Single end input high frequency amplifier

A single end input type high frequency amplifier of the present invention decreases noise superposed on a power supply of a pre-amplifier in a read amplifier which amplifies a signal from MR head. The single end input type high frequency amplifier includes a transistor connected to an input side terminal of a differential input amplifier. A single end type resistor is connected to the transistor and a loop amplifier is connected to the single end type resistor to feedback a difference voltage between a reference side terminal and an input side terminal of the differential input amplifier.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an amplifier used for a hard disc drive 
apparatus. In particular, the present invention relates to a single end 
input type high frequency amplifier mainly used as a pre-amplifier in a 
read amplifier which drives a head (referred to MR head) comprising a 
magneto-resistance (MR) effect element. This single end input type high 
frequency amplifier is less influenced by noise superposed to a power 
supply, in other words, it has better power supply voltage fluctuation 
removal ratio (referred to PSRR). 
2. Description of the prior art 
FIG. 3 shows a conventional pre-amplifier 80 used in a read / write (R / W) 
amplifier 300. In FIG. 3, signals read out from disc 58 by MR head 12 are 
amplified by pre-amplifier 80 and second amplifier 82 through input 
terminal 100 and outputted to an output terminal 200. 
FIG. 4 shows an example of a conventional single end input type high 
frequency amplifier. In FIG. 4, the conventional single end input type 
high frequency amplifier comprises, a resistor 11 one end of which is 
connected to a power supply 2 having potential Vcc and the other end of 
which is connected to a collector of a transistor 21. A constant voltage 
source 3 is connected to a base of the transistor 21, and an emitter of 
the transistor 21 is connected to a collector of a transistor 22. A 
constant voltage source 4 is connected to a base of the transistor 22, and 
an emitter of the transistor 22 is connected to the input terminal 100. A 
parasitic capacitance Cs is connected between a collector of the 
transistor 21 and a substrate. The collector of the transistor 21 is also 
connected to one input terminal of a differential input amplifier 40. The 
other input terminal of the differential amplifier is connected to a power 
supply through a constant voltage source 5. In FIG. 4, a signal voltage is 
inputted from MR head 12 to an input terminal 100, and amplified by 
transistors 21, 22, further amplified by the differential amplifier 40, 
and then outputted from output terminals 41, 42. Assuming an amplitude of 
noise wave form appearing at nodes A is V.sub.A as shown in the figure, an 
amplitude of noise wave form at node B is V.sub.B as shown in the figure, 
and an amplitude of noise wave form at node C is V.sub.C as shown in the 
figure, the frequency characteristics of the noise voltages V.sub.B, 
V.sub.C for the noise voltage V.sub.A, respectively, are shown in FIG. 5. 
That is, the mount of V.sub.C /V.sub.A at node C is constant over all 
frequency range, and the amount of V.sub.B /V.sub.A at node B decreases 
from a certain frequency according to the frequency increases. 
An impedance of the power supply for actual use is more than zero because 
it includes resistance components. In addition, wiring from the power 
supply to this amplifier also includes various parasitic elements. 
Therefore, the noise of high frequency generated at the other circuits are 
applied to the power supply through the parasitic elements. 
On an ideal condition, the power supply voltage fluctuation V.sub.A caused 
by noise applied to node A has the same amplitude and the same phase at 
node B and node C. As a result, the power supply voltage fluctuation is 
canceled by the differential input amplifier 40, to avoid decreasing the 
gain. At the lower frequency, the voltages at node B and node C have the 
same amplitude and the same phase as the voltage at node A, and therefore 
the voltages at node B and node C arc substantially canceled. At the 
higher frequency area, however, the gain of node B decreases by the 
influence of the parasitic capacitance Cs on the collector side of the 
transistor 21, while the gain of node C hardly decreases. In other words, 
V.sub.B and V.sub.C are not consistent on the input side of the 
differential input amplifier 40 at the high frequency area, which degrades 
PSRR. The frequency characteristics at nodes B, C are explained in detail 
as follows. 
In FIG. 4, the phase at node B for node A is generally calculated in the 
following. Assuming resistance of resistor 11 is R.sub.11, and the 
parasitic capacitance of the collector of the transistor 21 is Cs, the 
sign wave voltage V.sub.B appearing at node B when sign wave V.sub.A 
enters into node A is expressed as follows. 
##EQU1## 
Therefore, phase .theta..sub.A of the voltage at node B for node A is 
expressed as follows. 
EQU .theta..sub.A =tan.sup.-1 (.omega.R.sub.11 Cs) 
As easily understood from the above formulas, the phase of the voltage at 
node B is influenced by the parasitic capacitance Cs. Therefore, the 
voltage at the node B decreases as the frequency increases according to 
the phase delay of the parasitic capacitance Cs, as shown in FIG. 5. 
FIG. 6 shows another conventional single end input type high frequency 
amplifying circuit. In FIG. 6, capacitor 75 having the same capacitance as 
that of coupling capacitor 73 is connected to the reference side terminal 
of a differential input amplifier 40, in order to cancel the effect of 
coupling capacitor 73 connected between an amplifier 72 and the input side 
of the differential input amplifier 40 (Laid-open Japanese Patent 
Publication 64-77206). In this circuit, since the capacitor having the 
same impedance as that connected to the output side of the amplifier 72 is 
connected to reference side terminal, the phase of the noise from the 
power supply becomes the same as that of the referential input of the 
differential input amplifier 40, which cancels the noise component. 
In this circuit, it is possible to cancel the effect of the coupling 
capacitance connected at the outside of amplifier 72 as explained above. 
However, since the parasitic capacitance caused by PN junction between the 
collector and P substrate varies by the voltage and temperature between 
the PN junction, it is difficult to cancel the noise voltage by means of 
connecting the above mentioned capacitor at the reference side terminal of 
the differential input amplifier. 
SUMMARY OF THE INVENTION 
According to one aspect of the invention, a single end input type high 
frequency amplifier comprising a transistor, having the same output load 
as that of a transistor of preceding stage connected to an input side 
terminal of the differential input amplifier, which is connected to a 
reference side terminal of the single end type transistor; where the loop 
amplifier feedbacks a difference voltage between a reference side terminal 
and an input side terminal of the differential input amplifier to the 
single end type transistor. 
According to another aspect of the invention, a single end input type high 
frequency amplifier which comprises a transistor, whose collector is 
connected to a power supply through a resistor, whose base is connected to 
a constant-voltage source, and whose emitter is connected to a collector 
of a single end type transistor, a differential input amplifier, and a 
loop amplifier, for canceling a noise applied to a power supply 
comprising, a transistor whose collector is connected to a power supply 
through a serial circuit of a constant-voltage source and a resistor, 
whose base is connected to a constant-voltage source, wherein the 
differential input amplifier comprises an input side terminal connected to 
a collector of the transistor, and a reference side terminal connected to 
a collector of transistor, and the loop amplifier feedbacks a difference 
voltage between a reference side terminal and an input side terminal of 
the differential input amplifier to the base of the single end type 
transistor. 
According to further aspect of the invention, a single end input type high 
frequency amplifier which comprises a transistor whose collector is 
connected to a power supply through a resistor, whose base is connected to 
a constant-voltage source, and whose emitter is connected to a collector 
of a single end type transistor, a differential input amplifier, and a 
loop amplifier, for canceling a noise applied to a power supply comprises 
a transistor whose collector is connected to a power supply through a 
serial circuit of resistors, whose base is connected to a constant-current 
source, and whose emitter is connected to a constant-voltage source; 
wherein the differential input amplifier comprises an input side terminal 
connected to a collector of the transistor, and a reference side terminal 
connected to a collector of transistor, and the loop amplifier feedbacks a 
difference voltage between a reference side terminal and an input side 
terminal of the differential input amplifier to the base of the single end 
type transistor. 
According to further aspect of the invention, resistance of the resistor is 
the same as that of the resistor. 
According to further aspect of the invention, an output terminal of the 
amplifier is grounded through a capacitor. 
According to further aspect of the invention, a capacitor is connected to 
both ends of the resistor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Embodiment 1 
FIG. 1 shows a single end input type high frequency amplifying circuit of a 
first embodiment of the present invention. A single end input high 
frequency amplifier of the first embodiment of the present invention is 
explained as follows using FIG. 1. In FIG. 1, a collector of a transistor 
21 is connected to a power supply 2 through a resistor 11, an emitter of 
the transistor 21 is connected to a collector of a single end input type 
transistor 22, and a base of a transistor 21 is connected to a constant 
voltage source 3. An emitter of the single end input type transistor 22 is 
connected to a MR head through an input terminal 100. A capacitor Cs 
connected to the collector of transistor 21 is a parasitic capacitance. 
Transistor 23 operates to cancel the parasitic capacitance Cs connected to 
the collector of the transistor 21. A collector of the transistor 23 is 
connected to a power supply 2 through a resistor 13 and a constant voltage 
source 5, an emitter of the transistor 23 is grounded through constant 
voltage source 7, and a base of the transistor 23 is grounded through 
constant voltage source 6. The transistor 23 is formed such that it has 
the same size and the same parasitic capacitance as that of the parasitic 
capacitor Cs connected to the transistor 21. And also the base potential 
(by constant voltage source 6) and the emitter potential (by constant 
voltage source 7) of the transistor 23 are designed so that they are the 
same as those of transistor 21, respectively. Furthermore, resistor 13 is 
designed so that it has the same resistance as that of the resistor 11. 
One input terminal of the differential input amplifier 40 is connected to 
the collector of transistor 21 and also one input terminal of a loop 
amplifier 30 through node B, and the other input terminal of the 
differential input amplifier 40 is connected to the collector of the 
transistor 23 and also the other input terminal of a loop amplifier 30 
through node C. A difference between the two input voltages of the 
differential input amplifier 40 are outputted to the output terminals 
having opposite polarity. An output from the loop amplifier 30 is 
outputted to the base of the transistor 22. 
In the first embodiment, a time constant comprised of the resistor 11 and 
the parasitic capacitance Cs of transistor 21 at the input side (node B) 
is the same as that comprised of the resistor 13 and parasitic capacitance 
Cs of transistor 23 at the input side (node C), which makes the frequency 
characteristics of both the reference side and the input side of 
differential input amplifier 40 equal to improve PSRR. 
In the pre-amplifier of the present invention, since the collector of the 
transistor 21 is connected directly to the differential input amplifier 40 
without using any coupling capacitors, the voltages at the reference side 
and the input side of the differential input amplifier 40 are preferably 
equal to each other. Next, it is explained how a feedback loop comprised 
of the loop amplifier 30 and single end input type transistor 22 equalizes 
DC potential on the reference side with that on the input side. Assuming 
the voltage at node B is higher than that at node C, an output voltage of 
loop amplifier 30 increases, then a base voltage becomes higher and a 
collector current of single end input type transistor 22 increases. As a 
result, the voltage drop of resistor 11 increases and then the voltage at 
node B decreases, which makes the voltage of the input side of the 
differential input amplifier 40 reach the voltage of the reference side of 
the differential input amplifier 40. 
On the contrary, if the voltage at node B decreases below that of node C, 
the output voltage of loop amplifier 30 decreases, then a base voltage 
becomes lower and a collector current of single end input type transistor 
22 decreases. As a result, the voltage drop of resistor 11 decreases and 
then the voltage at node B increases, which makes the voltage of the input 
side of the differential input amplifier 40 reach the voltage of the 
reference side of the differential input amplifier 40. In this way, the 
voltages at nodes B and C are controlled to be equal by the feed-back 
loop. 
Here, capacitor 50 attenuates high frequency components of the output 
signal so that the feedback may not operate at high frequency. 
The present invention relates to an apparatus for decreasing noise, in 
spite of phase delay caused by the parasitic capacitance in case of the 
single end input high frequency amplifier. In case of the double end input 
amplifier, on the other hand, since parasitic capacitances Cs are inserted 
into both nodes B, C in the same way, as shown in FIG. 7 for instance, 
noise from the power supply is canceled at the input of differential input 
amplifier 40, which prevents the problem occurred in case of the single 
end type transistor amplifier. 
The pre-amplifier treating the small signals uses a single end input type 
transistor 22 having a low noise characteristic. The transistor having a 
low noise characteristic is generally larger in size, and has the larger 
parasitic capacitance. For this reason, if the resistor 11 is connected 
directly to the transistor 22 having a larger parasitic capacitance, 
characteristic at high frequency area becomes worse. Therefore, in 
general, transistor 21 having smaller parasitic capacitance connected in 
series with single end input type transistor 22, to prevent the phase 
delay of the signal at node B. Generally, the emitter resistance of 
transistor 21 has a resistance value of several .OMEGA., while resistance 
11 has a resistance value of around hundreds .OMEGA., though it varies by 
the current which flows through the resistors. 
Embodiment 2 
FIG. 2 shows another embodiment of the present invention. The circuit of 
FIG. 2 is similar to that of FIG. 1. Therefore, only the difference of the 
circuit from FIG. 1 is explained below. In FIG. 2, resistors 14, 13 are 
connected to the collector of the transistor 23, a constant-current source 
10 is connected to an emitter of the transistor 23, and a constant-voltage 
source 5 is connected to a base of the transistor 23. Reference side 
terminal (node C) of the loop amplifier 30 is connected to a collector of 
transistor 23, and an input side terminal (+) of the loop amplifier 30 is 
connected to the collector of transistor 21. A capacitor 31 is connected 
in parallel to resistor 14. Two input terminals (node B side, node C side) 
of the differential input amplifier 40 are connected to the collector of 
transistor 21 and the collector of transistor 23, respectively. 
The current of constant-current source 10 flows through resistors 14, 13, 
and the constant voltage drop of the resistor 13 is provided to the node C 
side of the differential input amplifier 40. That is, transistor 23 and 
resistor 13 are inserted in order to equate impedances at nodes B and C. A 
loop amplifier 30 is provided in order to equate the DC voltages at 
respective input terminals (node B side, node C side) of the differential 
input amplifier 40. Since the operation of the loop amplifier 30 is the 
same as that in the first embodiment, further explanation is omitted. The 
transistor 23 is formed such that it has the same emitter size and the 
same parasitic capacitance of that of the parasitic capacitor Cs connected 
to the transistor 21. And also the base potential and the emitter 
potential and current of the transistor 23 are designed so that they are 
the same as those of transistor 21, respectively. In this way, it is 
possible to obtain the single end input type high frequency amplifier 
having improved PSRR. The capacitor 31 is connected to the resistor 14 in 
parallel in the preamplifier treating small signal in the hard disc drive 
apparatus which is required less noise. The capacitor 31 removes the high 
frequency noise generated at resistance 14 and constant-current source 10.