Preamplifier circuit

Abstract: A preamplifier circuit for amplifying a video signal reproduced by a magnetic head includes a positive feedback circuit for reducing an input capacitance thereof. A negative feedback circuit may be added thereto for stabilizing an operation of the preamplifier.

Description: BACKGROUND OF THE INVENTION The present invention relates to an amplifier circuit and, particularly, to a preamplifier circuit for use in a magnetic recording and reproducing apparatus such as video tape recorder. In magnetic recording and reproducing, the frequency characteristic of a reproducing video signal is generally determined by a resonance characteristic defined by an inductance and capacitance of a video head circuit and it is very difficult to obtain a desired frequency characteristic therefor. In order to remove this problem, it has been usual to improve the resonance characteristics of the video head circuit by using a damping resistor and Figs. 1 and 2 show one of the conventional circuits for this purpose as an example, which is disclosed in U.S.Patent No. 3,513,267, and another conventional circuit, respectively. In these figures, the letter L depicts an equivalent inductance of a video head measured at an input side of a preamplifier whose gain is A and C is a parallel capacitance including a stray capacitance of the video head. The high cutoff frequency of a reproduced video signal is substantially determined by a resonance frequency defined by L and C according to the following equation fo = 1/2π√ LC In order to reduce such resonance and to make the frequency characteristic flat within the video signal frequency band, damping resistors R2 and R1 are employed in the circuits shown in Figs. 1 and 2, respectively. Among them, since the circuit in Fig. 1 includes the damping resistor R2 inserted in the circuit as a negative feedback resistor, it is possible to reduce the thermal noise produced by the resistor. For this reason, the circuit shown in Fig. 1 has been commonly used in current video tape recorders. In these prior art documents, when it is desired to make the frequency band of the reproduced video signal wider to obtain a higher video signal quality, the resonance frequency must be increased by reducing the inductance L or capacitance C of the video head. In order to reduce the inductance, the number of turns of the head coil must be reduced, resulting in a reduced induction voltage and degraded S/N ratio. Alternatively, a reduction of the parallel capacitance is practically impossible since it is substantially a stray capacitance of the head coil. therefore, at present, it is impossible to obtain a flat frequency characteristic for video signals over a substantially wider frequency range. SUMMARY OF THE INVENTION An object of the present invention is to provide a preamplifier according to claim 1 for use in a video signal recording and reproducing apparatus in which the reproducing frequency characteristic of the video signal is made substantially flat over a substantially wider frequency range while reducing neither the inductance nor the parallel capacitance of the video head. The above object of the present invention is achieved by a provision of a positive feedback circuit composed of a capacitor. With the positive feedback circuit provided between an output terminal of the preamplifier and an input thereof connected to an output of the video head, a capacitance component of an input side impedance of the preamplifier, which includes the stray capacitance of the video head is at least reduced substantially. This technique has been revealed for much lower frequency applications in: INTERNATIONAL JOURNAL OF ELECTRONICS, vol. 34, no. 3, 1973, pages 423-427; R. NATH et al.: "A high input impedance buffer amplifier using bipolar transistors", but was plagued with stability problems which required manual adjustments. In the present invention, negative feedback is used in addition to positive feedback. When the parallel capacitance is made zero equivalently, there is no resonance. When the capacitance is reduced equivalently, the resonance frequency is made higher and a damping of the resonance becomes easier. Therefore, it becomes possible that the resonance damping is performed by an input resistance of the preamplifier itself without using any special damping resistor, so that it is possible to widen the video signal frequency band as desired. BRIEF DESCRIPTION OF THE DRAWING Figs. 1 and 2 are circuit diagrams of conventional preamplifiers, respectively; Fig. 3 is a circuit diagram of a preamplifier with positive capacitive feedback; Fig. 4 is an equivalent circuit of the circuit in Fig. 3; Fig. 5 shows the circuit in Fig. 3 in more detail; Fig. 6 is a circuit diagram showing the general operation of an embodiment of the present invention; Fig. 7 is a circuit diagram showing a concrete example of the circuit in Fig. 6 Fig. 8 is a graph showing a gain-frequency characteristic of the embodiment in Fig. 7; and Fig. 9 is a further embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In Fig. 3, the inductance L, the parallel capacitance C and the preamplifier having gain A are the same as in the conventional circuit shown in Fig. 1 or 2. In addition, a positive feedback capacitor Cx is provided between an output of the amplifier and an input terminal thereof. In this circuit, an equivalent input resistance is depicted by Rin, and Vo and V1 are the reproduced signal voltage induced in a video head and the input terminal voltage of the preamplifier, respectively. Assuming branch currents flowing into and from a junction point p at the input terminal of the preamplifier as i1,.. i4 as shown, the following equation is established: i4 = i1 - ( i2 - i3 ) (1) where i2 = jω CV1 (2) i3 = jω Cx(A-1)V1 (3) Therefore, the equation (1) is rewritten as follow: i4 = i1 - jω C'V1 (4) where C' = C - (A-1)Cx (5) As is clear from the equation (4), the input circuit in Fig. 3 can be shown to be equivalent to that in Fig. 4. Assuming the following relation Cx = C/(A-1) (6) the equivalent capacitance C' becomes zero according to the equation (5). In such case, i4 = i1 and the input terminal voltage V1 of the preamplifier can be represented as follow: V1 = [1/(1 + j ω /ω o ]Vo (7) where ω o = Rin/L. Thus, with the positive feedback circuit composed of the capacitor Cx, it is possible to make the equivalent capacitance in the input side of the preamplifier zero, so that it is possible to eliminate the resonance of the video signal reproducing circuit. In this circuit construction, the high cutoff frequency of the video signal reproducing circuit is substantially determined by a value of ω o in the equation (7). However, since the input resistance Rin of the preamplifier can generally be made large compared with the inductance L of the video head, the high cutoff frequency mentioned above can be very large compared with that of the conventional circuit. As mentioned above, the circuit including the capacitor Cx must be a positive feedback circuit. Otherwise, the term (A-1)Cx in the equation (5) becomes negative, resulting in that the equivalent capacitance C' is larger than C. Fig. 5 shows a circuit diagram which is an example of concrete construction of the circuit in Fig. 3. In Fig. 5, a video head 1 has an inductance L and a parallel capacitance C and a positive feedback capacitor 4 has a capacitance Cx. Transistors 7, 8 and 9 which are cascaded constitute the preamplifier, the output of which is derived from an emitter 10 of the transistor 9. A collector output of the transistor 9 is connected to the base of a transistor 6, the emitter 5 of which positively feeds back a signal through the capacitor 4 to the base of the transistor 7 as in-phase voltage to the input voltage of the preamplifier. Thus, assuming that the gain of this circuit which is provided by the transistors 6, 7, 8 and 9 is A, the equivalent parallel capacitance with respect to the inductance of the video head can be set to zero according to the equation (6). Therefore, the resonance frequency of the preamplifier becomes much higher than that of the conventional circuit and thus the reproduction frequency characteristics of the video signal can be made flat over a wider frequency band, resulting in a high quality video signal reproduction. Further, since the resonance peak can be damped by the input resistance of the preamplifier itself, the problem produced by the use of a special damping resistor which is a source of thermal noise is resolved, i.e., the S/N ratio of the reproduced video signal is improved with simple circuit construction. Described hereinbefore is a circuit by which the equivalent capacitance C' is made zero. It should be noted that the present invention is not limited thereto and includes circuit constructions by which the equivalent capacitance C' is reduced sufficiently. In such sense, the value of the capacitance Cx is not always that satisfying the equation (6) and it is possible to set it as any of smaller values. For example, i may be possible to set the capacitance Cx such that the latter satisfies the following equation: Cx =α·C/(A-1) (8) where α < 1. In the latter case, the value of C' becomes as follows according to the equations (5) and (8): C' = ( 1-α ) C (9) The resonance frequency ω a is thus represented by the following equation: ω a = 1/√ L(1-α)C (10) That is, it is possible to increase the resonance frequency by a factor which is 1/√ 1-α times that of the conventional circuit. In this case, the input terminal voltage V1 of the preamplifier can be represented by the following transfer function: V1 = {1/ [(S/ω a )² + 2 ζ ( S/ω a ) + 1 ]}·Vo where ω a = 1/√ LC ζ = L/2Rin S = jω In order to make the frequency characteristic of V1 substantially flat, ζ , i.e., the damping coefficient, should be equal to or larger than 0.5. When the damping coefficient is 0.5, Rin becomes √ L/C' . Since, as mentioned previously, the equivalent capacitance C' can be made arbitrarily small by a selection of the value of α in equation (9), it is possible to satisfy the requirement of the damping coefficient easily. Fig. 6 shows a block circuit diagram of an embodiment of the present invention. In Fig. 6 in which same or corresponding components as those in Figs. 1 to 5 are depicted by same reference numerals, respectively, a positive feedback capacitor Cx is provided between a positive phase signal output terminal 17 of an amplifier circuit 27 of a preamplifier and an input terminal thereof and a negative feedback resistor Rx is provided between an inverse signal output terminal 18 and the input terminal of the amplifier circuit. Gains of the amplifier circuit 27 when looked at the positive phase output terminal 17 and the negative phase output terminal 18 are represented by Ap and An, respectively. This embodiment solves a possible problem of the aforementioned circuit, namely that,although the positive feedback gain of the preamplifier in Fig. 3 or 5 must be smaller than unity, when the positive feedback loop gain of the preamplifier becomes unity the circuit in Fig. 3 or 5 becomes unstable. That is, in the embodiment in Fig. 6, the negative feedback circuit composed of the resistor Rx is provided. Therefore, the input resistance Rin of the amplifier circuit 27 can be represented by the following equation: Rin = Rx/( An + 1 ) (11) Therefore, by changing the value of the negative feedback resistor Rx, the input impedance of the amplifier circuit 27 can be set arbitrarily. Thus, in order to make the input voltage V1 at the input terminal of the amplifier circuit constant regardless of frequency, the negative feedback resistance Rx should be defined as follow, by taking the equivalent input capacitance C' into consideration: Rx = ( An + 1 )√ L/C (12) Thus, the frequency characteristics of the preamplifier can be made substantially flat over a wider frequency range due to a combination effect of the reduced input capacitance due to positive feedback and the regulation of input resistance due to negative feedback. Fig. 7 is a circuit diagram showing a circuit construction based on Fig. 5. In Fig. 7, a reproduced video signal from a magnetic head is applied to the input terminal of the amplifier circuit, i.e., the base of transistor Q1 and voltage-amplified thereby in opposition of phase with respect to the input signal. The voltage-amplified signal is divided into a positive and negative output sides. The signal in the side of the positive phase is further inverse-amplified by a common emitter circuit composed of a transistor Q2 and resistors Rc2 and Re1 to obtain a signal in phase with the input signal. A capacitor Ce connected to the emitter of the transistor Q2 is to compensate for phase of the positive feedback loop. The output signal of the transistor Q2 which is in phase with the input signal is current-amplified by an emitter follower circuit composed of a transistor Q3 and a resistor Re2 and provided at the positive phase output terminal 17. The signal in the negative output side is current-amplified by an emitter follower circuit composed of a transistor Q4 and resistors Re3 and Re4 and provided at the negative output terminal 18. The in phase signal at the positive phase output terminal 17 is positive-fedback by the capacitor Cx to the input terminal to reduce the input capacitance C of the transistor Q1 which should have a large emitter region for noise limitation and for that reason the input capacitance thereof becomes very large compared with standard transistors. Therefore, it is very important to reduce the input capacitance, 30 to 50 pF, of the transistor Q1 in order to widen the frequency characteristics of the preamplifier. The signal at the negative output terminal 18 is negative-fedback through the resister Rx to the input terminal. Therefore, the input resistance of the transistor Q1 is set to a suitable value with respect to the equivalent input capacitance obtained by the magnetic head and the positive feedback, so that the input impedance of the preamplifer becomes constant regardless of frequency, resulting in a flat gain-frequency characteristics over a wide frequency range. In the shown embodiment, it is possible to connect a further capacitor between the input terminal of the preamplifier and a grounding point GND to regulate the positive feedback gain. Fig. 8 shows gain-frequency characteristics of the preamplifer shown in Fig. 6. In Fig. 8, a dotted curve 25 is the characteristic when there is neither positive nor negative feedback provided. This curve has a peak at around 5.5 MHz due to a resonance of the circuit composed of the inductance of the video head and the input capacitance of the amplifier and thus the frequency range of the reproduced signal is limited thereby, causing the widening of the frequency range to be impossible. A curve 26 is the characteristic when both the positive and negative feedback circuits are used according to the present invention. As is clear from the curve 26, it is possible to amplify the reproduced signal up to around 10 MHz with substantially flat characteristics. When it is unnecessary to widen the frequency range, it is possible to employ a larger inductance of the head due to the reduced input capacitance caused by the positive feedback, which means that it is possible to obtain a larger output of the head and to improve the S/N ratio. Fig. 9 shows another embodiment of the present invention which is similar to that shown in Fig. 7 except that the transistor Q4 in Fig. 7 is removed and the function thereof is performed by the transistor Q2 concurrently. In Fig. 9, the signal from the emitter of the transistor Q1 is the inverted output and the signal from the collector thereof is the in-phase-output. The output terminal 19 is derived from the emitter of the transistor Q2. This circuit construction is simpler than that shown in Fig. 7. The circuits shown in Figs. 7 and 9 have no regulating portion and the capacitor used therein is as small as several pF. Therefore, they can be manufactured in the form of IC. As described hereinbefore, according to the present invention, it is possible to at least reduce the input capacitance of the preamplifier and therefore it is possible to exclude the resonance frequency defined by the magnetic head inductance and the input capacitance from the frequency range of the preamplifier. Since the equivalent input resistance can be regulated by an addition of the negative feedback circuit, it is possible to select the sharpness of the resonance suitably and thus it is possible to realize a preamplifier having a very flat gain characteristic over a widened frequency range.

Claims: A preamplifier circuit for amplifying a signal reproduced by a magnetic bead comprising an amplifier circuit having an inverted input terminal and a non-inverted input terminal connected to said magnetic bead and an output terminal, wherein said preamplifier circuit comprises a positive feedback circuit and a negative feedback circuit, characterized in that said signal is a video signal, said positive feedback circuit includes a capacitive impedance (Cx) provided between said non-inverted input terminal of said amplifier circuit and said output terminal (17) of said amplifier circuit, whereby a capacitance component (C) of an input impedance of said amplifier circuit, including a stray capacitance of said magnetic bead, is at least reduced by a positive feedback provided by said positive feedback circuit, and said negative feedback circuit includes a resistive impedance (Rx) and wherein said amplifier circuit includes an inverted output terminal (18), said negative feedback circuit being connected between said non-inverted input terminal and said inverted output terminal of said amplifier circuit. The preamplifier circuit as claimed in claim 1, characterized in that said capacitive impedance has a value not larger than C/(Ap-1) and not less than C/10(Ap-1) and said resistive impedance has a value substantially equal to (An + 1) √ L/C' , wherein C is a value of an imput capacitance of said input terminal of said amplifier circuit, Ap is a voltage gain looked at said output terminal of said amplifier circuit from said input terminal thereof, An is a voltage gain looked at said inverted output terminal from said input terminal, L is an inductance of said magnetic head and C' is a value of said capacitive component.

Inventors: FUKUSHIMA ISAO, MINAMI NORIO, TAKAMIYA TADASHI, TOEDA HIROSHI, YATSUGI TOMISHIGE, FUKUSHIMA, ISAO, MINAMI, NORIO, TAKAMIYA, TADASHI, TOEDA, HIROSHI, YATSUGI, TOMISHIGE
Applicants: HITACHI LTD, HITACHI VIDEO ENG, HITACHI VIDEO ENGINEERING, INC., HITACHI, LTD.
Application year: 1987
Main classification: H03F1
IPC codes: H03F3, H03F1, G11B5