signal υi
When Vf becomes larger than the forward voltage drop Vf of the silicon diode 2, a current flows through the resistor 62, the capacitor 63, the silicon diode 2, and the capacitor 32. In this case, a delay time is generated by the resistor 62 and the capacitor 63 to alleviate sudden changes in gain and prevent abnormal noise from occurring in the subsequent stage. When the capacitor 32 is charged, current flows through the Darlington-connected transistors 41 and 42, and the internal resistance r between the collector and emitter of the transistor 43 becomes small. As the input signal υi becomes larger than the forward voltage drop υf of the silicon diode 2, the internal resistance r of the transistor 43 becomes smaller, and accordingly, the combined resistance R with the resistor 60 in the above equation becomes smaller. The gain G(r) of the equation becomes smaller. Therefore, as shown in FIG. 4, when the input voltage υi exceeds 1V, the output voltage υ0 will be saturated at about 0.75V.

Note that if the voltage charged in the capacitor 63 remains after that, the output voltage υ0 will be about 0.75V.
This will cause saturation, so please use a silicon diode 64
The capacitor 63 is discharged via. Furthermore, although the transistors 42 and 43 are Darlington-connected in this embodiment, the same objective can be achieved even if a single transistor is used. The diodes used in the rectifier section of conventional variable gain amplifiers require linear input/output characteristics, but the diodes used in this example do not require such linearity, but rather have non-linear characteristics. The gain can be easily varied by utilizing the forward voltage drop.

[0015]

[Effects of the Invention] As explained above, according to the present invention, the gain of the operational amplifier is made variable using the forward voltage drop of the silicon diode, so overflow of the analog-to-digital converter can be easily prevented. become able to.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the principle configuration of the present invention.

FIG. 2 is a diagram showing a variable gain amplifier according to an embodiment of the present invention.

FIG. 3 is a diagram showing forward voltage drop of a silicon diode.

FIG. 4 is a diagram showing input/output characteristics of a variable gain amplifier according to an embodiment of the present invention.

FIG. 5 is a diagram showing a conventional variable gain amplifier.

FIG. 6 is a diagram showing an example of the rectifying section in FIG. 5;

FIG. 7 is a diagram showing input/output characteristics of a variable gain amplifier.

[Explanation of symbols] 1...Amplifier 2...Silicon diode 3...Voltage holding part 4...Variable gain section

Claims (1)

[Claims] 1. A variable gain amplifier comprising an amplifier (1), which reduces the gain by feedback to maintain the output voltage constant when the output voltage exceeds a certain value, the anode of which is connected to the output of the amplifier (1). a silicon diode (2) and a voltage holding part (3) connected to the cathode of the silicon diode (2).
and a variable gain section (4) that reduces the gain of the amplifier (1) according to the amount exceeding the threshold voltage of the silicon diode (2).