JPH04237204A - Amplifier circuit - Google Patents

Abstract

PURPOSE:To implement the frequency characteristic compensation of a sound signal reproduced by a magnetic head without use of an electrolytic capacitor. CONSTITUTION:An operational amplifier 1 is formed to have a high output impedance being a multiple of (k) of a resistance R1. Thus, a filter circuit 2 having a low pass filter characteristic is formed by a capacitor C1 with a small capacitance of 1[muF] or less. An operational amplifier 4 applies high frequency characteristic compensation to a reproduction signal of a magnetic head 3 and the filter circuit 2 applies a low frequency characteristic compensation to the signal. Moreover, an output of a buffer amplifier comprising an operational amplifier 5 is applied with DC negative feedback through a resistor R2 so as to reduce an output offset voltage.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an amplifier circuit, and more particularly to an amplifier circuit used for preamplifying audio signals in tape recorders, video tape recorders, etc.

0002

2. Description of the Related Art Conventionally, the following circuits are generally known as amplifier circuits for amplifying audio signals output from a magnetic head of a tape recorder or the like. FIG. 5 is a circuit diagram of an example of a conventional amplifier circuit. In the figure, 3 is a magnetic head;
4 designates operational amplifiers each consisting of a differential amplifier. The magnetic head 3 has one end connected to ground and the other end connected to a non-inverting input terminal of a first operational amplifier (hereinafter abbreviated as operational amplifier) 4. The magnetic head 3 reproduces and outputs an audio signal recorded on a magnetic tape (not shown).

Resistors R4 and R5 are connected in series, and one end of resistor R4 is connected to the inverting input terminal of operational amplifier 4.
One end of the resistor R5 is connected to the output terminal of the operational amplifier 4, respectively. A capacitor C2 is connected to both ends of the resistor R5.

One end of the resistor R3 is connected to the inverting input terminal of the operational amplifier 4, and the other end is connected to the positive terminal of the electrolytic capacitor C4. One terminal of the electrolytic capacitor C4 is connected to ground. Further, a resistor R16 is connected to the connection point between the resistor R3 and the capacitor C4 and the output terminal of the operational amplifier 4.

The high frequency characteristics of the output signal of the magnetic head 3 are compensated by the time constant circuit consisting of the resistors R4 and R5 and the capacitor C2. Further, a time constant circuit consisting of a resistor R3 and a capacitor C4 also compensates for low frequency characteristics. The resistor R16 reduces the offset voltage caused by the bias current of the operational amplifier 4 by direct current feedback. Capacitor C4
This also prevents fluctuations in the output voltage due to the offset voltage of the operational amplifier 4.

Since the operational amplifier 4 is generally set to a high gain of about 70 dB, the resistance R3 has a value of several tens of Ω. Therefore, capacitor C4 has a frequency of 100 [Hz]
100-200 to achieve the desired low frequency compensation below.
[μF] electrolytic capacitors are commonly used.

[0007]

However, according to the amplifier circuit having the above configuration, an electrolytic capacitor having a capacitance of 100 [μF] or more must be used. 100 [μF]
The above electrolytic capacitors generally have a diameter of about 6.3 [mm]
The height of the tape recorder when mounted on a printed circuit board is approximately 5.5 mm or more, which significantly impedes the miniaturization of tape recorders and the like, particularly the slimming down of tape recorders and the like.

The present invention has been made in view of the above-mentioned drawbacks, and provides an amplifier that can compensate for desired frequency characteristics without using electrolytic capacitors, and can make tape retarders etc. thinner. The purpose is to provide circuits.

[0009]

[Means for Solving the Problems] In order to solve the above problems, the present invention includes a first operational amplifier for amplifying a reproduced audio signal, and a time constant circuit connected to a feedback path of the first operational amplifier. an amplifier circuit comprising: a second operational amplifier connected to the output terminal of the first operational amplifier and configured to have a high output impedance; and an output impedance setting connected to the input terminal of the second operational amplifier. a filter circuit that selects a low frequency component, and includes a resistor and a capacitor connected to the output terminal of the second operational amplifier;
A buffer amplifier having a third operational amplifier connected to the output terminal of the filter circuit, and a feedback resistor connected to the output terminal of the buffer amplifier and the feedback input terminal of the first operational amplifier were provided.

0010

[Operation] The reproduced audio signal is compensated for its high frequency characteristics by the first operational amplifier.

The filter circuit includes the high output impedance set by the output impedance setting resistor and the capacitor. The filter circuit compensates for low frequency characteristics of the reproduced audio signal.

The output signal of the buffer amplifier is DC-feedbacked to the input terminal of the first operational amplifier circuit by the feedback resistor.

[0013]

[Example] Next, an example of the present invention will be described. FIG. 1 is a circuit diagram showing the configuration of an embodiment of the present invention. In the figure, the same components as those in FIG. 5 are denoted by the same reference numerals, and the explanation thereof will be omitted as appropriate. In FIG. 1, numerals 1 and 5 respectively indicate second and third operational amplifiers (hereinafter abbreviated as operational amplifiers) each comprising a differential amplifier.

Resistor R3 that sets the gain of operational amplifier 4
has one end connected to the inverting input terminal of the operational amplifier 4, and the other end connected to ground. The output terminal of the operational amplifier 4 is connected to the non-inverting input terminal of the operational amplifier 1.

A resistor R1, which is an output impedance setting resistor, is connected to the inverting input terminal of the operational amplifier 1.
The other end of resistor R1 is connected to ground. The output impedance of the operational amplifier 1 is set to a high impedance, and the impedance value can be set by a resistor R1. A detailed explanation of this configuration will be given later. A capacitor C1 is connected to the output terminal of the operational amplifier 1, and the other end of the capacitor C1 is connected to ground. This constitutes a filter circuit 2.

The output terminal of the filter circuit 2 is connected to the non-inverting input terminal of the operational amplifier 5. Resistors R6 and R7 are connected to the inverting input terminal of the operational amplifier 5. The other end of resistor R6 is grounded, and the other end of resistor R7 is
They are respectively connected to the output terminals of the operational amplifier 5, thereby forming a buffer amplifier 6.

A resistor R is connected to the output terminal of the buffer amplifier 6.
2 is connected. The other end of the resistor R2 is connected to the inverting input terminal of the operational amplifier 4. The output impedance of the filter circuit 2 configured with high impedance is
The buffer amplifier 6 converts the impedance into low impedance, and direct current feedback is applied via the resistor R2.

In the above configuration, the resistors R2, R6, R7
The amount of DC negative feedback is adjusted according to the value of to reduce the output offset voltage. Further, high frequency compensation is performed by resistors R4, R5 and capacitor C2 as before, and low frequency compensation is performed by filter circuit 2.

Next, a specific example of the configuration of this filter circuit 2 will be explained with reference to the circuit diagram of FIG. 2. In the figure, the same components as those in FIG. 1 are denoted by the same reference numerals, and the explanation thereof will be omitted. In FIG. 2, 1 is an operational amplifier, and a specific circuit diagram thereof is shown. Power terminals 7 and 8 each have
Power supply terminals 7 and 8 are connected to operational amplifiers 1, 4, and 5, respectively, which are connected to a positive power supply voltage VCC and a negative power supply voltage -VEE, and are supplied with power supply voltages VCC and -VEE.

The NPN transistor Q12 is diode-connected, with its emitter connected to the power supply voltage -VEE and its collector connected to the resistor R9 and the base of the NPN transistor Q11. The other end of resistor R9 is the power supply voltage VCC
It is connected to the. The collector of transistor Q11 is N
The common emitters of the PN-type differential pair transistors Q1 and Q2 are connected to a resistor R8, respectively. The other end of resistor R8 is connected to power supply voltage -VEE.

The collectors of the differential pair transistors Q1 and Q2 are connected to the collectors of a diode-connected PNP transistor Q3 and a PNP transistor Q5, respectively. The emitters of transistors Q3 and Q5 are connected to power supply voltage VCC.

A PNP type transistor Q9, which is an output transistor, is connected to the transistor Q3 in a current mirror connection. Transistor Q3 and transistor Q9
The current ratio is set to k1 times by the emitter area ratio of each transistor. The emitter of transistor Q9 is connected to power supply voltage VCC. Further, the collector of transistor Q9 is connected to the collector of NPN type transistor Q10.

PNP type transistor Q4 is connected to transistor Q3 in a current mirror connection. The current ratio between transistor Q3 and transistor Q4 is set to k2 times by the emitter area ratio of each transistor, and the value is selected so that k1 < k2. The emitter of transistor Q4 is connected to power supply voltage VCC. Further, the collector of the transistor Q4 is connected to the base of the differential transistor Q2 and the collector of the NPN transistor Q7.

PNP type transistor Q6 is connected to transistor Q5 in a current mirror connection. The current ratio between transistor Q5 and transistor Q4 is set to k2 times by the emitter area ratio of each transistor. The emitter of transistor Q6 is connected to the power supply voltage VC
Connected to C. Further, the collector of the transistor Q6 is connected to the collector of the NPN transistor Q8.

Transistor Q8 is diode-connected, and the emitter of transistor Q8 is connected to the power supply voltage -V.
Connected to EE. Further, the base of transistor Q8 is connected to the bases of transistor Q7 and transistor Q10, respectively. Transistor Q7,
The emitter of each transistor Q10 is connected to the power supply voltage -VE
Connected to E.

The output terminal of the operational amplifier 4 is connected to the base of the differential transistor Q1, which serves as the non-inverting input terminal of the operational amplifier 1. A resistor R1 is connected to the base of the differential transistor Q2, which serves as an inverting input terminal. A capacitor C1 is connected to the collector of a transistor Q9 which serves as an output terminal. The resistor R1 and capacitor C1 are each connected to ground. Also,
The collector of transistor Q9 is connected to the non-inverting input terminal of operational amplifier 5 constituting a buffer amplifier.

Next, the filter circuit 2 configured as described above will be explained in detail. In the figure, resistance R1
Assume that the current flowing through is i, and the base voltages of differential transistors Q1 and Q2 are V1 and V2, respectively.

[0028] If the offset voltage is 0, V1, V
2 relationship is V1 = V2 = R1 i (
1) Also, the output impedance ROUT at the collector of the transistor Q9 is determined by the current ratio k1, k2 of each current mirror circuit, as follows:

[0029]

[Math 1]

It is expressed as: As an example, R1 = 1 [
MΩ], if you choose the current ratio k1 = 0.5, k2 = 5, then

[0031]

[Math 2]

[0032] In this way, the output impedance ROUT of the operational amplifier 1 is changed to the resistance R1
It can be k2/k1 times the value of. Therefore, the capacitor C1 has a small capacity and can constitute a low-pass filter circuit having a low-frequency time constant of 100 [Hz] or less.

Note that the voltage gain of the operational amplifier 1 is, for example, 1
The total gain is adjusted using resistors R2 and R5, and the total gain is adjusted using resistors R2 and R5 and the operational amplifier 5, so that the overall operation is stable and the desired frequency characteristics are achieved. Adjust as desired.

For example, set ROUT = 10 [MΩ], C1 = 1 [μF], C2 = 0.033 [μF]
, R3 = 50 [Ω], R4 = 4 [kΩ], R5 =
120 [kΩ], R6 = 33 [kΩ], R7 = 10
The results of a computer simulation with a selected value of R2 when 0 [kΩ] are shown below.

FIG. 3 is a diagram showing the results of a computer simulation of the frequency characteristics of an embodiment of the present invention. In the figure, the horizontal axis shows frequency on a logarithmic scale, and the vertical axis shows gain on a linear scale. In the figure, the curve indicated by I shows the frequency characteristics of the amplifier circuit when the feedback resistance R2 = 1 [kΩ]. Similarly, II, III, IV
The respective curves shown are R2 = 750, 500, 250
It shows the frequency characteristics when [Ω]. As the respective curves show, ROUT = 10 [MΩ], C1 = 1 [
A frequency characteristic with a low frequency time constant of 3180 [μsec] or less is obtained.

As explained above, according to this embodiment, by setting the output impedance of the operational amplifier 1 high, desired frequency characteristics can be achieved using the capacitor C1 of 1 [μF] or less. For this reason,
Without using conventional electrolytic capacitors,
It becomes possible to construct an amplifier circuit using a ceramic chip capacitor, a resistor, and a semiconductor.

Therefore, by converting all circuit components into chip components, the amplifier circuit board can be made smaller and thinner. Therefore, tape recorders and the like can be made smaller and thinner. In addition, there are several chip capacitors [
μF] to several tens of μF can be used, but by reducing this value to 1 μF or less, it is possible to reduce component costs.

Next, FIG. 4 is a circuit diagram showing the configuration of another embodiment of the present invention. In the figure, the same components as those in FIG. 1 are denoted by the same reference numerals, and the explanation thereof will be omitted. In FIG. 4, 4a and 5a each indicate an operational amplifier. The output terminal of the magnetic head 3 is connected to the inverting input terminal of the operational amplifier 4a. Also, for the non-inverting input terminal and output terminal,
A time constant circuit is connected.

Output terminal of operational amplifier 1 and operational amplifier 5a
A resistor R11 is connected to the inverting input terminal of. A resistor R12 and a capacitor C3 are connected to the output terminal of the operational amplifier 1 and the output terminal of the operational amplifier 5a. In addition, a resistor R10 is connected to the non-inverting input terminal of the operational amplifier 5a.
is connected. The other end of resistor R10 is connected to ground. The DC feedback resistor R2 is connected to the output terminal of the operational amplifier 5a and the inverting input terminal of the operational amplifier 4a.

In the above embodiment, the operational amplifier 5a,
A low-pass filter circuit is constituted by a buffer circuit consisting of resistors R10, R11, R12 and a capacitor C3, and an operational amplifier 1.

[0041]

[Effects of the Invention] As described above, according to the present invention, an amplifier circuit with desired frequency characteristics can be constructed using small chip capacitors without using electrolytic capacitors, so tape recorders, etc. can be made smaller and thinner. It has the advantage of being able to do

[Brief explanation of the drawing]

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

FIG. 2 is a circuit diagram showing a specific example of the configuration of a filter circuit according to an embodiment of the present invention.

FIG. 3 is a diagram showing computer simulation results of frequency characteristics according to an embodiment of the present invention.

FIG. 4 is a circuit diagram showing the configuration of another embodiment of the present invention.

FIG. 5 is a circuit diagram of an example of a conventional amplifier circuit.

[Explanation of symbols]

1 Second operational amplifier 2 Filter circuit 5 Third operational amplifier 6 Buffer amplifier R1 Impedance setting resistor R2 Feedback resistance C1, C3 capacitor

Claims (2)

[Claims] 1. An amplifier circuit that amplifies and imparts predetermined frequency characteristics to a reproduced signal, comprising: a filter circuit that selects a low frequency component of an output signal from an output terminal; and a buffer that buffers and amplifies the output signal of the filter circuit. An amplifier circuit comprising an amplifier and a feedback resistor that supplies an output signal of the buffer amplifier to an input terminal of the amplifier circuit. 2. In an amplifier circuit comprising a first operational amplifier for amplifying a reproduced audio signal and a time constant circuit connected to a feedback path of the first operational amplifier, an output terminal of the first operational amplifier a second operational amplifier connected to and configured to have a high output impedance, an output impedance setting resistor connected to the input terminal of the second operational amplifier, and a resistor connected to the output terminal of the second operational amplifier. a filter circuit that selects low frequency components, consisting of a capacitor;
A buffer amplifier having a third operational amplifier connected to the output terminal of the filter circuit, and a feedback resistor connected to the output terminal of the buffer amplifier and the feedback input terminal of the first operational amplifier. An amplifier circuit characterized by: