JPS6330013A - Amplifier circuit - Google Patents

Abstract

PURPOSE:To prevent the reduction in the S/N due to the attenuation of a signal voltage by cancelling the base current of the 1st or the 2nd transistor (TR) forming a differential pair by the base current of the 4th or 3rd TR so as to keep the input impedance high. CONSTITUTION:Since the resistance R12 is sufficiently smaller than a resistance R13, the signal inputted to an input terminal iN1 is fed to bases of the 1st and 4th TRs Q12, Q21 in phase. A collector current of the 2nd TR Q22 in cascade connection to the 4th TR Q21 in opposite phase to the collector current of the TR Q12 is multiplied by 1/hfe (hfe is a current amplification factor) by the TR Q21 and fed back to the emitter of the TR Q12. Thus, the base current of the TR Q12 is cancelled and the input impedance of the amplifier circuit viewed from the input terminal IN1 is increased. Since the resistance R22 is sufficiently smaller than the resistances R21, R23, the input impedance of the amplifier viewed from the input terminal IN2 is increased. Since the input impedance of the amplifier is increased, the reduction in the S/N due to the attenuation of the signal voltage is prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention (Field of Industrial Application)] The present invention relates to an amplifier circuit suitable for amplifying minute signals.

(Prior art) Generally, in order to amplify the signal voltage generated by elements with low output voltage, such as the magnetic head of a video tape recorder or the moving coil type cartridge of a record player, electric field effect is used to improve the S/N ratio. Bipolar transistors, which can increase mutual conductance more easily than transistors, are often used in the first stage of amplifiers. An example of a conventional amplifier circuit using the bipolar transistor is shown in FIG. In FIG. 5, Q1□ and Q22 are bipolar transistors that constitute a differential amplifier circuit, and each base is connected to the input terminal I via capacitors C11 and C21.
They are connected to N1 and IN2, respectively.

Transistor Q12. The emitters of Q22 are each resistor R3
1 and grounded via R32, and the emitters are connected via a capacitor C3.

The collectors of the transistors Q12' and Q22 are provided with bipolar transistors Q11. Q21 are each connected in cascode. Transistor Q11. A voltage V of a bias DC power supply (not shown) is applied to the collector of Q21 via load resistors R33 and R34. 0 is printed as 0. The transistor Q
11. o1゜, Q21. Q22 is a resistor R11°R12,
It is biased by R13, R21, R22, and R23, respectively, and the input load is connected between input terminals IN and IN2. However, in the case of unbalanced input, the input load is connected between either input terminal (IN or IN2) and ground, and the input load is connected between either input terminal (IN2 or
A feedback signal etc. is supplied to N1). The output signal is transmitted by transistor Q11. It is taken out from output terminals 0UT1 and oUT2 connected to the collector of Q21, respectively.

In the differential amplifier circuit configured as described above, bipolar transistors Q11. Q12- Q21゜Q2
The transconductance of 2 is proportional to the collector current. On the other hand, since the magnitude of the collector shot noise is proportional to the square root of the collector current, the relative value of the collector shot noise to the output signal can be reduced by increasing the collector current.

However, the bipolar transistor Q11. Q
12. Q21. Since the input impedance of Q22 is inversely proportional to the mutual conductance, there is a drawback that increasing the collector current as described above causes the input impedance to decrease. If the input impedance of the amplifier is thus much lower than the input load (for example, the magnetic head of a video tape recorder), the signal voltage will be attenuated at the input side of the amplifier. Therefore, the level of thermal noise generated by the base diffused resistance, input resistance, etc. of the first stage of the amplifier increases relative to the signal, and the S/N ratio deteriorates. Therefore, even in the first-stage amplifier circuit using bipolar transistors, it is impossible to unnecessarily increase the collector current and use the circuit in a state where the mutual conductance is sufficiently large.

(Problems to be Solved by the Invention) The present invention was made in view of the problem that input impedance decreases when the collector current is increased.The purpose of the present invention is to increase the input impedance even if the collector current is increased. The purpose is to provide an amplifier circuit that can maintain high performance.

[Structure of the Invention] (Means for Solving the Problems) The present invention focuses on the fact that the signal currents flowing through the collectors of two transistors constituting a differential amplifier circuit have opposite phases to each other. a third transistor whose emitter is connected to the collector of the first transistor and whose base is connected to the base of the second transistor at low impedance; is connected to the collector of the second transistor, and a fourth transistor whose base is connected to the base of the first transistor at low impedance, and the collectors of the third and fourth transistors each have a fourth transistor connected to the collector of the second transistor. It is characterized by obtaining an output signal.

(Operation) Since the base of the first transistor and the base of the fourth transistor are connected with low impedance,
Signals of approximately the same level are input to the first and fourth transistors. The collector current of the second transistor, which has the opposite phase to the collector current of the first transistor, is
The fourth transistor connected to the collector of the second transistor amplifies the signal by 1/hfe (Me is the current amplification factor) and feeds it back to the input of the first transistor. Since this feedback current has an opposite phase to the base current of the first transistor, the base current of the first transistor is canceled out.

Furthermore, since the base of the second transistor and the base of the third transistor are connected at low impedance, signals of approximately the same level are input to the second and third transistors. The collector current of the first transistor, which is in opposite phase to the collector current of the second transistor, is amplified by 1/hfe times by the third transistor connected to the collector of the first transistor, and then the collector current of the first transistor is amplified by 1/hfe. is fed back to the input. Since this feedback current has a reverse phase with respect to the base current of the second transistor, the base current of the second transistor is canceled out.

This increases the input impedance of the entire amplifier circuit, reduces attenuation of the signal voltage, and reduces the level of noise such as thermal noise generated on the amplifier circuit side.

(Example) Hereinafter, an example of the present invention will be described with reference to the drawings. In FIG. 1, the same parts as in FIG. 5 are designated by the same reference numerals, and the explanation thereof will be omitted. The difference in FIG. 1 from FIG. 5 is that the base of the third transistor Q11 connected in cascode to the first transistor Q12 is connected to the common connection point a of resistors R21 and R22, and the base of the third transistor Q11 is connected to the common connection point a of the resistors R21 and R22. The base of the cascode-connected fourth transistor Q21 is connected to the common connection point of the resistors R11 and R12, and the resistor R12 is set to a sufficiently smaller resistance value than the resistors R11 and R13, and the resistor R22 is connected to the common connection point of the resistors R11 and R12.
, R23, and the other parts are configured the same as in FIG.

Note that the resistance values of the resistors R11, R13, R21, and R23 are set to high resistance values that do not affect the input impedance of the amplifier.

In the circuit configured as described above, since the resistor R12 is sufficiently smaller than the resistors R11 and R13, the input terminal IN
The signal input to 1 is supplied to the bases of transistors Q1° and C21 in the same phase. The collector current of transistor Q22, which has the opposite phase to the collector current of transistor Q12, is generated by transistor Q21 as r1/hfe.
(hfe is the current amplification factor) and fed back to the emitter side of the transistor Q12.

Therefore, the base current of the transistor Q12 is canceled out, and the input impedance of the amplifier circuit as seen from the input terminal IN1 increases. Also, the resistor R22 is the resistor R
21-R23, the signal input to the input terminal IN2 is transmitted to the transistor Q22. It is supplied in phase to the base of Q11. The collector current of transistor Q12, which is in reverse phase to the collector current of transistor Q22, is multiplied by 1/hfe by transistor Q11 and fed back to the emitter side of transistor Q22. Therefore, the base current of the transistor Q22 is canceled out, and the input impedance of the amplifier circuit as seen from the input terminal IN2 increases. As the input impedance of the amplifier circuit increases in this way, the S/P due to signal voltage attenuation increases.
A decrease in the N ratio can be prevented.

The resistance values of the resistors R12 and R22 are determined by the resistors R11 and R13.
- If the resistance values cannot be set sufficiently smaller than the resistance values of R21 and R23, the configuration shown in FIG. 2 or 3 may be used. In other words, in FIG. 2, an AC bypass circuit consisting of a resistor R14 having an inductance and a low resistance value and a capacitor C12 is connected in series between the common connection point a of the circuit in FIG. 1 and the input terminal IN1, and the common connection point a An AC bypass circuit consisting of an inductance L21, a resistor R24 having a low resistance value, and a capacitor C22 is connected in series between the input terminal IN2 and the input terminal IN2. In addition, in FIG. 3, diodes D11 to D1o are connected in series in the forward direction instead of the resistor R1□ of the circuit in FIG. 1, and the DC level is shifted.
Instead of the resistor R, diodes D21 to D2n are connected in series in the forward direction to shift the DC level.

The circuits shown in FIGS. 2 and 3 also have the same functions and effects as the circuit shown in FIG. 1. Note that the resistance R14=R24 and the inductance L11. of the circuit of FIG.
L21 is for adjusting feedback capacitance and element variations, and may be omitted. Further, reverse biased Zener diodes may be used instead of the diodes D11 to D1o and the diodes D21 to D2n shown in FIG.

By using PNPN upper type transistors as the third and fourth transistors which are roughly connected in cascode to the first and second transistors, and using two positive and negative DC power supplies as shown in Fig. 4, DC input can be achieved. can also be amplified. In FIG. 4, C12 and C22 are the first
, a second NPN type transistor, each base of which is connected to the input terminals IN1 and IN2, respectively. The collector of the first transistor Q12 is connected to the bias positive power supply (voo) via the resistor R41, and the collector of the first transistor Q12 is
It is connected to the emitter of NP type transistor Q11. The collector of the second transistor Q22 is connected to the bias positive power supply (voo) via a resistor R42, and is also connected to the emitter of the fourth PNP transistor Q21. The base of the transistor Q21 is connected to the input terminal IN1, and the base of the transistor Q11 is connected to the input terminal N2. The transistor Q12
emitter of C21, collector of Qll, C
22 emitter is resistor R43-R44, R45=R4
They are connected to a bias negative power supply (VEE) via B, respectively. First transistor Q12 and second transistor Q
The 22 emitters are connected together. A resistor R15 is connected between the base of the transistor Q12 and the ground,
A resistor R2 is connected between the base of the transistor Q22 and the ground.
is connected. The output signal is transmitted by transistor Q11. Q
Output terminals OUT, each connected to the collectors of 21.

Extracted from 0UT2. In the circuit shown in FIG. 4, the transistor Q12 also operates in the same way as in the case of FIG.
．． The base current of Q22 can be canceled. Therefore, the input impedance of the entire amplifier circuit can be kept high. In FIG. 4, the transistor Q11. Q21
A diode may be additionally connected to the emitter of each to adjust the operating current.

In addition, in each of the embodiments shown in FIGS. 1 to 4, NP
Even if the N transistor and the PNP transistor are replaced with each other and the polarity of the power supply voltage is changed, the same operation and effect as described above can be obtained.

[Effects of the Invention] As described above, according to the present invention, the first or second
Since the base current of the fourth or third transistor is canceled out by the base current of the fourth or third transistor, the input impedance can be maintained high even if the collector current is increased to increase the mutual conductance.

Therefore, the decrease in the S/N ratio due to attenuation of the signal voltage is reduced, and when used as an initial stage circuit for amplifying minute signals generated by a video head or the like, signal amplification with a good S/N ratio is possible.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram showing one embodiment of the present invention, Figures 2 and 3 are circuit diagrams showing one embodiment of the present invention.
Both FIG. 4 and FIG. 4 are circuit diagrams showing other embodiments of the present invention, and FIG. 5 is a circuit diagram showing an example of a conventional amplifier circuit. Qll, C12, C21, C22...transistor R
11~R15・R21~R25・R31~R34・R4
1~R4B...Resistance C3, C11, C12, C21, C22°°°:]'' Densa agent Patent attorney Nori Chika Yudo Hiroshi Uji Figure 1 Figure 4 Figure 5

Claims (1)

[Claims] First and second transistors forming a differential pair and to which first and second input signals are respectively supplied; an emitter connected to the collector of the first transistor; , a third transistor having a base connected to the base of the second transistor in a low impedance manner, and an emitter connected to the collector of the second transistor and having a base connected to the base of the first transistor in a low impedance manner. and a fourth transistor connected to each other, wherein an output signal is obtained from the collectors of the third and fourth transistors, respectively.