In a circuit including an amplifier for amplifying a small signal from a signal source wherein the inductance of a transducer and the capacitance of a capacitor form a resonance circuit, the output from the amplifier is negatively fed back through a resistor or a combination of resistors to the input of the amplifier, whereby the noise in the demodulated signal may be suppressed.

BACKGROUND OF THE INVENTION 
The present invention relates to a preamplifier for amplifying a small 
signal from a transducer having an inductive signal source impedance with 
an improved signal-to-noise ratio S/N of the signal demodulated from the 
amplified signal and more particularly a preamplifier especially adapted 
for use in a system for recording and reproducing angularly modulated 
television signals. 
A preamplifier in a home type video tape recorder consists of an amplifier 
with an input terminal grounded through a parallel circuit of a resistor 
and a capacitor connected to a magnetic head. The inductance of the 
magnetic head and the capacitor form a parallel resonant circuit, and the 
resistor serves as a damping resistor of resonant circuit. The gain of the 
amplifier is maximum at the resonant frequency fr of the resonance 
circuit. 
The demands upon the performance of this preamplifier will become apparent 
from the following description of the process for recording and 
reproducing the television signal. When the sinusoidal waveforms recorded 
on the magnetic tape and reproduced across the magnetic head, the maximum 
output voltage is obtained at the frequency fo (approximately 1 MHz or 
less) which is dependent upon the gap width of the magnetic head and the 
relative velocity between the magnetic tape and head. 
Since the bandwidth of the video signal of the television signal is 3 MHz, 
the bandwidth of 3 MHz is also required in the frequency modulation of the 
video signal when the latter is recorded. Therefore the resonant frequency 
of the resonant circuit of the preamplifier is set at 5 MHz so that the 
output voltage-frequency characteristic curves of the preamplifier may be 
made flat. In general, the upper and lower limits of the bandwidth in the 
frequency modulation of the video signal are 4.5 MHz and 3.5 MHz, 
respectively, and the frequency modulation is made in such a way that the 
sync clip level may have a low frequency while the white level may have a 
high frequency. The carrier and the lower sideband are recorded on the 
magnetic tape, but the upper sideband is only partially recorded. When the 
video signal is reproduced with a video tape recorder with a narrow 
bandwidth, a peculiar phenomenon called "reversal" tends to result. In 
order to avoid this phenomenon, the resonant frequency fr is set higher 
than the upper limit (4.5 MHz). 
As a result of ever improving magnetic recording and reproducing 
techniques, information or data may now be recorded on magnetic tape at an 
extremely high density. Consequently, while by the EIAJ (Electronic 
Industrial Association of Japan) standards, the relative velocity between 
the magnetic head and tape and the track width were 11 m/sec and 120 .mu.m 
respectively, the standards for recently developed devices are 6 m/sec and 
30 .mu.m, respectively. This means that the recording density per unit 
area on the magnetic tape has been increased by 8 times as high as before. 
However, the slower the relative velocity and the shorter the track width, 
the lower the signal voltage reproduced across the magnetic head becomes. 
Even though the decrease in output voltage has been avoided by the 
improvements of the magnetic tapes and heads to some extent, it results in 
serious degradation of the signal-to-noise ratio S/N as will be described 
below. 
First, in order to obtain a high S/N ratio with a low output voltage, noise 
must be suppressed as much as possible. However, the causes and nature of 
noise produced in a system consisting of a magnetic tape, a magnetic head 
and a preamplifier had not been thoroughly investigated and analyzed. 
Furthermore the impedance of the signal source including the magnetic head 
varies in response to the frequency. As a result, very complex yet 
unsatisfactory methods had been used in determining the noise factor NF of 
the preamplifiers. 
So far the dominant noise was the so-called modulation noise from a system 
including a magnetic head and a magnetic tape. The lower the level of the 
reproduced signal, the less the modulation noise becomes as the results of 
the extensive studies and experiments conducted by the inventor prove. 
Since the modulation noise contains many amplitude modulated components, 
the modulation noise may be suppressed to some extent by passing the 
signal through a limiter prior to the frequency modulation. 
Therefore the inventors made extensive studies and experiments in order to 
clarify the source of noise which greatly influences the signal-to-noise 
ratio S/N in the signal frequency demodulated from the signal reproduced 
from the magnetic tape upon which the signals have been recorded at a high 
density, and the inventors have succeeded in finding out the cause of 
noise and means for eliminating it or suppressing it as much as possible, 
to an extent hitherto unattainable by conventional preamplifiers. 
SUMMARY OF THE INVENTION 
Accordingly, one of the objects of the present invention is to reduce the 
noise components in the upper and lower sidebands of the output signal 
from a limiter circuit so that the noise in the demodulated signal may be 
reduced. Therefore, the variation in signal-to-noise ratio S/N of the 
demodulated signal in response to the d-c component level of the 
demodulated signal (which corresponds to the white level, the gray level 
or the black level in case of the video signal) may be minimized. 
To the above and other ends, according to the present invention, the output 
from a preamplifier is negatively fed back through a resistor to the input 
of the amplifier, whereby the gain in the vicinity of the resonance 
frequency of a resonance circuit formed by the inductance of a transducer 
and the capacitance of a capacitor may be controlled by the damping effect 
and the noise components in the vicinity of the resonance frequency may be 
considerably suppressed.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Prior Art 
Prior to the description of the preferred embodiments of the present 
invention, an example of preamplifiers will be described. FIG. 1 (a) shows 
a typical example of a preamplifier employed in the home type video tape 
recorders. The video signal reproduced through a magnetic head H from a 
magnetic tape is applied to an input terminal Tin of an amplifier A. One 
ends of a resistor Ro and a capacitor C.sub.H are connected to the input 
terminal Tin while the other ends are grounded. FIG. 1 (b) shows an 
equivalent circuit of the circuit shown in FIG. 1 (a). Inductance L.sub.H 
of the magnetic head H and capacitance C.sub.H form a parallel resonance 
circuit, and the resistor Ro functions as a damping resistor for this 
resonance circuit or system. The gain-frequency characteristic curve 2 of 
this circuit is shown in FIG. 2 (a). The highest gain is obtained at the 
resonance frequency fr. 
The broken line curve 1 in FIG. 2 (a) shows the output voltage vs. 
frequency characteristic curve when the sinusoidal waveforms recorded on 
the tape are reproduced. The maximum output voltage is obtained at a 
frequency fo which is dependent upon a gap width of the magnetic head and 
the relative velocity between the magnetic head and the tape. In general, 
the frequency fo is approximately 1 MHz or less. 
The bandwidth of the video signal is 3 MHz so that the 3 MHz bandwidth is 
also required for frequency modulation. Therefore the resonance frequency 
fr of the resonance system of the preamplifier is approximately set to 5 
MHz, and the output voltage from the preamplifier is flattened against the 
variation in frequency. In the frequency modulation of the video signal, 
the upper and lower limits of the frequency deviation are selected in 
general 4.5 MHz and 3.5 MHz, respectively, and the frequency modulation is 
made in such a way that the sync chip level may have a low frequency and 
the white level may have a high frequency. While the carrier and the lower 
side band are recorded on the magnetic tape, almost all the upper side 
band is lost. 
As described elsewhere, in the video tape recording with a narrow bandwidth 
the resonance frequency is so that the output voltage vs. frequency 
characteristic may be corrected and that "the reversal" may be avoided. 
However, the frequency fs of the signal reproduced across the magnetic 
head is in general lower than the resonance frequency; that is, 
fs.ltoreq.fr so that the noise is produced. As described above, the gray 
and black levels have frequencies lower than the frequency of the white 
level in the frequency modulated video signal so that the gray and black 
levels are more adversely affected by the noise than the white level. It 
had not been made clear yet where the noise which becomes maximum in the 
proximity of the resonance frequency fr is, and it had been considered 
that the noise is due to the impedance of the magnetic head. However, the 
inventors have found out that the major cause of noise is due to the 
damping resistor in the resonance circuit as will be described below. 
Referring back to FIG. 1 (b), the noise is due to the real component of the 
impedance of the resonance circuit consisting of inductance L.sub.H, 
capacitance C.sub.H and resistance Ro, and becomes maximum at the 
resonance frequency fr. 
The degradation of the signal-to-noise ratio after frequency demodulation 
results because of the reasons described below. Referring to FIG. 2 (b), 
the output voltage-frequency characteristic curve 3 is obtained when the 
video signal at fs is reproduced across the magnetic head and amplified by 
the preamplifier. The components in the upper and lower sidebands are 
noise, and the noise becomes maximum in the vicinity of the resonance 
frequency fr. When the signal containing such noise [with the 
characteristic curve 3 in FIG. 2 (b)] is made to pass through a limiter 
for processing the signal prior to the frequency demodulation, the 
characteristic curve 4 as shown in FIG. 2 (c) is obtained. The noise 
output reaches its peaks in the upper and lower sidebands. This is similar 
to the phenomenon observed in vestigial sideband transmission wherein when 
the carrier and one vestigial sideband are made to pass through a limiter 
circuit, the other vestigial sideband is reproduced. When such signal 
(with the characteristic curve 4 in FIG. 2c) is frequency modulated, the 
noise is increased. When the video signal is recorded or reproduced with 
the resonance circuit with the resonance frequency fr, the specific 
phenomenon called "reversal" tends to result. In order to avoid this 
phenomenon, the resonance frequency fr must be set higher than the upper 
limit about 4.5 MHz. 
With the ever increasing recording density of the magnetic 
recording-reproducing equipment, the EIAJ standards have been so revised 
that the relative velocity between the magnetic head and tape and the 
track width are defined 6 m/sec and 30 .mu.m which were 11 m/sec and 120 
.mu.m. in other words, the recording density per unit area on the magnetic 
tape has been increased by approximately 8 times. However, the slower the 
relative velocity and the shorter the track width, the lower the voltage 
of the signal reproduced across the magnetic head becomes. The 
considerable reduction in output voltage which is compensated to some 
extent by the improvements of the magnetic tapes and heads results in the 
degradation in S/N ratio which adversely affects the magnetic recording 
and reproducing systems as will be described below. 
Since the output voltage is decreased, the noise must be suppressed as much 
as possible in order to obtain a high S/N ratio. However, so far the cause 
of noise from magnetic tapes and heads and preamplifiers have not been 
thoroughly investigated, and the impedance of the signal source or the 
reproduced video signal varies in response to the frequency thereof so 
that the circuit designs for selecting optimum noise frequency have not 
been available and have been extremely complex. Furthermore the major 
noise had been the so-called modulation noise produced from the system 
consisting of the magnetic tape and head, but as the output level of the 
reproduced signal is reduced, the modulation noise is also decreased. 
(This has been confirmed by the experiments conducted by the inventors.) 
However, since the modulation noise has many amplitude modulated 
components, it may be reduced to some extent with a limiter preceding to a 
frequency demodulation stage. 
Thus, there had been a strong demand for clarifying the source of noise 
which dominantly adversely affects the signal-to-noise ratio S/N in case 
of the frequency demodulation of the signal recorded in a high density and 
for means for eliminating or suppressing the noise. 
THE INVENTION, FIGS. 3, 4 and 5 
Referring first to FIG. 3 (a), L.sub.H is the inductance of the magnetic 
head and Es is the signal induced across the magnetic head. The 
capacitance C.sub.H and the inductance L.sub.H form a resonance circuit. A 
capacitor C.sub.1 is connected in order to avoid the direct current flow 
between the amplifier A and the resonance circuit, and R.sub.1 is a 
feedback resistor. A first amplifier stage in the amplifier A is d-c 
biased. The signal applied to the input terminal Tin of the amplifier is 
amplified with a gain or an amplification factor a (a&gt;+1), inverted in 
phase and appears at the output terminal Tout. 
The feedback factor .beta. is 
##EQU1## 
wherein Zin=the parallel impedance of the impedance Zs of the resonance 
circuit and the input impedance Rin of the amplifier. When viewed from the 
magnetic head to the amplifier A, the resistor R.sub.1 is a negative 
feedback resistor so that it equals to 
##EQU2## 
The resistor is interconnected between the input terminal Tin and the 
ground in parallel with the impedance Zs and Rin so that the resistor 
R.sub.1 becomes same with the resistor Ro in the circuit shown in FIG. 1 
(b). Therefore, 
EQU R.sub.1 =(1+a) Ro. 
The difference between the circuit shown in FIG. 3 (a) and the circuit 
shown in FIG. 1 (b) will be described below with reference to FIGS. 5 (a) 
and 5 (b). FIG. 5 (a) shows an equivalent circuit including the noise 
source of the circuit shown in FIG. 1 (b). The signal voltage Es' with the 
frequency fs induced across the magnetic head is transmitted to the 
terminal Ts through the impedance Zs, i.e. the resonance circuit 
consisting of L.sub.H and C.sub.H. (Es'=a proportionality factor.times.the 
voltage Es because the circuit shown in FIG. 1 (b) is redrawn as shown at 
(a) and (b) in FIG. 5.) The damping resistor Ro is interconnected between 
the terminal Ts and a voltage source En which represents the thermal noise 
produced by the resistor Ro. The signal and noise are transmitted from the 
terminal Ts to the input terminal Tin, and a voltage is obtained across an 
input resistor Rin of the amplifier A. The signal with the frequency fs at 
the terminal Ts is 
##EQU3## 
and the noise En due to the resistor Ro which has a peak in the vicinity 
of the resonance frequency fr is 
##EQU4## 
where Zs(fs) and Zs(fr) are impedance Zs at fs and fr, K=Boltzman's 
(constant), 
T=temperature of the resistor, and 
.DELTA.f=a narrow frequency band in the vicinity of the frequency fr. 
The ratio of the signal at fs to the noise in the vicinity of fr is 
##EQU5## 
FIG. 5 (b) shows an equivalent circuit including a noise source of the 
circuit shown in FIG. 3 (a). En' is the thermal noise of the resistor 
R.sub.1. The signal at the terminal Ts is 
##EQU6## 
and the noise at the terminal Ts is 
##EQU7## 
The ratio between them is 
##EQU8## 
Inserting into this ratio R.sub.1 =(a+1)Ro and dividing it by the ratio 
obtained in the circuit shown in FIG. 5 (a), we have 
In practice, the magnetic head has resistive loss, and the absolute value 
of the impedance Zs of the resonance circuit consisting of the inductance 
L.sub.H of the magnetic head and the capacitance C.sub.H is in general 
EQU Ro.apprxeq..vertline.Zs.vertline. or Ro&gt;.vertline.Zs.vertline. 
The absolute value of the gain a of the amplifier A is 
EQU a&gt;+1 
(In practice, a=10 to 1000). 
Hence, 
EQU R.sub.1 =(a+1)Ro&gt;.vertline.Zs.vertline. 
Therefore, 
##EQU9## 
In the second term, 
EQU .vertline.Zs(fr).vertline.&gt;.vertline.Zs(fs).vertline. 
since the resonance frequency is fr. 
Therefore, 
##EQU10## 
Since the gain a is by greater than 3, 
EQU Ratio'&gt;Ratio 
Therefore the noise is suppressed by far considerably in the circuit shown 
in FIG. 5 (b) than in the circuit shown in FIG. 5 (a). That is, the noise 
in the vicinity of the frequency fr is more suppressed in the circuit FIG. 
3 (a) than in the circuit shown at (b) in FIG. 1. 
Referring back to FIG. 2 (b), while the output voltage vs. frequency 
characteristic curve for the circuit shown at (b) in FIG. 1 is 3, the 
preamplifier shown in FIG. 3 (a) has the characteristic curve 5 which 
shows that the noise is considerably suppressed in the vicinity of the 
frequency fr. Furthermore as shown in FIG. 2 (c) at 6, the noise is 
suppressed at its peaks in the upper and lower sidebands when the signal 
is made to pass through the limiter. Thus the S/N ratio may be remarkably 
improved after the frequency demodulation. 
The preamplifier shown in FIG. 3 (b) is substantially similar in 
construction to the preamplifier shown in FIG. 3 (a) except that the first 
stage in the amplifier A is biased with a d-c voltage fed back through the 
resistor R.sub.1 from the output terminal Tout of the amplifier A. 
In general, in the amplifier A a predetermined d-c voltage is divided by 
resistors, and a divided voltage is supplied to the first stage. Therefore 
the voltage dividing resistors also become the noise source. To remove 
this noise source, not only the a-c component but also the bias voltage 
are negatively fed back from the output terminal Tout of the amplifier so 
that in addition to the noise from the resistor Ro, the noise from other 
sources may be effectively suppressed. 
In the preamplifier shown in FIG. 3 (c), the method for supplying the first 
stage in the amplifier A with the d-c bias voltage is also modified based 
on the fact that the magnetic head exhibits almost no resistance to direct 
current. A d-c voltage E.sub.DC is divided by resistors R.sub.B1 and 
R.sub.B2, and the a-c component is by-passed to the ground through a 
capacitor C.sub.B. Therefore a very stable d-c voltage may be supplied to 
the amplifier A through the magnetic head, whereby the noise in the bias 
voltage may be eliminated. Thus, the problems to be solved are the 
elimination of the noise produced from the magnetic head and the noise 
figure NF of the amplifier A. 
In the embodiments shown in FIGS. 4 (a) and 4 (b), adverse effects due to 
the floating capacitance of the feedback resistor are eliminated. In the 
circuit shown in FIG. 3 (a), the resistor R.sub.1 has a floating 
capacitance which is in parallel with the capacitance C.sub.H due to the 
so-called Miller effect. As a result, in order to obtain a desired 
resonance frequency fr, the capacitance C.sub.H must be reduced. In an 
extreme case, a negative capacitance is needed. The first reason is that 
the gain a of the amplifier A is very high because of the purpose of the 
provision of this preamplifier, and the second reason is that the floating 
capacitance of the feedback resistor may be high. Higher floating 
capacitance is due to the poor design of the resistors, and the variable 
resistors have in general higher floating capacitance than the fixed 
resistors. 
In the preamplifier circuit shown in FIG. 4 (a), the output from the 
amplifier A is divided by resistors R.sub.3 and R.sub.4, and a divided 
voltage is fed back through a resistor R.sub.2. An equivalent gain by this 
negative feedback is 
##EQU11## 
Therefore, the miller capacitance which is 
EQU Cs.times.(1+a) 
becomes 
##EQU12## 
That is, the miller capacitance is reduced. Since the values of the 
resistors R.sub.3 and R.sub.4 and the output impedance of the amplifier A 
are so small that their floating capacitance may be negligible. 
The preamplifier shown in FIG. 4 (b) is substantially similar in 
construction to the preamplifier shown in FIG. 4 (a) except that instead 
of the resistors R.sub.3 and R.sub.4, a variable resistor R.sub.VAR is 
used. This arrangement is advantageous in that the negative feedback 
resistance may be arbitrarily selected. The output voltage from the 
amplifier A is divided by the variable resistor R.sub.VAR and a fixed 
negative feedback resistor R.sub.5 so that a damping resistor equivalent 
to the combination of the variable and fixed resistors may be a variable 
resistor. Furthermore the floating resistance Cs' of the fixed resistor 
R.sub.5 is very low. 
In summary, according to the present invention, the design of the 
preamplifiers may be so improved that the production of the preamplifiers 
may be much facilitated.