JP2557398B2 - Amplifier circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Object of the Invention (Industrial field of application) The present invention relates to an amplifier circuit suitable for use in, for example, a reproducing first-stage amplifier of a tape recorder having a recording / reproducing head. The present invention relates to a device in which adverse effects due to the temperature change are improved.

(Prior Art) As is well known, for example, in a tape recorder or the like, the same head that is used for both recording and reproduction has one end of the head 11 grounded and the other end of the head 11 as shown in FIG. Is connected to the input end of the reproducing first-stage amplifier 12 and the output end of the recording amplifier 13.

At the time of reproduction, the reproduction signal obtained from the head 11 is amplified by the reproduction first-stage amplifier 12 and then guided to the reproduction circuit system (not shown) via the output terminal 14. At the time of recording, the reproducing first-stage amplifier 12 is set in a non-operating state to increase its input impedance, and the recording signal supplied to the input terminal 15 is amplified by the recording amplifier 13 and guided to the head 11.

FIG. 4 shows a specific structure of the reproducing first-stage amplifier 12. Reference numeral 16 is an input terminal to which a reproduction signal obtained from the head 11 is supplied. This input terminal
16 is connected to the base of an NPN type transistor Q1 via a coupling capacitor C1. Also, the connection point between the capacitor C1 and the base of the transistor Q1 is a resistor
It is connected to the power supply terminal 17 to which the reference voltage V1 is applied via R1, and is connected to the power supply terminal 18 to which the reference voltage V2 is applied via a resistor R2.

The transistor Q1 has an emitter connected to the power supply terminal 18 via a constant current source 19 and a bypass capacitor C2 in parallel, a collector connected to the power supply terminal 17 via a resistor R3, and an NPN transistor. It is connected to the emitter of Q2.

The base of the transistor Q2 is connected to the power supply terminal 17 via the resistor R4 and is connected to the power supply terminal 18 via the constant current source 20 to form a cascode connection. Also, the collector of the transistor Q2 is the output terminal
21 as well as to the power supply terminal 17 via the resistor R5.

The resistors R3 to R5 are set so that the temperature coefficients thereof are substantially equal to each other, and the constant current sources 19 and 20 are also set to have substantially the same temperature coefficient.

The reproduction signal supplied to the input terminal 16 has a direct current component cut off by the capacitor C1, and then a direct current bias obtained by dividing the differential voltage between the reference voltages V1 and V2 by the resistors R1 and R2 is applied to the reproduced signal, and the transistor Q1 Supplied to the base of. Therefore, the amplified output is taken out from the collector of the transistor Q2 via the output terminal 21.

In order to obtain a high gain in the amplifier circuit as described above, load resistor RL (here resistor R5) is used as transistor Q.
It may be set sufficiently larger than the internal resistance re of 1 (the reciprocal of the mutual conductance gm of the transistor Q1). Then, in order to obtain R5 >> re, the current is shunted to the resistor R3, and the value of the resistor R5 is set large.

In other words, the gain Gu of the amplifier circuit shown in FIG. 4 is Gu = R5 / (I / VT), where: I: output current of constant current source 19 VT; thermal voltage of transistor Q1 In order to reduce the value of re, the output current I of the constant current source 19 is increased and the gain Gu is increased. However, the voltage drop of the resistor R5 increases as it is and the operating power supply voltage range is increased. Since it becomes narrower, the current is shunted to the resistor R3 to suppress the voltage drop at the resistor R5.

FIG. 5 shows a modification of the amplifier circuit shown in FIG. 4 as a differential amplifier circuit configuration. Connect the NPN type transistor Q3 and the transistor Q1 in common
The output signal of 22 is DC biased with the reference voltage V3 applied to the power supply terminal 23.

In each of the amplifier circuits as described above, while a high gain amplification factor can be obtained, the current flowing through the resistor R3 depends on the base-emitter voltage Vbe (Q1) of the transistor Q2, and the constant current source 19,20. It will not correspond to the temperature counting of. Therefore, the current flowing through the resistor R5 is added to the current flowing through the resistor R3, and has a temperature coefficient corrected to have the same current value as the output current of the constant current source 19.

Temperature change of collector potential of transistor Q2 (V / ℃)
To widen the temperature range in which the transistor Q2 can actively operate by
This is extremely difficult, and there arises a problem that the temperature range in which the amplifier circuit can stably operate cannot be expanded.

As described above, at the time of recording, the amplifier circuit is in a non-operating state to set its input impedance high, but a large amplitude appears in the output of the recording amplifier 13, and the amplitude is at the base of the transistor Q1. When the collector junction is forward biased, the resistor R3 causes the amplifier circuit, that is, the reproduction first-stage amplifier 12
The input impedance of the head 11
There is also a problem that the current flowing through the memory is reduced and normal recording cannot be performed.

(Problems to be Solved by the Invention) As described above, in the conventional amplifier circuit, the temperature range in which stable operation can be performed is extremely narrow, and the recording / reproducing system having the recording / reproducing head is used for reproducing. When used as the first-stage amplifier, there is a problem that the input impedance becomes low depending on the amplitude of the recording signal and the normal recording operation cannot be performed.

This invention eliminates the above-mentioned problems and enables stable operation in a wide temperature range, and when used as a reproducing first-stage amplifier of a recording / reproducing system having a recording / reproducing head, It is an object of the present invention to provide an extremely good amplifier circuit capable of keeping the input impedance high regardless of the magnitude of the amplitude.

[Configuration of the Invention] (Means for Solving the Problems) That is, in the amplifier circuit according to the present invention, the input signal is supplied to the base, and the emitter is connected to the first reference potential point via the first constant current source. A first transistor connected to and a cascode connection with the first transistor,
The emitter is connected to the collector of the first transistor,
The base is connected to the second reference potential point via the first resistor and is connected to the first reference potential point via the second constant current source, and the collector is connected to the second reference potential point via the second resistor. A second transistor connected to the reference potential point of
The connection point between the collector of the transistor and the emitter of the second transistor is connected to the second reference potential point via the third resistor, and the temperature coefficients of the first to third resistors are substantially equal to each other, and The target is one in which the temperature coefficients of the first and second constant current sources are set to be substantially equal. Then, a diode is connected in series to the third resistor with a polarity such that the first and second transistors are forward biased, and the base potential and collector potential of the second transistor are set to be equal at a predetermined temperature. It was done.

(Operation) According to the above configuration, the diode is connected in series to the third resistor with the polarity such that the first and second transistors are forward biased, and the base potential of the second transistor at a predetermined temperature. Since the collector potential is set to be equal to the collector potential, the base potential and the collector potential of the second transistor are changed to be equal to each other even if the temperature is changed, so that stable operation is possible in a wide temperature range. It will be.

In addition, the base-collector junction of the first transistor is not biased in the forward direction even if any signal is supplied to the base, and the input impedance is always kept at a high impedance. It is suitable for use as a first-stage amplifier for reproduction of the recording / reproduction system that it has.

(Embodiment) Hereinafter, one embodiment of the present invention will be described in detail with reference to the drawings. In FIG. 1, the same parts as those in FIG. 4 are designated by the same reference numerals, and only different parts will be described here. That is, a resistor D3 is connected in series with a diode D1 with a polarity such that the transistors Q1 and Q2 are forward biased, and the base potential and the collector potential of the transistor Q2 are set to be equal at a predetermined temperature. It is a different point.

According to such a configuration, the potential of the emitter of the transistor Q2 is determined by the voltage drop of the resistor R4 and the base-emitter voltage Vbe (Q1) of the transistor Q2.
By connecting the diode D1, the current flowing through the resistor R3 has a temperature coefficient equal to that of the constant current sources 19 and 20. Therefore, the current flowing through the resistor R5 is
Furthermore, it has the same temperature coefficient as the current flowing through the resistor R3.

Therefore, if the base potential and the collector potential of the transistor Q2 are set to be equal at a predetermined temperature, both potentials change so that they become equal even if the temperature changes thereafter. The temperature range in which the amplifier circuit can operate is extended, and the amplifier circuit can stably operate in a wide temperature range.

Further, the base-collector junction of the transistor Q1 is not biased in the forward direction even if any signal is supplied to the base, and the resistor R3 does not appear as the input impedance of the amplifier circuit. Therefore, since the input impedance is always kept at a high impedance, as a recording / reproducing system having a recording / reproducing head, such as a digital audio tape recorder,
When used in the first stage amplifier for reproduction, it is suitable because it does not interfere with normal recording operation.

FIG. 2 shows an example of the case where the present invention is applied to the amplifier circuit of FIG. 5 described above, and the diode D1 connected in series with the resistor R3 has the same operation as the above embodiment.

The present invention is not limited to the above-described embodiments, but can be variously modified and implemented without departing from the scope of the invention.

[Effects of the Invention] As described in detail above, according to the present invention, stable operation in a wide temperature range is possible, and it is used as a reproducing first-stage amplifier of a recording / reproducing system having a recording / reproducing head. In this case, it is possible to provide an extremely good amplifier circuit that can keep the input impedance high regardless of the amplitude of the recording signal.

[Brief description of drawings]

FIG. 1 is a circuit configuration diagram showing an embodiment of an amplifier circuit according to the present invention, FIG. 2 is a circuit configuration diagram showing another embodiment of the present invention, and FIG. 3 is a recording / reproduction using a recording / reproducing head. Block diagram for explaining the system, FIG. 4 and FIG.
Each of the drawings is a circuit configuration diagram showing a conventional amplifier circuit. 11 …… Head, 12 …… Playback first stage amplifier, 13 …… Recording amplifier, 14 …… Output terminal, 15,16 …… Input terminal, 17,18…
… Power supply terminal, 19, 20 …… Constant current source, 21 …… Output terminal, 22
…… Signal source, 23 …… Power supply terminal.

Claims (1)

(57) [Claims] 1. A first transistor, whose base is supplied with an input signal and whose emitter is connected to a first reference potential point via a first constant current source, and is cascode-connected to this first transistor. An emitter is connected to the collector of the first transistor, a base is connected to a second reference potential point via a first resistor, and the first reference is connected via a second constant current source. A second transistor connected to a potential point and having a collector connected to the second reference potential point through a second resistor, the collector of the first transistor and the emitter of the second transistor Is connected to the second reference potential point via a third resistor, the temperature coefficients of the first to third resistors are substantially equal, and the temperatures of the first and second constant current sources are If the coefficients are set to be approximately equal, In the circuit, the third resistor,
The first and second transistors are forward biased in polarity and diodes are connected in series, and the base potential and collector potential of the second transistor are set to be equal at a predetermined temperature. And an amplifier circuit.